Arrow Spine & Setup Calculator

Spine follows the standard chart logic (one step stiffer per inch over 28″, half-step per 25 gr of point weight); speed derives from your bow's IBO rating with the accepted adjustments (−10 fps/inch of draw, −1 fps per 3 gr, −2 fps/lb) — real bows with accessories typically shoot 5–10 fps under the estimate. Hunting FOC sweet spot: 10–15%. Manufacturer spine charts trump this general one where they differ.

Six rungs, four thousand inputs

The ladder has 6 entries. Sweeping every draw weight from 30 to 80 lb, every arrow length from 26 to 32 inches in half inches, and six point weights — 3,978 combinations — produces exactly 6 distinct answers:

Nearly half land on 250, the stiffest rung, and 51% sit on one end of the ladder or the other where the clamp is holding them. That matters, because a clamped answer is not the same kind of answer as an interior one: the arithmetic wanted to go further and could not, so two setups that genuinely differ get handed the same shaft.

And the edges are hard

A single pound crosses a class at 39 to 40. A tenth of an inch crosses one at 28.4 to 28.5. Meanwhile 27.5 and 28.4 — nearly a whole inch apart — give the same answer, because the length rule rounds to the nearest whole inch first. None of that is unreasonable for a starting recommendation, since spine charts really are bucketed. It does mean a number that swings a full class for a tenth of an inch should be read as "you are near a boundary, try both" rather than as a precise answer.

And boundaries are not rare: 31% of draw weights in the swept range sit within a pound of one.

The point-weight adjustment is asymmetric, and here is why

The code computes a half-step per 25 grains and then rounds it to a whole rung, which quietly makes it a step per 50:

So 125 and 150 give the same recommendation, and so do 175 and 200. It is not symmetric about 100 either, and the reason is a corner of JavaScript most people never meet: Math.round(0.5) is 1 but Math.round(-0.5) is -0, because the language rounds halves toward positive infinity rather than away from zero. A 125 gr point therefore moves a rung stiffer and a 75 gr point moves nothing at all.

That asymmetry used to show up as a wrong sentence. The tool printed "75 gr point (lighter than 100) → weaker: 340" next to a spine of 340 — a step claiming a change that had not happened. It now only prints the step when the ladder actually moves. Every recommendation is unchanged; the asymmetry itself is documented here rather than papered over.

Read the whole thing as a starting point, and a coarse one. If your numbers sit near a bucket edge — within a pound of a ten-pound boundary, within a tenth of a half inch, or anywhere between 100 and 150 grains — two adjacent spines are both defensible, and paper will settle it faster than arithmetic will.