Understanding Linen Thread Weights for Leathercraft

Understanding Linen Thread Weights for Leathercraft

If you've ever stood in front of a display of linen threads wondering which one is right for your leatherwork, you're not alone. Understanding thread weights is fundamental to achieving professional results in leathercraft, whether you're hand-stitching a wallet, saddle, or bespoke bag.

Why Linen Thread for Leather?

Before diving into weights, it's worth understanding why linen remains the preferred choice for many leather artisans. Linen thread offers exceptional strength for its weight, resists rot and mildew, and has a natural wax affinity that makes it ideal for hand-stitching leather. Unlike synthetic alternatives, linen slightly swells when wet, helping to seal stitch holes and create watertight seams.

The Linen Thread Numbering System

Linen thread weights follow a numbering system that seems counterintuitive at first: the higher the number, the finer the thread. This is because the first number indicates how many kilometres of single-strand thread would weigh one kilogram. A 60/3 thread is therefore much finer than an 18/3, as it takes 60 kilometres of that single strand to reach one kilogram.

The second number after the slash denotes the ply count—how many individual strands are twisted together. Most linen threads for leatherwork are 3-ply or 4-ply, providing the strength needed for durable leather goods.

Thread Weight Comparison Chart

Here's a practical reference for the most common linen threads used in leathercraft:

Note: Diameters are approximate and can vary between manufacturers and depending on how tightly the thread is twisted.

Matching Thread to Leather Weight

The golden rule in leathercraft is to match your thread weight to your leather weight. Using thread that's too fine for heavy leather risks breakage and weak seams, whilst thread that's too heavy for delicate leather can tear through stitch holes and create unsightly, bulky seams.

Heavy Leather (3mm and above)

For belts, saddles, and heavy bags, reach for 18/3 or 18/4 thread. These robust threads can handle the stress of thick leather and large stitch holes without breaking. Your stitching will be visible and should be considered a design feature.

Medium Leather (2-3mm)

Bags, briefcases, and medium-weight goods work beautifully with 25/3 or 30/3 thread. These weights offer excellent strength whilst remaining proportional to the leather thickness. They're the workhorses of leathercraft.

Light Leather (1-2mm)

Wallets, card holders, and small leather goods call for 35/3 or 40/3 thread. These finer threads allow for neat, professional-looking seams that don't overpower delicate work.

Very Light Leather (under 1mm)

For watch straps, thin linings, or exceptionally fine work, 50/3 thread provides the refinement needed without sacrificing too much strength.

Stitching Technique Considerations

Saddle Stitching

The traditional hand-saddle stitch, where two needles work from opposite directions through the same hole, benefits from slightly heavier threads like 18/3 or 25/3. The crossing threads in each hole create a distinctive, strong seam that's become synonymous with quality leatherwork.

Stitch Length

Your thread weight should complement your stitch length. Heavier threads look best with longer stitch lengths (4-5mm or more), whilst finer threads suit tighter stitching (2-3mm). Mismatched combinations can appear either clumsy or weak.

Pricking Iron Size

Your stitching chisel or pricking iron teeth must accommodate your thread. As a general guide, 18/3 thread requires 3-4mm spacing, 25/3 works with 3-3.5mm, and 35/3-40/3 suits 2.5-3mm spacing. Holes that are too small make threading difficult; too large and the thread gets lost in the holes.

The Importance of Quality

In leathercraft, thread quality is paramount. Premium linen thread should be evenly twisted with minimal slubs or thin spots. It should accept wax readily and pull smoothly through leather without fraying. Barbour's, Fil Au Chinois, and traditional European manufacturers produce exceptional threads that are worth the investment for serious leatherwork.

Waxing Your Thread

Most linen thread for leatherwork benefits from additional waxing beyond any factory treatment. Beeswax or specialized thread wax helps the thread glide through leather, binds the plies together to prevent fraying, and seals stitch holes for weather resistance. Run your thread through the wax 2-3 times, then burnish it between your fingers or a piece of canvas to work the wax in evenly.

Colour Considerations

Whilst natural or ecru linen thread is traditional, coloured threads can dramatically enhance your leatherwork. Consider using contrasting thread colours to highlight your stitching as a design element, or matching threads for a subtle, refined look. Remember that natural linen takes dye beautifully and darkens attractively with age and handling.

Common Mistakes to Avoid

Using Synthetic Thread

Whilst polyester or nylon threads are strong, they lack linen's desirable qualities for leathercraft. They don't swell when wet, can cut through leather over time, and lack the traditional aesthetic that hand-stitched leather goods deserve.

Choosing Thread by Colour Alone

Many beginners select thread based solely on colour without considering weight. Always prioritize the appropriate weight for your leather thickness, then find the best colour within that weight range.

Insufficient Thread Length

Always cut generous lengths when saddle stitching—approximately four times your seam length plus extra for needles. Running short mid-seam necessitates joining, which weakens the seam and disrupts the rhythm of your stitching.

Building Your Thread Collection

If you're just starting out, consider these essential weights:

• One spool of 18/3 for heavy work
• One or two spools of 25/3 or 30/3 in neutral colours (your most-used threads)
• One spool of 35/3 or 40/3 for fine work

This collection will cover the vast majority of leathercraft projects whilst you develop your preferences and techniques.

For UK-based leather workers, British Leather Supplies offers an excellent range of quality linen hand sewing threads in various weights and colours, making it easy to build your collection and find exactly what you need for your projects.

Final Thoughts

Understanding linen thread weights elevates your leathercraft from functional to exceptional. The right thread, properly matched to your leather and technique, creates seams that are not only strong but beautiful—seams that will outlast the leather itself. Take time to experiment with different weights, develop your eye for proportion, and remember that the subtle details of well-chosen thread separate amateur work from professional craftsmanship.

Your thread is as much a part of your leather piece as the hide itself. Choose wisely, stitch carefully, and your work will reflect the pride you take in every detail.

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Post authored by Becky Cocker, leather artisan and owner of British Leather Supplies. If you'd like to explore a curated selection of quality leather tools and sewing supplies, check out our collection at British Leather Supplies. We’re here to help you take your leatherworking skills to the next level.