Stitch Pitch Converter & Thread Matcher
Turn millimetres into stitches per inch (and back), then get the right thread diameter and irons for the job.
Quick answer
To convert stitch pitch to stitches per inch, divide 25.4 by the pitch in millimetres (SPI = 25.4 ÷ pitch); to go back, divide 25.4 by SPI. For most beginner leather goods a 3.38–4.5 mm pitch pairs with 0.6–0.8 mm thread. Enter your pitch or SPI below to get thread size, holes per foot and matching irons.
Your stitch numbers
Typical use at this pitch: wallets.
See the stitch at true scale
Top row is your pitch; bottom row is the nearest shop iron size, both against a real one-inch ruler. On a calibrated screen this is life-size.
Thread diameter for this pitch
How the conversion works
Leather stitch spacing is quoted two ways. European makers use pitch — the millimetres between hole centres. US patterns often use SPI, the number of stitches per inch. Because there are 25.4 mm in an inch, the two are simple reciprocals: SPI = 25.4 ÷ pitch and pitch = 25.4 ÷ SPI. Holes per foot is just 12 × SPI, handy for estimating how long a seam will take.
Thread should match the hole. Too thick and the stitches bunch and pucker; too thin and the seam looks weak and can loosen. The chart below is the field-tested pairing most makers settle on.
Irons & thread that fit this pitch
Matched to your pitch (3.85 mm) within ±0.3 mm.
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Diamond Stitching Chisels 3.85 mm (1+2+4+6)
- Size
- 3.85 mm pitch | 1/2/4/6 prong set
- Pitch
- 3.85 mm
- SPI
- 6.6
- Prongs
- 1+2+4+6
- Hole
- Diamond
- Weight
- 3-7 oz
- Blade
- hardened steel
- Level
- Beginner
The most forgiving all-round beginner pitch. Big enough holes to see, tight enough to look refined on wallets and small bags.
Reviewed 2026-06-30 -
French Pricking Irons 4.0 mm (2+8)
- Size
- 4.0 mm pitch | 2 & 8 prong
- Pitch
- 4 mm
- SPI
- 6.4
- Prongs
- 2+8
- Hole
- French
- Weight
- 4-8 oz
- Blade
- hardened steel
- Level
- Intermediate
General-purpose pitch for bags and straps. Angled prongs give the slanted saddle-stitch look prized on higher-end goods.
Reviewed 2026-06-30 -
Ritza 25 Tiger Thread 0.6 mm
- Size
- 0.6 mm | 25 m or 500 m
- Pitch
- 3.85 mm
- Thread
- 0.6 mm
- Material
- Polyester
- Weight
- 3-7 oz
- Level
- Beginner
The default beginner thread — flat, pre-waxed polyester that lies neat and rarely frays. Great for wallets and small bags.
Reviewed 2026-06-30 -
Ritza 25 Tiger Thread 0.8 mm
- Size
- 0.8 mm | 25 m or 500 m
- Pitch
- 4.5 mm
- Thread
- 0.8 mm
- Material
- Polyester
- Weight
- 5-10 oz
- Level
- Beginner
Heavier flat polyester for bags, totes and straps stitched at 4.0–4.5 mm. Bold but still refined.
Reviewed 2026-06-30 -
Linen Thread 18/5 (approx 0.8 mm)
- Size
- 18/5 waxed linen | ~50 m
- Pitch
- 4.5 mm
- Thread
- 0.8 mm
- Material
- Linen
- Weight
- 5-10 oz
- Level
- Intermediate
Heavier linen for bags and belts. Choose linen for restoration and heritage work; polyester for durability and ease.
Reviewed 2026-06-30
Pitch → thread quick chart
| Pitch | SPI | Thread | Ritza 25 | Typical use |
|---|---|---|---|---|
| 2.7 mm | 9.4 | 0.45–0.45 mm | Meisi 0.45 | watch straps |
| 3.0 mm | 8.5 | 0.45–0.55 mm | 0.6 (snug) | card holders |
| 3.38 mm | 7.5 | 0.55–0.63 mm | 0.6 | wallets |
| 3.85 mm | 6.6 | 0.63–0.8 mm | 0.6 / 0.8 | wallets |
| 4.5 mm | 5.6 | 0.8–1.0 mm | 0.8 | bags |
| 5.0 mm | 5.1 | 1.0–1.2 mm | 1.0 | belts |
| 6.0 mm | 4.2 | 1.2–1.2 mm | 1.0 / 1.2 | heavy duty |
Frequently asked questions
What size thread do I need for 3.85 mm irons?
At 3.85 mm pitch, 0.6 mm thread (such as Ritza 25 / Tiger 0.6) is the classic beginner choice; step up to 0.8 mm if you want a bolder line on small bags.
How many stitches per inch is 3 mm pitch?
3 mm pitch is about 8.5 SPI (25.4 ÷ 3). That is a fine, dressy spacing suited to card holders and wallets.
Is pitch the same as SPI?
They are two ways of saying the same thing. Pitch is the millimetres between holes; SPI is how many stitches fit in one inch. SPI = 25.4 ÷ pitch in mm.
Why do my irons say 4 mm but the holes look wider?
Many budget brands quote the gap between teeth (tip-to-tip) rather than hole centres (centre-to-centre). Always check listing photos with a ruler before buying.
Should thicker leather change my thread size?
Yes — for a combined stack over about 10 oz, step up one thread diameter (capped at 1.2 mm) so the stitch is strong enough to hold the bulk.