Linear Guide Rail Sizes Explained: 15, 20, 25, 30, 35, 45 and 55

Sep 24

Linear guide sizes such as 15, 20, 25, 30, 35, 45, and 55 are nominal size designations used to identify the physical scale of a profile rail system. As the nominal size increases, the rail, carriage, load ratings, permissible moments, and overall stiffness generally increase—but the number does not by itself tell you the exact rail width, carriage dimensions, or load capacity.

That last point is critical.

A size 25 linear guide is not simply “a 25 mm-wide rail” across every manufacturer and every series.

For example, THK’s HSR25 is a nominal size 25 guide, but its published HSR rail width is 23 mm. HSR30 uses a 28 mm rail, HSR35 uses a 34 mm rail, and HSR55 uses a 53 mm rail. THK nevertheless identifies these as nominal sizes 25, 30, 35, and 55.

So when someone says:

“I need a size 25 linear rail,”

that is only the beginning of the identification or selection process.

Key Takeaways

15, 20, 25, 30, 35, 45, and 55 are common nominal profile rail size designations.
The nominal size is a family or size designation—not a universal statement of every physical dimension.
Some series have a rail width equal to the nominal number; others do not.
A larger nominal size generally means a larger rail and block with greater potential load and moment capacity.
Load capacity cannot be determined from nominal size alone.
Standard and long blocks within the same nominal rail size can have different C and C₀ ratings.
Flanged and narrow blocks can share the same nominal rail size while having very different external dimensions.
“Size 25” does not prove that a THK, Schaeffler, SBC, WON, HIWIN, or other carriage is interchangeable.
Rail width, rail height, mounting-hole pitch, bolt size, block height, block width, block length, preload, and accuracy all matter.
Nominal rail size should be treated as an initial classification—not a complete part number.
Replacement applications should be identified from the complete manufacturer and model whenever possible.
New designs should be sized from actual machine loads and moments rather than choosing a rail size first.

What Do Linear Guide Sizes Mean?

Profile rail manufacturers organize their product families into standardized or semi-standardized nominal sizes.

Common industrial sizes include:

Larger and smaller guide sizes also exist.

For example, THK’s current HSR global-standard product family spans sizes from 8 through 150, while the common HSR C-style blocks include sizes 15, 20, 25, 30, 35, 45, 55, and 65.

These size numbers help organize:

rails,
matching carriages,
dimensional tables,
load-rating tables,
accessories.

They are best thought of as nominal frame sizes.

Does the Size Number Equal Rail Width?

Sometimes.

Not always.

Consider THK’s HSR family.

Nominal SizeTHK HSR Rail Width1515 mm2020 mm2523 mm3028 mm3534 mm4545 mm5553 mm

These are published dimensions for the HSR series.

Notice:

size 15 uses a 15 mm rail,
size 20 uses a 20 mm rail,
size 25 uses a 23 mm rail,
size 30 uses a 28 mm rail,
size 35 uses a 34 mm rail,
size 45 uses a 45 mm rail,
size 55 uses a 53 mm rail.

Therefore:

Do not infer exact rail width from the nominal size designation without checking the manufacturer’s dimensional table.

Why Manufacturers Use Nominal Sizes

Nominal size designations make it easier to organize related guide components.

Within a particular series, a nominal size commonly identifies a compatible group of:

rails,
standard blocks,
long blocks,
flange blocks,
narrow blocks.

THK’s HSR family, for example, offers several carriage styles within many of these same nominal sizes.

That means a size number usually tells you the general platform size, but the remaining part number determines the actual carriage configuration.

Size Is Only One Part of a Linear Guide Part Number

Consider the THK HSR family.

A customer could encounter model designations such as:

HSR25C,
HSR25LC,
HSR25R,
HSR25LR.

All belong to nominal size 25.

But they are not the same carriage.

THK describes:

C as a flange-style block,
LC as a longer flange-style block,
R as a narrower block,
LR as a longer narrow block.

This is why saying only:

“I need a 25.”

does not fully identify the component.

Why Block Style Matters

Different blocks in the same nominal size can differ in:

overall width,
overall length,
mounting-hole pattern,
dynamic load rating,
static load rating,
permissible moment.

For example, THK describes its HSR-LC long block as having the same general cross-sectional shape as HSR-C but a longer overall block length and greater load rating.

