Linen Thread Sizes Explained: What Does 18/3 Mean in mm?

Linen Thread Sizes Explained: What Does 18/3 Mean in mm?

If you’ve ever searched for “18/3 linen thread in mm” or wondered what the numbers on a spool of linen thread actually mean, you’re not alone.

When buying linen thread for leatherwork, you’ll commonly see sizes such as 18/3, 25/3, 30/3 and 40/3. These numbers describe the construction and traditional count of the thread — but they don't translate directly into a single, exact diameter in millimetres.

In this guide, we explain linen thread sizing, what 18/3 means, how linen thread sizes compare in mm, and which sizes are commonly used for different leatherworking projects.

Linen thread size chart

Here’s a quick reference for some of the linen thread sizes commonly used in leatherwork:

Linen thread size	Approx. diameter*	Typical leatherworking applications
18/3	0.55–0.65 mm	Heavy leatherwork, belts, bags and robust stitching
25/3	0.45–0.55 mm	General leatherwork, bags, cases and belts
30/3	0.40–0.50 mm	Medium-weight leather goods
35/3	0.35–0.45 mm	Fine leatherwork and smaller goods
40/3	0.30–0.40 mm	Fine stitching, wallets and small leather goods

*These are approximate diameter ranges rather than exact conversions. Actual thread diameter can vary according to the manufacturer, construction, twist and finishing of the thread.

What does 18/3 linen thread mean?

The 18/3 designation describes the yarn count and the number of plies used to make the finished thread.

The 18 refers to the count of the individual linen yarn, while the 3 indicates that three yarns have been twisted together.

So:

18/3 = three plies of 18-count linen yarn

Similarly:

• 25/3 = three plies of 25-count linen yarn
• 30/3 = three plies of 30-count linen yarn
• 40/3 = three plies of 40-count linen yarn

One important point is that 18/3 does not mean 18 mm or 3 mm. The numbers are part of a traditional thread-counting system rather than a measurement of diameter.

Is 18/3 linen thread thicker than 40/3?

Generally, yes.

When comparing threads with the same number of plies, a lower first number indicates a heavier individual yarn.

Therefore, as a general guide:

18/3 → thicker

25/3 → finer

30/3 → finer still

40/3 → finest of these examples

This makes the thread count useful when comparing different sizes, although the exact finished diameter can vary between manufacturers.

How do you convert linen thread size to mm?

This is one of the most common questions we get about linen thread.

Unfortunately, there isn't a universal formula that will convert a linen thread size such as 18/3 into one precise measurement in mm.

That's because the traditional linen numbering system and millimetre measurements describe different characteristics.

The linen count is based on the yarn's traditional weight-to-length numbering system, whereas a measurement in millimetres refers to the physical diameter of the finished thread.

The diameter can also be affected by:

• The thickness and quality of the linen fibres
• How the yarn has been spun
• The number of plies
• The tightness of the twist
• Waxing or other finishing
• Compression of the thread during stitching

For this reason, an 18/3 linen thread might not measure exactly the same diameter as another manufacturer's 18/3 thread.

The mm figures in a linen thread size chart should therefore be treated as a useful guide rather than an exact mathematical conversion.

18/3 linen thread in mm

If you're specifically looking for 18/3 linen thread in mm, a useful working estimate is approximately:

18/3 linen thread: approximately 0.55–0.65 mm

The exact measurement will depend on the particular thread.

For leatherwork, however, the difference between individual products can be less important than the overall relationship between thread size, stitch spacing, leather thickness and the appearance you want to achieve.

25/3 linen thread in mm

As a general guide:

25/3 linen thread: approximately 0.45–0.55 mm

25/3 is a useful middle-ground size for many leatherworking applications. It provides a visible stitch without looking as heavy as a larger 18/3 thread.

It can be suitable for projects such as:

• Leather bags
• Cases
• Belts
• General hand stitching
• Medium-weight leather goods

30/3 linen thread in mm

A typical working range for:

30/3 linen thread: approximately 0.40–0.50 mm

This gives a slightly finer appearance while retaining the character and strength associated with traditional linen thread.

It's often a useful option where 25/3 looks slightly too heavy but a finer thread would be unnecessarily delicate.

40/3 linen thread in mm

For fine leatherwork, you may come across:

40/3 linen thread: approximately 0.30–0.40 mm

40/3 is considerably finer than 18/3 and can produce a neat, refined stitch line.

It's particularly useful for smaller leather goods where a large, heavy stitch could overpower the design.

What size linen thread should I use for leather?

There isn't one correct linen thread size for leatherwork.

The best choice depends on the project, the thickness of the leather, your stitch spacing and the finish you're trying to create.

As a general starting point:

18/3 linen thread

Consider 18/3 when you want a bold, substantial stitch.

