Arrow Spine Calculator (Compound & Recurve Bow Sizing)
Calculate your dynamic arrow spine deflection (300, 340, 400, 500) based on bow draw weight, carbon shaft length, and point weight.
How to Use the Arrow Spine Calculator (Compound & Recurve Bow Sizing)
Static arrow spine measures the stiffness of a carbon arrow shaft by suspending a 1.94-pound (880-gram) weight at the center of a 28-inch span and measuring how many thousandths of an inch the shaft bends. A 300-spine shaft deflects 0.300 inches, making it significantly stiffer than a 400-spine shaft (0.400 inches) or 500-spine shaft (0.500 inches).
Formula and Calculation Methodology
- W_draw: Actual peak bow draw weight measured on a bow scale (lbs)
- L_arrow: Carbon-to-carbon shaft length from throat of nock to end of carbon (inches)
- W_point: Combined front-end weight of broadhead/field point plus insert and collar (grains)
Dynamic spine—how the arrow actually flexes upon release—is governed by three primary variables: bow poundage, shaft length, and front-end point weight. Every additional inch of arrow length beyond 28 inches weakens dynamic spine by roughly 5 pounds of draw weight, and every additional 25 grains of point or brass insert weight above 100 grains weakens the shaft by roughly 3 pounds.
Carbon Arrow Spine Selection Chart (100–125 Grain Point)
| Bow Draw Weight | 26″ – 27″ Arrow | 28″ – 29″ Arrow | 30″ – 31″ Arrow |
|---|---|---|---|
| 45 – 55 lbs | 500 Spine | 400 Spine | 340 / 350 Spine |
| 55 – 65 lbs | 400 Spine | 340 / 350 Spine | 300 Spine |
| 65 – 75 lbs | 340 / 350 Spine | 300 Spine | 250 Spine |
| 75 – 85 lbs | 300 Spine | 250 Spine | 200 / 250 Spine |
Frequently Asked Questions
Is a lower spine number stiffer or weaker?
A lower spine number is stiffer. Because spine measures deflection in thousandths of an inch, a 300-spine arrow bends less (0.300″) than a 400-spine arrow (0.400″).
If I am borderline between two arrow spines on a compound bow, which should I choose?
With modern center-shot compound bows and mechanical releases, always err on the stiffer side (lower spine number), especially when shooting fixed-blade broadheads.
How does adding a 50-grain brass insert affect arrow spine?
Adding 50 grains to the front of the arrow increases Front-of-Center (FOC) balance but causes the shaft to flex more on launch, requiring you to step up one spine class stiffer.