So two carriages may both be size 25 but provide different performance.

Size 15 Linear Guides

Size 15 is a common compact industrial profile rail size.

Within THK’s HSR family, size 15 is offered in several carriage configurations.

For the HSR-C example, THK publishes approximately:

rail width: 15 mm,
rail height: 15 mm,
assembled height: 24 mm,
flange-block width: 47 mm.

For HSR15C, THK’s published basic load ratings are approximately:

dynamic C: 10.9 kN,
static C₀: 15.7 kN.

The longer HSR15LC increases those ratings to approximately:

dynamic C: 14.2 kN,
static C₀: 22.9 kN.

This immediately illustrates why nominal size alone is insufficient.

Both are:

size 15

but their load ratings differ.

Typical Reasons to Consider Size 15

Size 15 may be worth evaluating for:

compact automation,
instrumentation,
smaller positioning stages,
packaging equipment,
light machine assemblies.

The final decision still depends on actual loading and rigidity.

Size 20 Linear Guides

Size 20 moves into a somewhat larger physical platform.

In THK HSR-C dimensions:

nominal size: 20,
rail width: 20 mm,
rail height: 18 mm,
assembled height: 30 mm,
flange-block width: 63 mm.

HSR20C is published with basic ratings of approximately:

C = 19.8 kN,
C₀ = 27.4 kN.

The longer HSR20LC increases the corresponding ratings to approximately:

C = 23.9 kN,
C₀ = 35.8 kN.

Again, the block style changes capacity without changing the nominal rail size.

Size 25 Linear Guides

Size 25 is one of the most recognizable industrial profile rail designations.

But it also provides one of the clearest examples of why nominal size should not be confused with exact rail width.

For THK HSR25:

nominal size = 25,
published rail width = 23 mm,
rail height = 22 mm.

For HSR25C:

assembled height = 36 mm,
flange-block width = 70 mm,
block length = approximately 83.1 mm.

THK publishes approximately:

C = 27.6 kN,
C₀ = 36.4 kN

for HSR25C.

The longer HSR25LC increases those figures to approximately:

C = 35.2 kN,
C₀ = 51.6 kN.

So even within one manufacturer and one rail family:

size 25 does not equal one load capacity.

Why “25 mm Rail” Can Be Misleading

Some distributors may describe a product as a:

25 mm nominal rail

while separately listing an actual physical rail width.

For example, a current THK HSR25 listing identifies it as a nominal rail size of 25 mm while giving the overall rail width as 23 mm.

This distinction is extremely useful when identifying an unknown guide.

If you measure a rail at approximately:

23 mm wide

you should not immediately conclude:

“This is not a size 25.”

It may indeed be a nominal 25 guide.

Size 30 Linear Guides

In the THK HSR example:

nominal size = 30,
rail width = 28 mm,
rail height = 26 mm.

HSR30C dimensions include approximately:

assembled height = 42 mm,
flange-block width = 90 mm,
block length = 98 mm.

Published basic ratings are approximately:

C = 40.5 kN,
C₀ = 53.7 kN.

For HSR30LC:

C = 48.9 kN,
C₀ = 70.2 kN.

Compared with smaller sizes, the guide occupies more physical space but also provides substantially greater load-rating potential.

Size 35 Linear Guides

For THK HSR35:

nominal size = 35,
rail width = 34 mm,
rail height = 29 mm.

HSR35C dimensions include approximately:

assembled height = 48 mm,
flange-block width = 100 mm,
block length = 109.4 mm.

Its basic ratings are approximately:

C = 53.9 kN,
C₀ = 70.2 kN.

The longer HSR35LC increases them to approximately:

C = 65 kN,
C₀ = 91.7 kN.

Size 35 therefore represents another meaningful increase in both physical envelope and potential load capability.

Size 45 Linear Guides

Size 45 enters a much heavier physical guide class.

For THK HSR45:

rail width = 45 mm,
rail height = 38 mm.

HSR45C dimensions include approximately:

assembled height = 60 mm,
flange-block width = 120 mm,
block length = 139 mm.

Published HSR45C basic ratings are approximately:

C = 82.2 kN,
C₀ = 101 kN.

HSR45LC increases those ratings to approximately:

C = 100 kN,
C₀ = 135 kN.

