The saddle stitch: spacing and thread.

Stitches per inch → spacing

Spacing in mm = 25.4 ÷ SPI, and the irons come in real sizes: the 7 SPI pricking iron = 3.63 mm, 8 = 3.175, 6 = 4.23 (the maker's own listings). Small goods sit around 3 mm; bags and belts 4 to 6 mm.

The thread picks the number, the spacing picks the thread

The thread's diameter ≈ spacing ÷ 6, plus or minus 10% (the app's fitted rule of thumb, shown as such). 2.7 mm spacing pairs with a ~0.45 mm thread; 3.85 with ~0.65.

Leather weight: oz to mm

1 oz = 1/64 in ≈ 0.4 mm, sold as ranges: a 4–5 oz shoulder = 1.6–2.0 mm (the charts at the big suppliers). The app uses the range's middle per layer.

How much thread, the formula that never runs short

Per inch of seam: 2 + 2 × thickness × SPI inches of thread, the member formula the app checks (1/4 in thick at 5 SPI = 4.5 in of thread per inch of seam). Then add the backstitches, the working allowance, and the tail. The forum rule lands near 4× the seam for two layers of 2 mm, and grows with thickness, the same way the formula does.

The calculator runs this live. Cut generous: the second regret is always cheaper than the first.

A worked seam

Take a 20-inch seam on 9 oz leather (about 3.6 mm per layer — 1 oz is 1/64 in, about 0.4 mm, the suppliers' own chart) stitched at 7 SPI:

• Spacing: 25.4 ÷ 7 = 3.63 mm — the iron's own listed figure.
• Thread: about spacing ÷ 6 ≈ 0.6 mm, plus or minus a tenth either way. A rule of thumb, labelled as one in the app.
• Thread per inch of seam: 2 + 2 × (9/64) × 7 ≈ 3.97 in — the 2008 forum formula, per inch, before the working ends.
• The whole seam: about 3.97 × 20 ≈ 79 in per side, and the saddle stitch carries two sides — each half wants the seam plus every hole's stack, plus backstitches and a tail.

The stitch calculator runs the same line for whatever seam, weight and SPI you feed it, and lands where the forum rules point: about 4× the seam for two layers of 2 mm, growing with thickness.

Mistakes worth avoiding

the mistake	what it costs
thread run from a rule that ignores the stack	the seam runs out mid-row; the formula's holes × stack term is the whole point
spacing chosen by eye instead of the iron	stitches drift; the pricking iron sets 25.4 ÷ SPI and the bench should match it
one thread length for every thickness	4× is the two-layer 2 mm figure; thick work wants 4–5× — the calculator carries the thickness
skipping the backstitch and tail in the cut	the math says enough thread until the last two holes; the tail is not optional in the sum

The app helps

The Thread page carries this math: pick the leather weight (it uses the middle of the range), the SPI, and the seam, and it cuts the length with the backstitches and tail in the sum. The bench's calculator is the same line in the browser.

Sources: the spacing figures are the iron makers' own (Abbey England's 7 SPI listing); the thread rules come from the leatherworker.net forum threads (the 2008 per-inch formula, the 4× rule of thumb); the weight chart is Tandy's and Weaver's. Every figure in the app is checked against its source in the app's own craft-facts document.