Determine the correct arrow spine for your bow setup. Calculate required stiffness based on draw weight, arrow length, and point weight for perfect tuning.
Input your bow's actual peak draw weight, your arrow length, and the weight of the point/insert combination you plan to use.
Spine refers to the stiffness of the arrow shaft. Too weak, and the arrow will 'fishtail'; too stiff, and it won't clear the bow riser properly.
The calculator provides an estimated 'static spine' value (e.g., 400, 350, 300). Always refer to the manufacturer's spine chart for your specific shaft model.
Static spine is the physical stiffness of the shaft. Dynamic spine is how the arrow actually bends when fired. Draw weight and point weight significantly affect dynamic spine.
A weak spine causes the arrow to flex excessively (fishtailing), leading to poor accuracy and potentially dangerous shaft failure upon release.
A heavier point makes the arrow act weaker (increases dynamic spine). If you switch from a 100-grain to a 150-grain point, you may need a stiffer shaft.
It starts from your peak draw weight and applies adjustments: every inch your arrow length differs from 28 inches shifts the effective weight by 5 pounds, each full 100 grains of point weight beyond 150 adds another 5, a mechanical release adds 5 while a finger release subtracts 5, and a broadhead adds 7.5. The adjusted effective weight then maps to a static spine band — for instance, a 55-pound effective weight points to a 350 shaft, an effective weight just above 60 calls for a 300, and above 65 moves you to a 250.
Take a 50-pound bow, a 29-inch arrow, a 150-grain field point, and a mechanical release. The length term adds (29 − 28) × 5 = 5, the point term adds 0, the release adds 5, and the field point adds 0, giving an effective weight of 60 pounds. That falls in the band that recommends a 340 spine. Swapping to a broadhead would push the effective weight to 67.5 and suggest an even stiffer 250 shaft.