This is significantly larger than a size 15 or 20 guide.

But even here, selection still cannot be made from size alone.

Size 55 Linear Guides

Size 55 is a large industrial guide size.

For THK HSR55:

nominal size = 55,
rail width = 53 mm,
rail height = 44 mm.

HSR55C dimensions include approximately:

assembled height = 70 mm,
flange-block width = 140 mm,
block length = 163 mm.

THK publishes approximately:

C = 121 kN,
C₀ = 146 kN

for HSR55C.

HSR55LC increases those values to approximately:

C = 148 kN,
C₀ = 194 kN.

That illustrates the substantial scale difference between small and large guide families.

Size Comparison Using One Example Series

The following table uses THK HSR-C as a single controlled example.

It should not be treated as a universal dimensional table for every brand.

Nominal SizeRail WidthRail HeightAssembled HeightBlock WidthDynamic CStatic C₀1515 mm15 mm24 mm47 mm10.9 kN15.7 kN2020 mm18 mm30 mm63 mm19.8 kN27.4 kN2523 mm22 mm36 mm70 mm27.6 kN36.4 kN3028 mm26 mm42 mm90 mm40.5 kN53.7 kN3534 mm29 mm48 mm100 mm53.9 kN70.2 kN4545 mm38 mm60 mm120 mm82.2 kN101 kN5553 mm44 mm70 mm140 mm121 kN146 kN

Source: THK HSR dimensional data.

The trend is clear:

larger nominal size generally corresponds to a larger physical guide and higher load ratings.

But that does not create a universal cross-brand equivalency.

Why This Table Should Not Be Used for Other Brands

A manufacturer can use the same nominal size designation while having different:

rail width,
rail height,
block height,
block width,
mounting dimensions,
hole pitch,
load ratings.

Even different series from the same manufacturer can vary.

THK itself offers multiple guide families using familiar nominal sizes. Its HSR global-standard family and SR radial-type family both include many sizes such as 15, 20, 25, 30, 35, 45, and 55, but they do not represent identical guide architectures.

Therefore:

nominal size is not a universal interchange standard.

Is Every Size 25 Carriage Interchangeable?

No.

This is one of the most important points in this article.

Two carriages can both be described as:

size 25

and still differ in:

rail geometry,
raceway geometry,
block dimensions,
mounting-hole spacing,
installed height,
preload,
accuracy,
seals,
load rating.

Do not install a replacement simply because the number 25 appears in both part numbers.

Nominal Size vs. Interchangeability

A true interchange should evaluate at least:

manufacturer,
guide series,
nominal size,
rail width,
rail height,
block height,
block width,
block length,
block mounting pattern,
rail mounting-hole pitch,
rail bolt size,
raceway compatibility,
preload,
accuracy class,
load ratings.

This is why the full model number is so valuable.

Can a Block From One Size Fit Another Rail Size?

Normally, no.

A size 20 block is designed around the geometry of its corresponding rail family.

A size 25 block belongs to a different dimensional envelope.

Do not attempt to mix nominal rail sizes unless a manufacturer explicitly documents compatibility.

Can Blocks From Different Series Fit the Same Nominal Size Rail?

Do not assume so.

For example:

HSR25

and another THK size 25 family designation

may both contain the number 25 but belong to different rail architectures.

The complete series matters.

The same warning applies across manufacturers.

Why Rail Width Alone Is Not Enough

Suppose you measure an unknown rail and find:

23 mm width.

That measurement may point toward a nominal size 25 guide in certain series.

But you still need:

rail height,
mounting-hole dimensions,
carriage dimensions,
model markings.

Several products may have similar dimensions.

Identification should rely on multiple features.

Why Installed Height Matters

Installed height is the distance from the mounting surface under the rail to the primary mounting surface on top of the carriage.

This dimension is extremely important in replacement work.

If a replacement carriage is even several millimeters taller or shorter, it can change:

machine alignment,
actuator position,
tooling height,
coupling geometry.

THK’s HSR-C examples show installed heights increasing from:

24 mm at size 15,
to 70 mm at size 55.

Why Block Width Matters

Flange-style blocks can be considerably wider than the rail itself.

For example, in the same HSR-C family:

HSR15C block width = 47 mm,
HSR25C = 70 mm,
HSR35C = 100 mm,
HSR55C = 140 mm.

