Arrow FOC Calculator
This arrow FOC calculator works out the front of centre balance of a finished arrow, the single number that tells you how far forward the weight sits along the shaft. FOC, short for front of centre, matters because an arrow that is nose-heavy steers and recovers from the bow more cleanly, which usually means tighter groups and better penetration, while an arrow that balances too far back can fly erratically. You need just two measurements: the arrow length, taken from the throat of the nock to the end of the shaft and not counting the point, and the balance point, which is where the completed arrow, with point, insert, vanes and nock fitted, balances on an edge, measured from the same throat of the nock. The calculator subtracts the physical centre of the shaft from the balance point, divides by the length, and expresses the result as a percentage. It also shows the centre point of the shaft and how far ahead of it your balance sits, so you can see exactly what is driving the number. Target and field archers often run around 7 to 12 percent, while many bowhunters prefer 10 to 15 percent for heavier hitting arrows. Enter your two measurements in inches to see your FOC, with a worked example below. Use it to compare shafts, points and inserts as you tune a new setup.
An FOC of 7.5% sits within the commonly recommended 7 to 15% range, giving a good balance of forgiveness and trajectory.
Measure the arrow length from the throat of the nock to the end of the shaft, excluding the point, and take the balance point on the finished arrow. FOC is a guide to flight, not a rule: tune to your own bow and distance.
How it works
The physical centre of the shaft is simply half the arrow length. The balance point of a finished arrow sits ahead of that centre because the point and insert add weight at the front. FOC measures that offset as a share of the whole arrow: FOC percent equals 100 times the balance point minus half the arrow length, all divided by the arrow length. Because it is a ratio, the units cancel, so measuring in inches or centimetres gives the same percentage as long as you are consistent. A larger FOC means more weight forward, which improves steering and penetration but slows the arrow and lowers its point of impact at distance.
Worked example
Say your arrow measures 28 inches from the nock throat to the end of the shaft, and the finished arrow balances 16.1 inches from the nock throat. The shaft centre is 28 / 2 = 14 inches, so the balance sits 16.1 - 14 = 2.1 inches ahead of centre. FOC is 100 x 2.1 / 28 = 7.5 percent. That falls inside the common 7 to 15 percent window, so this arrow is well set up for general shooting. Adding a heavier point would push the balance further forward and raise the FOC.
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