Bow poundage vs arrow spine and why the wrong arrow flies sideways
The first time I shot a bow above 40 lb my arrows started flying like they were drunk — visibly wobbling, hitting the target sideways. Turned out I was shooting arrows spined for a much lighter bow. Draw weight and arrow spine are a matched pair and nobody tells beginners this.
Draw weight is the force in pounds it takes to draw the bow to full length. A '50 lb bow' at 28 inch draw takes 50 lb of force to hold at full draw. Above and below 28 inches the actual force changes — traditional bows gain roughly 2-3 lb per inch of draw beyond 28.
What arrow spine means
Spine is the arrow shaft's stiffness. Confusingly, lower spine numbers mean stiffer arrows. A 340 spine arrow is stiffer than a 500 spine. Spine is measured by hanging a standard weight from a shaft supported at two points and measuring the deflection in thousandths of an inch — hence the numbers.
The archer's paradox
When you release an arrow it flexes around the bow (or around the arrow rest on a compound). If the spine is too weak for the draw weight, the arrow flexes too much and never straightens out in flight — you get visible fishtailing. If it's too stiff, the arrow doesn't clear the bow cleanly and drifts left (right-hand shooter) or right. Correct spine flexes the exact right amount to snap back straight by the time it leaves the bow.
Rough match
For a 28-inch draw, 400 spine works for 40-50 lb, 340 spine for 50-60 lb, 300 spine for 60-70 lb. Longer draw needs stiffer spine (add ~50 spine numbers stiffer per extra inch of draw). Heavier field points also need stiffer spine. Compound bows shoot arrows harder than the same draw weight on a recurve — go one spine stiffer.
Recap
Draw weight in pounds and arrow spine in thousandths are a matched pair. Wrong pairing looks wrong in flight.
Try the converters mentioned in this article
FAQ
Do I really need a spine chart?+
Yes. Every arrow manufacturer publishes one. Buying spine by draw weight alone will get you close but the chart accounts for arrow length, point weight, and bow type.
What's GPI?+
Grains per inch — arrow weight per unit length. Total arrow weight in grains is GPI × length + point + nock + fletching.
This article was written by The Turbo Unit Converter engineering desk (Applied metrology & engineering units) and last reviewed on 2026-07-23 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.