This affects:

tabletop width,
mounting-hole placement,
machine clearance.

Why Block Length Matters

Block length changes:

available rolling-element contact,
permissible moments,
load ratings.

That is why manufacturers offer:

standard blocks,
long blocks.

THK explicitly states that HSR-LC uses a longer block than HSR-C and provides greater load rating.

So if a size 25 standard block does not meet the load requirement, the next step is not automatically a size 30.

A long size 25 block may also deserve evaluation.

Should You Always Move Up a Rail Size for More Capacity?

No.

Possible ways to increase application capability include:

selecting a long block,
using additional blocks,
increasing block spacing,
increasing rail spacing,
using a higher-capacity series,
reducing moments,
moving up a nominal size.

Choosing a larger rail is only one option.

Long Block vs. Larger Rail Size

Suppose a size 25 standard block does not provide enough:

fatigue life,
moment capacity.

Potential choices might include:

Option A

Use a long size 25 block.

Option B

Use two size 25 blocks.

Option C

Move to size 30.

Which is best depends on:

available length,
available width,
mounting pattern,
rigidity,
load distribution.

Why Larger Is Not Always Better

Oversizing a linear guide can increase:

component cost,
moving mass,
block dimensions,
rail dimensions,
mounting-bolt requirements.

It can also create unnecessary design changes.

The engineering goal is not:

choose the biggest rail possible.

It is:

choose the smallest practical guide configuration that meets load, life, rigidity, accuracy, and environmental requirements with appropriate margin.

How Load Ratings Change With Size

Within a comparable guide family, increasing nominal size generally increases:

dynamic load rating C,
static load rating C₀.

The HSR-C example demonstrates this clearly:

HSR15C C ≈ 10.9 kN,
HSR25C C ≈ 27.6 kN,
HSR35C C ≈ 53.9 kN,
HSR55C C ≈ 121 kN.

But those values are specific to that series and block style.

How Static Capacity Changes With Size

The same trend occurs with C₀.

Using HSR-C:

size 15 ≈ 15.7 kN,
size 25 ≈ 36.4 kN,
size 35 ≈ 70.2 kN,
size 55 ≈ 146 kN.

Again, this should not be interpreted as a universal chart for all size 15, 25, 35, or 55 guides.

How Moment Capacity Changes With Size

Larger blocks typically provide increased permissible moments because they have:

larger rolling contacts,
larger physical geometry,
greater internal support distances.

Block length also affects permissible moment.

Therefore, moment capacity should be checked from the exact manufacturer’s table rather than estimated from nominal rail size.

How Rigidity Changes With Size

Larger guide systems generally have greater stiffness potential.

But rigidity depends on more than rail size.

Important factors include:

ball vs. roller design,
preload,
block length,
number of blocks,
rail spacing,
block spacing,
mounting structure.

A smaller roller guide may, in some applications, provide different stiffness characteristics than a larger ball guide.

Size 15 vs. 20

If both sizes pass the load calculation, the decision may come down to:

available space,
rigidity,
moment loading,
life,
cost.

Do not move to 20 just because it sounds more industrial.

Size 20 vs. 25

Size 25 typically provides a larger platform with greater load capability.

But it also requires more space.

Check:

installed height,
block width,
rail geometry,
mounting bolts.

The physical difference may affect the whole machine layout.

Size 25 vs. 30

This is a particularly common design step.

In the HSR-C example:

size 25 dynamic rating ≈ 27.6 kN,
size 30 dynamic rating ≈ 40.5 kN.

But HSR25LC provides approximately 35.2 kN without changing nominal rail size.

That demonstrates why block configuration should be considered before automatically increasing rail size.

Size 30 vs. 35

Moving from 30 to 35 increases:

rail dimensions,
carriage dimensions,
ratings.

But improved guide geometry or an additional carriage may sometimes allow size 30 to remain adequate.

Size 35 vs. 45

The step from size 35 to 45 is physically significant.

In HSR-C:

block width increases from about 100 to 120 mm,
assembled height rises from about 48 to 60 mm,
dynamic rating rises from about 53.9 to 82.2 kN.

That is not a minor substitution.

It can require meaningful machine redesign.

Size 45 vs. 55

Likewise, size 55 occupies substantially more space.

