Arrow Build & GPI Weight Calculator

Guide

Understanding Arrow Spine, GPI & Build Weight

The workbench above gives you a verdict the moment you move a slider. This is what's actually driving that verdict.

What Is Arrow Spine & Why Getting It Wrong Ruins Your Groups

Spine is how much a shaft flexes under load, and every arrow needs to flex a very specific amount to fly straight off a given bow. Too weak (a spine number too high) and the shaft bends excessively as it's released, porpoising through the air and hitting left of the point of aim for a right-handed shooter. Too stiff (a number too low) and it barely flexes at all, typically printing right. Three things drive what spine you actually need: more draw weight demands a stiffer shaft, a longer draw length demands a stiffer shaft, and more weight hung on the front — point plus insert — also demands a stiffer shaft, because all three add more force trying to bend the arrow at the moment of release.

Recurve and traditional setups are the exception that trips people up: with no let-off and typically heavier front-of-center builds, trad archers commonly need a noticeably weaker (higher-numbered) spine than a compound shooter pulling the same poundage — which is why this workbench treats compound and recurve as separate calculations rather than one chart with a footnote.

Which shaft family you're starting from changes some of these defaults, too — Types of Arrows breaks down the five families this workbench draws its presets from.

Reading the Spine Verdict: Matched, Weak, or Stiff

Spine charts from shaft manufacturers give you a starting point, but the number that matters is how your specific bow and specific components behave together — which is exactly what changing a slider above is meant to show before you buy a dozen shafts.

How to Build & Tune an Arrow — Step by Step

1. Start with draw weight and draw length. These set your baseline spine requirement before a single component is chosen.
2. Pick a shaft and check total weight against grains per pound. The old IBO minimum is 5 grains per pound of draw weight — shoot lighter than that and you risk the bow the way a dry-fire does. For hunting, most experienced bowhunters stay in a 6.5–8.5 GPP sweet spot for a better balance of speed and penetration.
3. Add point and insert weight, then re-check the spine verdict. Front weight is the fastest way to push a build from Stiff toward Matched, or from Matched toward Weak if you overdo it.
4. Check FOC and kinetic energy together, not separately. A build can hit a great KE number and still carry a poor balance point, or vice versa — the verdict panel above shows both from the same inputs.
5. Confirm with a bareshaft or paper tune. This tool gets you to a very good starting shaft; the bow itself gives the final answer.

Frequently Asked Questions

What does GPI mean for arrows?

GPI is grains per inch — how much a bare shaft weighs for every inch of its length, before any point, insert, vanes or nock are added. It's a spec every shaft manufacturer publishes, and it's the main lever for total arrow weight: a longer arrow or a higher-GPI shaft both add finished weight without touching the components you hang on the end.

How many grains per pound should a hunting arrow weigh?

Never below 5 grains per pound of your bow's draw weight — that's the old IBO minimum, and shooting lighter risks stressing the limbs similarly to a dry fire. For hunting specifically, most bowhunters aim for 6.5 to 8.5 grains per pound, trading a little speed for meaningfully better penetration on game.

Why does draw length change what spine I need?

A longer draw length means more of the shaft is unsupported past the rest at full draw, and more energy is being loaded into it at release — both of which call for a stiffer shaft to keep the same flex behavior. Shooters with a longer-than-average draw length routinely need a stiffer spine than the chart's default 28 to 29 inch assumption suggests.