Why four times the seam is the wrong constant
A saddle stitch runs one length of thread with a needle at each end. Every stitch, each needle travels along the seam by one stitch space and through the leather by the full thickness of the stack. So the thread consumed per stitch is two lots of spacing plus thickness, and dividing that by the spacing gives the ratio directly:
ratio = 2 x (1 + thickness / spacing)
Put a quarter inch stack on a 3.85 mm iron and the thickness term alone is 1.65, so the ratio is about 5.3 to 1 before any allowance for slack. Put a thin lining of 1.6 mm on the same iron and the term falls to 0.42, giving about 2.8 to 1. The four times rule sits somewhere in between and is exactly right for nothing in particular.
This matters most on the jobs where being wrong hurts most. A holster or a thick belt keeper is where you are most likely to run short, because it is thick and because it is where the rule of thumb understates by the largest margin.
Making the spacing land on both ends
A ten inch seam at 3.85 mm does not divide evenly. It comes to just under 66 spaces, so if you mark from one end at the nominal pitch, the last space at the far corner is a fraction and it looks like a fraction. The fix is the one this page does: round to a whole number of spaces, then divide the seam by that number and use the result. The spacing shifts by a few hundredths of a millimetre, spread over sixty-odd holes, which nobody can see.
In practice you do this by starting an iron at each end of the seam and working toward the middle, letting the small discrepancy fall where the eye does not go. The arithmetic tells you how much discrepancy there is to hide, which is what decides whether you can absorb it or need to change the pitch.
Placements, and the overshoot
An iron with six teeth, dropped one tooth back into the last hole it made, adds five new holes each time. So the first placement gives six holes and every one after adds five. The count runs out unevenly almost always, and the last placement overshoots the end of the seam.
| Teeth | Overlap | New holes per placement | Placements for 67 holes |
|---|---|---|---|
| 6 | 1 tooth | 5 | 14 |
| 6 | none | 6 | 12 |
| 10 | 1 tooth | 9 | 8 |
| 2 | 1 tooth | 1 | 66 |
Butting the iron rather than overlapping needs fewer placements and drifts, because nothing registers one placement against the last. The overlap is not wasted effort; it is the registration.
What the slack factor covers
The geometry is a floor and it is honest about being a floor. Real stitching adds a little on every pull: the thread is cast around the needle on some stitch styles, the holes are punched at an angle rather than square so the path through the leather is longer than the thickness, and each stitch gets seated with a tug that takes up a fraction. Fifteen percent is a starting point, not a measurement.
If the number has to be right — because you are cutting thread for forty identical seams, or because the thread is expensive — stitch one measured seam, unpick it, and measure what comes out. That gives you a factor specific to your hand, your holes and your thread, and it will not match anyone else.
For machine sewing the model is different, because a lockstitch uses two separate threads and a bobbin runs out on a schedule of its own. The thread consumption calculator handles that case. If the seam is going round a piece you have yet to cut, the hide yield calculator comes first, and for a belt the belt blank length calculator settles the strap before either.
Questions people ask
How much thread should I cut for a saddle stitch?
Whatever this page says, plus a bit. The formula is two times one plus thickness over spacing, which for typical wallet work lands near three to one and for a thick holster seam near five to one, then a slack allowance and tails at each end. The four times rule that circulates is a middling case treated as a constant. Cutting long costs you a few inches of thread; cutting short costs you the seam, because a join mid-run is visible and weak.
What size iron should I use?
The page takes your pitch rather than recommending one, because it is an appearance decision tied to the thickness and the thread. Broadly, finer pitches suit small goods and thin thread, coarser pitches suit heavy straps and thick thread, and a stitch line looks wrong when the thread is too thick for the pitch to hold it or too thin to fill the hole. Punch a test line in an offcut of the actual leather with the actual thread before committing to a piece.
Why does the calculator change my spacing slightly?
Because a seam almost never divides into a whole number of iron pitches. If you mark from one end at the nominal spacing, the leftover appears as a short space at the far corner, which is where it is most visible. Rounding to the nearest whole number of spaces and dividing the seam by it spreads that leftover across every space instead. On a ten inch seam that is a change of a few hundredths of a millimetre per hole, which is invisible, replacing a fractional gap at a corner, which is not.
Should I overlap the iron or butt it up?
Overlapping by one tooth into the last hole is what keeps the line straight and the spacing even, because each placement is registered against the previous one. Butting the iron up against the last hole without going into it needs fewer placements, but nothing is holding the pitch, and small errors accumulate along the seam until the line visibly wanders. On a long seam that accumulation is exactly what people notice.
Does the thickness include the seam allowance or the layers?
The layers, all of them, as they sit stacked when you punch. Two pieces of six ounce leather glued together is about a quarter inch, and that is the number that goes in. If there is a lining, or a stiffener, or a folded edge inside the seam, it counts too. The thread does not care what the layers are, only how far it has to travel to get from one face to the other and back.