In HSR-C:

block width increases from approximately 120 to 140 mm,
assembled height from 60 to 70 mm,
dynamic rating from approximately 82.2 to 121 kN.

The additional capacity must justify the larger machine envelope.

What Size Linear Guide Do You Need?

Do not start with:

“I think I need a size 25.”

Start with:

moving mass,
center of gravity,
mounting orientation,
acceleration,
process forces,
pitch/yaw/roll moments,
rail arrangement,
block arrangement,
required service life,
rigidity,
accuracy,
environment.

Then determine the required:

C,
C₀,
permissible moments,
rigidity.

Only then choose a preliminary nominal size.

A Better Selection Sequence

Machine requirements

↓

Applied forces

↓

Moments

↓

Individual block loads

↓

Static safety

↓

Dynamic life

↓

Rigidity

↓

Rail/block arrangement

↓

Preliminary nominal size

↓

Exact guide series and block style

↓

Dimensional verification

This is much more reliable than choosing a size from payload weight alone.

How to Identify an Existing Rail Size

If the part number is readable, start there.

If not, measure:

rail width,
rail height,
rail mounting-hole pitch,
mounting-hole diameter/counterbore,
assembled height,
block width,
block length,
block mounting-hole spacing.

Also photograph:

block markings,
rail markings,
end caps,
grease fittings.

These details can help narrow the manufacturer and series.

Do Not Round a Measurement to the Nearest Nominal Size

Suppose you measure:

23 mm rail width.

Do not simply round that to:

20 or 25

and order something.

As the HSR25 example shows, 23 mm can correspond to nominal size 25.

Always compare the measurement against the manufacturer’s technical drawing.

What About Mounting-Hole Pitch?

Nominal size does not determine finished rail hole locations by itself.

Rail-hole pitch varies by:

series,
size,
rail design.

Cut rails also have end distances that matter.

Linear Automation USA provides a Linear Rail Cut Calculator for planning:

finished length,
hole pitch,
first-hole/end distance.

Can Rail Length Be Customized?

Yes, many profile rails can be supplied or cut to required lengths within manufacturer and application constraints.

However, cutting the rail does not change its nominal size.

size 25 × 500 mm rail

and a:

size 25 × 2,000 mm rail

remain the same nominal guide size.

Length is a separate specification.

Nominal Size vs. Rail Length

These two dimensions are often confused.

A part designation can contain numbers referring to:

nominal size,
rail length,
preload,
accuracy,
accessories.

For example:

25 may identify the guide size,

while another number identifies the rail's finished length.

Read the manufacturer’s model-number structure before interpreting the digits.

Nominal Size vs. Accuracy Class

Size also tells you nothing about accuracy grade.

The same nominal guide can be supplied in different accuracy classes depending on manufacturer and series.

Accuracy can affect:

running parallelism,
height tolerance,
dimensional matching.

A size 25 precision guide is still nominal size 25.

Nominal Size vs. Preload

Likewise, preload is separate.

Two visually identical size 25 carriages may have different preload classes.

That can change:

stiffness,
friction,
internal loading,
life.

Therefore, replacements should match preload where required.

Nominal Size vs. Seal Configuration

Guide blocks may also be supplied with different:

end seals,
side seals,
scrapers,
lubricators.

These options may affect overall carriage dimensions or running resistance.

Nominal size does not capture those details.

Nominal Size vs. Material

Some guide families offer:

standard bearing steel,
stainless options,
surface treatments.

Again, the size number does not identify these characteristics.

Common Linear Guide Size Mistakes

Mistake 1: Assuming Size Equals Exact Rail Width

It may not.

Mistake 2: Selecting From Size Alone

Load rating depends on the exact guide series and block.

Mistake 3: Assuming Every Size 25 Is Interchangeable

Nominal size does not establish compatibility.

Mistake 4: Ignoring Block Style

Standard and long carriages can have different ratings.

Mistake 5: Ignoring Installed Height

This can create serious replacement problems.

Mistake 6: Ignoring Rail Hole Pitch

A rail can be the right general size but still not match the mounting base.

Mistake 7: Choosing Larger Size Before Improving Geometry

Rail spacing and block spacing can reduce individual carriage loads.

Mistake 8: Assuming Bigger Always Means Better

Oversizing adds cost, weight, and space.

