Spools of leather stitching thread in several sizes beside bridle leather and a caliper on a workbench.
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What Thread Size Goes With What Leather

ByPeter Joeseph 'Briefcase Maker' Kalafatis

The hide gets the blame. The hole makes the call.

Thread size for leather starts with the hole, and the hole starts with the leather, which is why every chart you find gives you a range and not a number. Thin goods want fine thread. Heavy goods want heavy thread. Everything interesting happens in between, where the tool in your fist, the spacing on the iron and the stiffness of the hide all pull on the same decision. I hand stitch everything I make now, after years of owning the machines, so this question comes down to my hands every time a seam starts. People ask the same six things, and the answers come from the charts, from makers who have done the testing, and from my own bench where my practice is on record.

Video: How to Choose the Right Thread and Needle Size for Hand-Sewing Leather, from Tandy Leather on YouTube.

Does thread size follow the leather or the stitch spacing?

Both, but the spacing has the final word.

Hands using pliers to pull a harness needle and waxed thread through thick bridle leather.

Start with the hide. Tandy Leather’s guide to choosing thread and needle size, written by Nicole Bradford in 2025, gives a three-row table. Leather of 1 to 2 mm takes thread of 0.4 to 0.6 mm and a size 2 to 4 needle. From 2 to 4 mm, the thread goes to 0.6 to 0.8 mm and the needle to size 0 to 2. Past 4 mm, Tandy says 1.0 mm and up, with a size 000 or larger needle.

That table is a fine place to start. It is not where the decision ends.

The older Tandy piece on thread weight and chisel size puts the other half in plain terms: thicker thread wants a chisel with wider spacing, finer thread wants narrower spacing, and the hole the chisel makes should take the thread without distorting the leather. Nigel Armitage says the same thing from the saddler’s side in his post on pricking irons, stitching irons and chisels. Match the iron to the item, the thread to the stitches per inch, and the needles to the thread. In his words, “The larger hole suits a larger thread.”

So the order runs like a plumbing job. Pick the pipe for the load, then the fitting for the pipe, then the washer for the fitting. The leather sets the load. The iron sets the fitting. The thread is the washer, and a washer that is the wrong size leaks no matter how good the pipe was.

My opinion: buy the iron first, then the thread. Choose the spacing that suits the piece and the leather, and let the thread follow the hole that iron leaves.

What do the numbers on the spool mean?

Some are millimeters, some run backwards, and one is in inches.

This is where beginners get burned, because three makers can sell three threads of almost the same size under three numbers that look nothing alike.

The easy one is the braided polyester most beginners buy first. Rocky Mountain Leather Supply sells Ritza 25 Tiger thread on full spools labeled by diameter: 0.6, 0.8, 1.0, 1.2 mm and up. It is braided, flattened and lightly waxed, and the number on the spool is its thickness. That simple.

French linen runs the other way. The same shop lists Fil au Chinois Lin Cable in five sizes with diameters beside them. Size 332 is 0.77 mm. Size 832 is 0.43 mm. Bigger number, thinner thread. Buckleguy’s page on the same thread lists the identical diameters. Neither page explains the numbering, so I won’t either. Read the millimeters, ignore the code, and buy by the diameter printed beside it.

American waxed cord adds a third system. Tandy sells Maine Thread’s twisted waxed poly cord at 0.035 inch, which the page converts to 0.9 mm and calls an all-purpose light-medium weight.

Then there is linen counted the old way. In a leatherworker.net thread on thread size and stitches per inch, the English maker Matt S works in 18-lea linen, written as 18/3, 18/4, 18/5, where the last figure is the number of yarns twisted together. In the same thread a member called gary lines up 0.8 mm Ritza with roughly 18/3 linen. The original poster adds that machine thread sizes such as 138 or 207 are not millimeters at all, citing a source that puts them at denier divided by ten.

Four systems. One bench. Keep a card of samples and trust your calipers over the label.

What size for a wallet, a belt, a bag?

The charts agree more than the forums fight.

Macro view of a leather sampler with rows of saddle stitching in different thread sizes and spacings.

Tandy’s 2025 guide splits it by job. Thread of 0.4 to 0.6 mm for wallets, watch straps and small goods. Thread of 0.8 to 1.0 mm for belts, bags and medium-weight work. Saddles, thick straps and rugged gear get 1.2 mm and up.

Weaver Leather Supply’s page on choosing a stitching chisel gives the spacing that goes with those jobs. A 3 mm chisel at 9 stitches per inch for wallets and watch straps, a 4 mm at 7 for most general projects. Then a 5 mm at 6 for handbags, and a 6 mm at 5 for saddles and belts.

Put those two lists side by side and the working makers on the forums land right inside them. In a thread asking what size thread people use, a member called Effuse writes, “At 6 Stitches per inch I like to use 0.8mm thread,” and goes to 1.0 mm at five or fewer. Another, El Zipster, uses 1 mm at six to seven stitches per inch on straps, belts and cases. In a thread on matching thread to stitches per inch, a holster maker posting as wizard of tragacanth runs 0.8 or 1.0 mm at six or seven, while two saddle makers in the same thread work at five to six with heavier thread still.

The pattern is plain enough. Fewer stitches per inch, heavier thread. The finer the goods, the closer the holes and the thinner the strand.

