Bow Draw Length Calculator

Wingspan is fingertip to fingertip with your arms out level, relaxed rather than stretched — stretching adds an inch you cannot hold at full draw. Height is optional and is used only to show what the height-based rule would have told you.

Draw length is wingspan ÷ 2.5, rounded to the nearest half inch because that is how cam modules come. Speed follows the bow's advertised rating adjusted by 10 fps per inch either side of the 30″ reference. A pro shop with a draw-length arrow will beat all of this by a quarter inch; use it to walk in knowing roughly what you are looking for.

Why wingspan and not height

People who cannot find anyone to hold the tape substitute their height, and the substitution looks harmless because it is the same arithmetic. It is not harmless, and the size of the error has a closed form. Ape index is wingspan minus height, so the two rules differ by exactly ape ÷ 2.5 — and that gap does not depend on how tall you are at all. Swept across every height from 58″ to 82″ and every ape index from −6″ to +8″, the identity holds to floating-point precision.

Which matters because ape index is not small. Take three archers who are all exactly 70″ tall:

How carefully to measure

This page originally claimed the opposite of what is below. The reasoning was that the answer is quantised into half-inch modules long before your tape error matters, so a rough measurement is fine. Sweeping it said no, and the arithmetic agrees: each module owns a band of just 1.25″ of wingspan, and an error of ±e needs e of clearance at both edges. Only the middle 1.25 − 2e of the band is safe. At the half inch a person measuring alone can easily be out, that leaves a quarter inch of safety in one and a quarter — so 79.9% of wingspans would land on a different bow.

Two independent routes to the same column: a sweep over every wingspan in quarter-hundredths of an inch, and the closed form above. They agree to within a percentage point, and the residual is understood rather than waved at — the safe interval is closed at both ends, so a discrete sweep counts one extra safe sample in every band, always in the same direction. A genuine disagreement would not be one-sided. Past ±0.625″ the error spans a whole module and no measurement is safe at all.

So both things are true at once, and they are not in tension. Measure carefully — but note that the spread of ape index moves the answer 8× further than even a sloppy tape does. Measuring your height precisely and dividing it by 2.5 is a very accurate answer to the wrong question.

Which module your wingspan buys

The third column is the check you can make on a bow you already own. AMO draw length is measured from the nock groove to the throat of the grip plus 1.75″, which is roughly where the pivot point sits behind the throat. So a bow marked 29″ should measure 27.25″ at full draw. If yours does not, the module is not what the sticker says.

When you land between two modules, go down

The two errors are not equally bad, which is the one piece of judgement on this page that is not arithmetic. Half an inch too short costs exactly 5 fps and nothing else — the cost is linear in draw length, so it is the same 5 fps whatever bow you shoot and wherever on the range you are. Half an inch too long costs form: the string sits closer to your forearm, your head tilts back to reach the anchor, your bow elbow locks, and every one of those makes your groups worse by more than 5 fps ever could. A bow set too long is the most common fitting fault in archery and the hardest to unlearn, because by the time you notice it you have built a draw cycle around it. Five feet per second is a cheap insurance premium.