Saddle Stitch Thread Size Chart

The core idea: your pricking irons set the hole spacing (pitch), and the thread has to look proportional to those holes. Wider spacing can carry a fatter thread without crowding; tighter spacing needs a thinner thread so the stitches stay crisp and the leather between holes doesn't tear out.

Work from pitch, not gut feeling. Confirm your iron spacing with the stitch pitch converter (irons are sold by both SPI and millimeters, and the numbers don't always agree), then read the matching thread size off the chart below. When you're between two sizes, the thicker thread reads bolder and fills the hole more; the thinner one looks refined and sits flatter.

The most-asked pairing: 3.85 mm irons (roughly 6-7 SPI) take 0.6 mm thread for a classic, balanced look. That's the workhorse combination for wallets, belts, and bags. Other reliable starting points:

• 2.7 mm pitch (~9 SPI): 0.45 mm thread for fine, dressy work.
• 3.0-3.38 mm pitch (~7-8 SPI): 0.5 mm thread.
• 3.85 mm pitch (~6-7 SPI): 0.6 mm thread.
• 4.0-5.0 mm pitch (~5-6 SPI): 0.8 mm thread for bold, heavier seams.

These are starting points, not laws. If you like a fuller stitch, size up one step; if you want it subtle, size down. See the full reference table below.

This is the decision most beginners actually face, because both sizes cover a lot of common projects. Choose 0.6 mm when your pitch is around 3.0-3.85 mm and you want stitches that look neat and proportional. Choose 0.8 mm when your pitch is 4.0 mm or wider, the leather is thick, or you specifically want a bold, hand-made-looking seam.

The failure modes tell you when you've gone wrong. Too thick (0.8 mm in 3.0 mm holes): the thread crowds each hole, the awl or needle drags, and the stitches bunch up on the surface. Too thin (0.6 mm in 4.5 mm holes): the seam looks sparse and under-filled, and it's structurally weaker than it should be. When in doubt on a first project, 0.6 mm is the more forgiving default.

Thread gets labeled three different ways, which is why sizing feels confusing:

• Millimeters (mm): the finished diameter of braided polyester thread like Ritza 25 / Tiger and Meisi. This is the easiest number to match to pitch. Ritza 25 sizes run 0.45, 0.6, 0.8, 1.0, and 1.2 mm.
• Tex: the weight of 1,000 meters of thread in grams. Higher Tex means thicker thread. You'll see it on cones of bonded polyester and nylon; roughly, Tex 70 is thin, Tex 135 is medium, Tex 210 is heavy.
• Linen ply, like 18/3: the first number is the yarn's fineness and the second is how many plies are twisted together. A 3-ply linen is common for hand stitching. Ply notation doesn't translate cleanly to millimeters, so match linen by look and feel against a known mm thread.

For a deeper split between the two materials, see polyester vs linen thread.

Thickness of the leather changes how a given thread reads. On a thick stack (multiple layers, a belt, a heavy bag gusset) the holes are longer and can visually swallow a thin thread, so it's common to size up one step from the chart, say 0.8 mm where 0.6 mm would be the flat-leather pick. On thin, single-layer pieces, size down so the seam doesn't overpower the leather.

Also budget your length before you start: thicker thread and thicker stacks both eat more thread per stitch. The thread length calculator gives you a cut length for your seam so you're not splicing mid-row. A rough hand-stitch rule is about 3.5 to 4 times the seam length, but the calculator accounts for stack thickness.