Mistake 9: Identifying a Rail From Width Only

Use multiple dimensions and the complete model number when possible.

Mistake 10: Ignoring Preload and Accuracy During Replacement

Dimensional similarity does not prove functional equivalence.

Linear Automation USA’s Perspective

At Linear Automation USA, we treat sizes such as:

15, 20, 25, 30, 35, 45, and 55

as useful starting points—but never as complete specifications.

The biggest misconception we want buyers to avoid is:

“It measures about 25 mm, so any size 25 block should fit.”

That is not a safe replacement strategy.

Even within a respected manufacturer such as THK, the nominal number is not always identical to actual rail width. THK’s HSR25, for example, uses a published 23 mm rail width, while HSR30 uses 28 mm and HSR35 uses 34 mm.

More importantly, the same nominal rail size may have:

standard blocks,
long blocks,
flange blocks,
narrow blocks

with different dimensions and ratings.

For new machines, our preference is:

Do not choose size first. Calculate the machine first.

Determine:

maximum carriage loading,
moments,
required static safety,
dynamic life,
rigidity,
rail arrangement.

Then identify the smallest practical guide family that satisfies the requirements.

For replacements, we prefer the opposite approach:

Identify what is installed before selecting anything.

The best information is:

manufacturer,
complete part number,
rail number,
block number.

If those are unavailable, accurate measurements and photographs can often help.

Linear Automation USA supplies industrial profile rail systems from Schaeffler, SBC Linear, and WON Linear, along with replacement and cut-to-length support.

Our rule is:

Nominal size narrows the search. The complete geometry and part number determine the solution.

Frequently Asked Questions

What Does Size 25 Mean on a Linear Guide?

It identifies the nominal size family of the guide. It does not universally mean every physical dimension is exactly 25 mm.

Is a Size 25 Rail Always 25 mm Wide?

No. THK HSR25, for example, has a published rail width of 23 mm.

Is a Size 30 Rail Always 30 mm Wide?

No. THK HSR30 uses a published rail width of 28 mm.

Is a Size 35 Rail Always 35 mm Wide?

No. THK HSR35 uses a published rail width of 34 mm.

Is a Size 55 Rail Always 55 mm Wide?

No. THK HSR55 uses a published rail width of 53 mm.

Does a Larger Linear Guide Size Carry More Load?

Within comparable product families, generally yes. But use the exact C and C₀ ratings rather than estimating capacity from nominal size.

Can Two Blocks With the Same Size Number Have Different Load Ratings?

Yes. Standard and long blocks within the same nominal size can have different ratings.

Are All Size 25 Linear Guides Interchangeable?

No. Manufacturer, series, block geometry, rail geometry, mounting dimensions, preload, and accuracy must be verified.

Can a THK Size 25 Block Fit Another Brand's Size 25 Rail?

Do not assume so. Compatibility must be explicitly verified.

Should I Choose Size 30 Instead of Size 25 for More Capacity?

Possibly, but first evaluate long blocks, additional blocks, rail spacing, block spacing, and the exact required load and life.

What Is the Most Common Linear Guide Size?

There is no single best size for all machinery. Sizes 15 through 55 are widely represented across industrial product families, but application requirements determine the appropriate selection.

How Do I Know What Size Rail I Already Have?

Use the model number if available. Otherwise measure rail width, rail height, hole pitch, assembled height, block dimensions, and mounting pattern.

Can I Determine Rail Size From Rail Width Alone?

Not reliably. Use multiple dimensions and manufacturer documentation.

Does Rail Size Determine Rail Length?

No. Nominal size and rail length are separate specifications.

Does Rail Size Determine Preload?

No.

Does Rail Size Determine Accuracy?

No. Accuracy class is a separate specification.

Need Help Identifying a Linear Guide Size?

For an existing system, provide:

manufacturer,
complete block number,
complete rail number,
rail width,
rail height,
block width,
block length,
assembled height,
mounting-hole spacing,
rail hole pitch,
clear photographs.

For a new application, provide:

moving mass,
center of gravity,
acceleration,
mounting orientation,
process forces,
rail spacing,
block spacing,
required life,
rigidity requirements.

Contact Linear Automation USA for help identifying, sizing, sourcing, cutting, or replacing industrial profile rail systems.