Hong Life CalculatorsUnderstanding the arrow FOC calculator — concept and principle
FOC (front of centre) describes how far an arrow's balance point sits ahead of its middle. It affects how stable the arrow is in flight and how it behaves at longer distances. This calculator finds FOC from the arrow length and the measured balance point, and optionally adds kinetic energy and momentum from the arrow weight and speed.
How it is calculated
FOC (%) = (B − L/2) ÷ L × 100
Kinetic energy (ft·lbf) ≈ grains × fps² ÷ 450,437
Momentum (slug·ft/s) ≈ grains × fps ÷ 225,218
- L is the arrow length from the bottom of the nock groove to the end of the shaft (in).
- B is the distance from the nock groove to the balance point of the finished arrow, point included (in).
- Arrow weight in grains (1 grain = 64.79891 mg) and speed in feet per second.
The principle
An arrow flies point-first because its weight is concentrated toward the front while drag acts mostly on the fletching at the back. The further the balance point is ahead of the drag centre, the stronger the "weathervane" effect that straightens the arrow after release. FOC measures this as a percentage of length, so arrows of different lengths can be compared. Heavier points, inserts or outserts move the balance forward and raise FOC; heavier fletching or nocks move it back.
Worked example
Default inputs: length 29 in, balance point 17.6 in, weight 420 grains, speed 280 fps.
- Centre of the arrow = 29 ÷ 2 = 14.5 in.
- Balance point ahead of centre = 17.6 − 14.5 = 3.1 in.
- FOC = 3.1 ÷ 29 × 100 = 10.7%, which falls in the 10–15% — recommended range.
- Kinetic energy = 420 × 280² ÷ 450,437 = 73.1 ft·lbf.
- Momentum = 420 × 280 ÷ 225,218 = 0.522 slug·ft/s.
How to read the result
Easton Archery's guidance, as described by the Archery Trade Association, recommends roughly 10–15% FOC. Below 10%, arrows may take longer to stabilise; above about 15–16%, they can drop more at long range and be harder to tune, though some bowhunters deliberately use higher FOC for penetration. Kinetic energy and momentum help compare setups for hunting; local hunting regulations sometimes set minimum requirements.
Common mistakes
- Measuring length to the end of the point instead of the end of the shaft.
- Finding the balance point without the point or broadhead fitted.
- Comparing FOC values measured by different methods.
- Ignoring that a heavier point also changes spine behaviour.
Frequently asked questions
What is a good FOC for target archery?
Around 10–15% is the commonly recommended range.
How do I raise FOC?
Use a heavier point, a weighted insert or lighter fletching and nocks.
Does FOC affect arrow speed?
Not directly, but adding front weight raises total arrow weight, which lowers speed.
Why measure from the nock groove?
It is the standard reference point used in Easton's FOC method.
Sources: Archery Trade Association (Bowhunters United), E. Barber, "What Is FOC? Your Questions Answered" (2019) · Archery 360, "Nerd Out with Archery Math" (2021)