How this calculator works
Balance an assembled arrow on a knife edge and mark where it balances. Measure from the bottom of the nock groove to that point, then compare it with the midpoint of the arrow. An arrow that balances exactly at its midpoint has 0 percent FOC. Moving weight toward the point, such as a heavier insert or broadhead, raises the balance point and increases FOC. The measure is a fraction of overall arrow length, so it can be compared between arrows of different sizes.
Formula
FOC = (balance point from the nock - arrow length / 2) / arrow length x 100. To find the balance point: arrow length / 2 + FOC x arrow length. To find the length: balance point / (0.5 + FOC).
Worked example
A 75 cm arrow balances 42 cm from the nock. Its midpoint is 37.5 cm, so the balance point is 4.5 cm forward. FOC is 4.5 / 75 = 0.06, or 6 percent. To reach 10 percent with the same length, the balance point would need to be 37.5 + 7.5 = 45 cm from the nock.
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Frequently asked questions
How should I measure arrow length?
Measure from the bottom of the nock groove to the end of the shaft, excluding the point. Include the nock and vanes in the balance test, because their weight affects where the arrow balances.
What FOC should I aim for?
It depends on the use. Easton suggests 10 to 15 percent for hunting and general accuracy, while some target setups run lower. Fixed-blade broadheads usually benefit from at least 10 to 12 percent.
Does higher FOC always mean better?
No. More front weight can improve wind resistance and penetration, but it also makes the arrow drop faster and may require a stiffer spine. Very high values can hurt trajectory at long range.