About the Arrow Spine Calculator
This arrow spine calculator estimates how stiff your arrows should be. Spine is the static deflection of a shaft in thousandths of an inch, so a 400 arrow bends 0.400" under the standard test and is stiffer than a 500. Too weak a spine makes arrows flex and fly erratically; too stiff a spine makes them kick to one side and group poorly.
Enter your bow type, peak draw weight, draw length, finished arrow length and point weight. The calculator works out an “effective” bow weight — what the bow feels like to the arrow — and converts it to a recommended spine size plus the next weaker and stiffer options. It is aimed at beginners buying their first dozen, recurve archers whose draw weight changes with draw length, and anyone switching point weights.
Treat the result as a starting point, not a guarantee. Every manufacturer publishes its own chart, and cam aggressiveness, string material, release style and fletching all shift the ideal spine. Use it to narrow your choice, then confirm with the arrow maker’s chart and paper or bare-shaft tuning.
With the default inputs, the recommended spine is 350. Change any value above to recalculate instantly.
How to use the arrow spine calculator
- 1Choose compound or recurve.
- 2Enter the draw weight marked on the limbs (peak weight for a compound).
- 3Enter your draw length and the finished arrow length.
- 4Enter the point weight you plan to shoot.
- 5Read the recommended spine and check it against the arrow maker’s chart.
Formula and method
Arrow makers’ charts move about one weight band (roughly 5 lb) for every inch of arrow length and about 3 lb for every 25 grains of point weight away from the 28-inch, 100-grain baseline. This calculator applies those rule-of-thumb adjustments to an effective bow weight. Recurve weight is first corrected to your draw length (about 2 lb per inch from the 28" marking), and compounds are multiplied by f = 1.15 because cams store more energy than a recurve of the same peak weight.
The effective weight is then converted to static spine with an inverse relationship fitted to typical published carbon-arrow charts and rounded to the nearest common shaft size. Results are an estimate; brand charts differ by one size either way.
- Wbow
- Draw weight at your draw length (lb)
- f
- 1.15 for compound bows, 1.0 for recurves and longbows
- L
- Finished arrow length (in)
- P
- Point weight (grains)
Worked examples
60 lb compound, 28" arrows, 100 gr points
Effective weight is 60 × 1.15 = 69 lb with no length or point adjustment. 24,000 ÷ 69 ≈ 348, so a 350-spine shaft (often sold as 340/350) is the starting point.
70 lb compound, 29" arrows, 125 gr points
70 × 1.15 = 80.5 lb, plus 5 lb for the extra inch and 3 lb for the extra 25 grains = 88.5 lb. 24,000 ÷ 88.5 ≈ 271, nearest the stiff 250 size; a 300 is the weaker alternative.
32 lb recurve, 27" draw, 28" arrows
Drawing an inch short of 28" takes about 2 lb off, so the bow pulls 30 lb. 24,000 ÷ 30 = 800, a typical beginner target spine.
Frequently asked questions
What does arrow spine mean?+
Spine is how much an arrow bends. Static spine is measured by hanging a 1.94 lb weight from the centre of a shaft supported 28 inches apart; a 500 spine bends 0.500 inches. Lower numbers are stiffer.
What happens if my arrow spine is wrong?+
An under-spined (too weak) arrow flexes too much and tends to group erratically, while an over-spined (too stiff) arrow kicks sideways out of the bow. Both widen groups and make broadheads plane.
Does point weight change spine?+
Yes. A heavier point makes the arrow act weaker, so it needs a stiffer shaft. As a rule of thumb, each 25 grains above 100 behaves like about 3 lb more draw weight.
Does arrow length affect spine?+
Yes. Longer arrows act weaker. Most charts move up about one weight band, roughly 5 lb, for every inch of arrow length beyond 28 inches, so cutting arrows shorter makes them stiffer.
Why does my recurve need a different spine than a compound of the same weight?+
Compound cams store and release more energy than recurve limbs at the same peak weight, so a compound launches the arrow harder and needs a stiffer spine.
Is this calculator a substitute for the manufacturer chart?+
No. It gives a sensible starting size. Each shaft maker publishes its own spine chart, and paper tuning or bare-shaft tuning is the final check.