What this calculator does
FOC, front of centre, describes how far forward of an arrow's exact midpoint its balance point sits, expressed as a percentage of the arrow's length. A higher FOC means more of the arrow's weight sits toward the point end, which archers generally associate with better penetration and more stable flight, at some cost to speed.
Working it out needs two measurements: the arrow's total length, and the balance point measured from the nock end (the end the string sits against, not the point). This calculator takes those two figures and returns the FOC percentage, using the same formula bow technicians and arrow builders use to tune hunting and target setups.
The formula
Divide the balance point (measured from the nock) by the total arrow length, subtract 0.5, then multiply by 100. Subtracting 0.5 centres the result on the arrow's true midpoint, so a balance point exactly in the middle gives 0% FOC, and any shift toward the point end shows as a positive percentage.
| Term | Meaning |
|---|---|
| FOC | Front of centre: how far forward of the arrow's midpoint the balance point sits, as a percentage of arrow length. |
| Balance point | The single point along the arrow where it balances level, measured as a distance from the nock end. |
| Midpoint | The exact centre of the arrow's length: half of total length, measured from either end. |
The inputs explained
| Field | What to enter |
|---|---|
| Arrow length (in) | The total length of the arrow, nock end to point tip. |
| Balance point, measured from the nock end (in) | Where the arrow balances on a level edge or FOC tool, measured from the nock end (not the point). |
When to use it
Tuning a hunting arrow
Hunters often add point weight or move components to push FOC into the 10-15% range or higher, aiming for the extra penetration and flight stability that a forward balance point provides.
Comparing arrow builds
Two arrows can weigh the same overall but fly differently if their balance points differ, which is exactly what FOC captures and raw weight does not.
Checking a component change
Swapping a heavier point, insert or nock shifts the balance point; recalculating FOC after the change confirms how far it moved.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How FOC changes with balance point at a fixed arrow length
A fixed 29-inch arrow, with the balance point moved progressively forward from the nock.
How FOC changes with arrow length at a fixed balance point
The same 17.5-inch balance point, applied to arrows of different total lengths.
Questions
What is a good FOC percentage?
Roughly 10-15% is a common target for hunting and field arrows, balancing penetration and stability against speed. Target and 3D archers often run lower FOC for flatter trajectories, while some hunting setups push higher for maximum penetration.
Can FOC be negative?
Yes. A negative FOC means the balance point sits behind the arrow's midpoint, closer to the nock than the point. This is unusual for hunting arrows and generally considered undesirable for flight stability.
How do I measure the balance point?
Balance the bare arrow (with all its final components fitted) on a level edge or a dedicated FOC balancing tool, then measure the distance from the nock end to that balance point.
Does FOC affect arrow speed?
Yes, indirectly. Adding weight toward the point to raise FOC also adds total arrow weight in most builds, which typically reduces speed somewhat in exchange for the penetration and stability benefits of a more forward balance point.
For the speed and energy side of an arrow build rather than its balance, see the arrow kinetic energy calculator.