It can work well for heavier leather, traditional-style projects, belts and larger items where the stitching is intended to be visually prominent.

25/3 and 30/3 linen thread

These sizes offer a good balance between strength, visibility and refinement.

They're useful general-purpose options for many bags, cases, belts and other leather goods.

35/3 and 40/3 linen thread

These finer sizes are worth considering when you want a smaller, more refined stitch.

They can be particularly appropriate for wallets, watch straps and other smaller leather goods.

These aren't hard rules. Experienced leatherworkers often choose thread based on the particular aesthetic they're trying to achieve.

Linen thread diameter vs thread size

It's important to distinguish between thread size and thread diameter.

A thread labelled 18/3 has a traditional count and three plies. A measurement such as 0.6 mm describes its approximate physical diameter.

The two systems shouldn't be treated as interchangeable.

This is why you may see slightly different mm specifications for apparently similar linen threads.

Natural fibres such as linen aren't perfectly uniform, and the manufacturing and finishing processes can influence the final diameter.

Does waxing change the diameter of linen thread?

Yes, potentially.

Waxed linen thread can appear and measure slightly different from the un-waxed yarn because the wax adds material to the surface and can alter the way the fibres sit together.

More importantly for leatherworkers, waxing can change how the thread behaves during stitching. It can make the thread easier to control, help prevent the individual fibres from separating and contribute to a smooth finished stitch.

When comparing thread sizes, it's therefore worth checking whether the quoted diameter refers to the thread before or after finishing.

Thread size and stitch spacing

Choosing a thread isn't simply about finding the right diameter.

Thread size and stitch spacing should be considered together.

A thick thread with widely spaced holes creates a bold, traditional appearance.

A finer thread combined with closer stitching produces a more understated and refined result.

For example:

Heavy thread + wider spacing = bold and traditional

Fine thread + closer spacing = refined and delicate

The ideal combination will depend on the project and your personal preference.

Is thicker linen thread stronger?

Not necessarily in a simple, like-for-like sense.

A thicker thread will generally contain more material and can be appropriate for heavier-duty stitching, but thread strength depends on much more than diameter.

The quality of the linen, fibre construction, twist, finishing and manufacturing process can all affect performance.

The strongest-looking stitch isn't always the strongest stitch, either. The quality of the stitching technique, the size and preparation of the holes, the leather itself and how the finished item is used all contribute to the durability of a seam.

Linen thread sizes: the key points

If you're trying to remember how linen thread sizing works, there are a few simple points to keep in mind:

• 18/3 means three plies of 18-count linen yarn.
• The /3 refers to the number of plies, not the diameter.
• A lower first number generally means a heavier thread when comparing the same ply construction.
• 18/3 is generally thicker than 40/3.
• Linen thread sizes cannot be converted into one universally accurate mm measurement.
• Approximate diameter ranges can still be useful when choosing thread.
• The actual diameter can vary between manufacturers and products.
• Thread size should be considered alongside leather thickness, stitch spacing and the desired appearance.

Find the right linen thread for your leatherwork

Understanding linen thread sizing makes it much easier to choose the right thread for your next leatherworking project.

Whether you're looking for a substantial 18/3 linen thread, a versatile 25/3 or 30/3, or a finer 40/3 linen thread, the right choice depends on the balance you want between appearance, handling and performance.

At British Leather Supplies, we stock materials and tools for leatherworkers, saddlers and makers. If you're unsure which thread size is right for your project, comparing the available sizes alongside your leather and stitch spacing is a good place to start.

Frequently Asked Questions

What does 18/3 mean in linen thread?
18/3 means the thread is made from three plies of 18-count linen yarn. The 3 refers to the number of plies, not a 3 mm diameter.

What is 18/3 linen thread in mm?
As a general guide, 18/3 linen thread is approximately 0.55–0.65 mm in diameter, although the actual measurement can vary by manufacturer and finishing.

Is 18/3 thicker than 40/3 linen thread?
Generally, yes. When comparing the same ply construction, 18/3 is substantially heavier than 40/3.

What is the finest linen thread size?
There are linen threads finer than 40/3, depending on the manufacturer and sizing system. For leatherwork, 35/3 and 40/3 are commonly encountered finer sizes.

What linen thread should I use for a leather wallet?
A finer thread such as 35/3 or 40/3 may be appropriate for a refined wallet stitch, while 25/3 or 30/3 can give a more prominent stitch. Your leather thickness and stitch spacing should also be considered.

Is linen thread measured in mm?
Linen thread is traditionally specified using a count-and-ply system such as 18/3. Its diameter can also be measured in millimetres, but there is no universal one-to-one conversion between the two systems.

Does waxing affect linen thread size?
Waxing can alter the apparent or measured diameter and also changes the handling characteristics of the thread. When comparing products, check the manufacturer's specifications for the finished thread.