Where does a bridle leather briefcase sit?

At the heavy end, with less room than the chart suggests.

A briefcase is not a wallet. Its seams run through stiff bridle, often two layers and sometimes three where a pocket joins, and the handle carries every pound of paper the owner jams inside, every day. My opinion: a briefcase seam lives at the heavy end of the charts, closer to the thick straps than to the handbags.

Here is the part the charts leave out. On my page about the art of hand stitching, I say hand stitching tends to leave slightly bigger holes, because they are punched by hand and sewn with hand needles, and “The hand is limited to how small it can get.” The page adds the condition that matters: especially with thick bridle saddlery leather. Thick bridle sets a floor under the hole. The thread has to be sized to fill that floor, not the finer hole you might want.

That is why I pre-punch and line up every hole on thick work before a needle goes near it. Heavy bridle will not stitch any other way. The post on punching stitch holes in thick bridle leather covers the punching itself.

Armitage gives the old saddler’s standard for the fit. A traditional saddle-stitch hole is barely big enough to pass one needle at a time, so the thread cannot ride or slip in it. The thread should sit in the hole like a gasket that fits. Snug, pulled down, with no play.

What goes wrong when the match is off?

Too thick fights you. Too thin hides nothing.

Heavy saddle-stitched gusset seam on a finished bridle leather briefcase.

Too heavy first. In the same thread on what size thread people use, a member called Piedmont found that a 4 mm iron and Tandy’s waxed thread came out too thick for the work, and dropped to a 2 mm iron with a lighter bonded nylon. That is the common mistake. Heavy thread looks strong on the spool and then chews the grain, piles up at the corners and fights every pull.

Too light is quieter and worse. A thin strand in a big hole lets the stitch wander, shows daylight around the thread and leaves the seam loose in the leather instead of locked into it. The post on why a broken stitch doesn’t sink a saddle-stitched seam explains why each stitch locking tight is the whole point of the saddle stitch. A size 4 bolt in a size 6 hole holds nothing.

There is a test for it. In a leatherworker.net thread on hand sewing thick leather, a member called Martyn says the awl, needle and thread must be matched, and the thread should fill the hole “with no daylight visible.” He goes further. If you need pliers to pull the needle, he says, something is wrong, and you want a smaller needle, lighter thread or a bigger awl.

Now the reversal, because I owe it to you. He is right about most work and I still don’t follow him. On very thick bridle I wax the needles and pull them through with pliers, and I have no plan to stop. Three layers of the heaviest bridle do not care about a forum rule. A thread fat enough to fill a hand-punched hole in that stack is fat enough to need help getting through it, and if I sized down to spare my fingers, the seam would pay for my comfort for the rest of its life.

So both things are true. On a wallet, pliers mean you chose wrong. On a gusset, pliers may mean you chose right. The trick is knowing which of the two is sitting on your bench, before you blame the thread or your hands.

How do you test before you commit?

Thread is the cheapest thing on the bench. Spend some.

Matt S, in the thread on stitches per inch, gives the best advice in this whole post, and it costs almost nothing. Make a sampler. Prick a line at each spacing you plan to use, sew each one with a few different thread sizes, label them, and keep the strip for the next project. He is honest about the rest too: past a point, the ratio of thread to stitch length is “largely an aesthetic choice.”

Do it on the leather you will actually sew. A sampler on soft scrap tells you nothing about bridle, the same way a test drive in an empty lot tells you nothing about the highway. Cut a strip from the same hide, stack it the way the real seam will stack, and sew it with the real needles.

Then read it with your hands as much as your eyes. Most of stitching skill lives in the fingertips. You learn to feel the thread as it goes, and to know the moment a strand has popped somewhere inside the layers where no eye can follow it. A thread that is too heavy for the hole tells you early, through the pull. A thread that is too light tells you late, when the seam is done and sits a little loose.

Check both faces before you decide. The Maker page puts the standard in one line: “The side nobody sees is where a weak stitch gets found out.” A sampler that looks good on the front and ragged on the back is a size that fails.

My opinion: once you find the match for one leather and one iron, write it on the sampler and stop shopping. The post on why bridle is a hard first leather is worth reading before you choose the hide that the whole test depends on.

RITZA Tiger Thread, Brown: A braided waxed polyester thread for hand-stitching leather. See it on Amazon (affiliate link).

Mandala Crafts Waxed Linen Thread: A spool of waxed linen thread for hand sewing. See it on Amazon (affiliate link).

Maine Thread .040" Brown Waxed Polycord: A spool of brown waxed polyester cord, .040 inch. See it on Amazon (affiliate link).

Weaver Leather Supply Diamond Leather Stitching Chisel Set: A set of 4 mm diamond stitching chisels for punching saddle-stitch holes. See it on Amazon (affiliate link).

John James Saddlers Harness Blunt Point Needles: A set of blunt-point harness needles, five sizes with five needles of each. See it on Amazon (affiliate link).

General Tools Scratch Awl with Hardwood Handle: A scratch awl with a hardwood handle. See it on Amazon (affiliate link).

As an Amazon Associate I earn from qualifying purchases. A commission may be earned if you buy through this link, at no extra cost to you.

Sources

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