Arrow Spine Calculator

This calculator applies a simplified AMO-style spine approximation: it converts your Draw Weight, Draw Length, Point Weight, and String Type into an "effective draw weight," then estimates a matching spine deflection value as roughly 28,000 divided by that effective weight, rounded to the nearest standard 50-unit spine group (e.g. 350, 400, 450). Draw Weight has the largest influence on the recommendation across each input's full allowed range: sweeping the full 15-100 lb Draw Weight range moves the recommended spine several times more than sweeping the full 20-34" Draw Length range does. Draw Length still shifts effective weight meaningfully -- about 2 lbs per inch away from the 28" baseline, since bow draw weight is itself rated at a specific length and stroke length changes how much energy the arrow actually absorbs -- but its declared 20-34" range is narrower than Draw Weight's, so end to end it moves the result less.

Raising either Draw Weight or Draw Length always calls for a stiffer (lower-numbered) spine, never a weaker one. Point Weight only shifts the recommendation once it climbs past 100 grains -- lighter tips are treated as neutral in this model. Dacron strings add a flat 5 lb equivalent versus modern Fast Flight material, reflecting their lower energy transfer. This is a starting-point estimate, not a replacement for a manufacturer's individual arrow spine chart: it does not account for arrow shaft material, diameter, or length beyond what Draw Length implies, and real-world tuning (paper tuning, bare-shaft testing) should always confirm the final spine choice.