FOC (front of center) is how far forward of the arrow's middle its balance point sits, as a share of the arrow's length. Easton's formula: FOC = (balance point − length ÷ 2) ÷ length × 100, with both measured from the bottom of the nock groove and the length taken to the end of the shaft. A 29-inch arrow that balances 18 inches from the nock groove: (18 − 14.5) ÷ 29 × 100 = 12.1% FOC.
From components, the calculator places each part's weight along the arrow (point and insert at the front, the shaft spread over its length, nock and vanes about an inch from the back) and finds where they balance. A 28.5-inch shaft at 8.5 grains per inch with a 100-grain point, 15-grain insert, 10-grain nock and 24 grains of vanes weighs 391 grains and balances 17.5 inches from the nock: 11.4% FOC. A 125-grain point moves it to 13.9%.
Easton recommends 10–15% FOC for hunting arrows, and the check on this page uses that range. Setups at 19% and up, which the Ashby Bowhunting Foundation calls extreme FOC, give up some trajectory for penetration; the arrow setup guide covers that debate.
Reference
For what FOC does for broadhead flight and penetration, and how it trades against spine and trajectory, see the arrow setup guide.
READ THE GUIDE →To see what extra point weight costs in speed, use the arrow speed calculator.
