Arrow Foc Calculator

What the arrow FOC calculator does

FOC, short for front of center, describes how front-heavy an arrow is, and it has a real effect on how the arrow flies. This tool calculates it. You enter the arrow's length and the distance to its balance point, and it returns the FOC as a percentage.

It is a number archers use to fine-tune arrow flight, stability, and how well a broadhead-tipped hunting arrow holds its line, especially at longer distances.

How to use it

1. Measure the arrow length, from the throat of the nock to the end of the shaft, not counting the point, and enter it.
2. Find the balance point by balancing the finished arrow on a thin edge, then measure from the nock throat to that spot and enter it.

Both measurements use the same unit. Press Calculate for the FOC percentage, or Reset to clear the fields.

How FOC is worked out

FOC measures how far the balance point sits ahead of the arrow's physical centre, expressed as a percentage of the arrow's length:

FOC % = ((balance point − half the arrow length) ÷ arrow length) × 100

You find the physical centre by halving the length, see how much further forward the real balance point is, and express that as a fraction of the whole length. An arrow that balanced exactly at its centre would have an FOC of zero. The more weight sits up front, the further forward the balance point, and the higher the FOC.

Why front weight matters

There is a nice way to picture this. If you dropped a plain stick from a height, it would tumble end over end all the way down. But tie a weight to one end of that stick and drop it again, and the heavy end leads while the light end trails behind, so the stick falls straight and steady. An arrow works the same way. Concentrating weight toward the front makes the point lead and the back follow, which keeps the arrow pointed the right way and lets it correct small wobbles in flight. FOC is simply the number that captures how much of that forward bias your arrow has.

An example with real numbers

Say you have a 28.5-inch arrow, so its physical centre is at 14.25 inches, and the finished arrow balances at 17.75 inches.

• FOC = ((17.75 − 14.25) ÷ 28.5) × 100
• = (3.5 ÷ 28.5) × 100 = 12.28%

So this arrow has an FOC of about 12.3 percent, with its balance point sitting 3.5 inches forward of centre.

How much FOC, and how to change it

There is a trade-off to balance. A higher FOC makes an arrow more stable and helps it penetrate, but the extra nose weight slows it down and makes it drop faster, so the trajectory curves more. A lower FOC gives a flatter, faster arrow, but its flight can get erratic, especially at distance. Easton Archery recommends roughly 10 to 15 percent FOC for hunting setups and accuracy, while many target archers sit a little lower, around 9 to 12 percent. To raise FOC, add weight to the front with a heavier point, broadhead, or insert, or cut the arrow a touch shorter. To lower it, move weight rearward with heavier vanes or a heavier nock. Bear in mind that adding much point weight also weakens the arrow's spine, so a big change may call for a stiffer shaft.

Questions people ask

How do you calculate arrow FOC?

Subtract half the arrow's length from the balance point, divide by the full length, and multiply by 100. The arrow length is measured from the nock throat to the end of the shaft, not including the point.

Why does FOC matter?

Forward weight makes the arrow's front lead and its back follow, like a stick dropped with a weight on one end, which keeps it flying straight and self-correcting. FOC measures how much of that forward bias the arrow has.

What FOC should I aim for?

Easton recommends about 10 to 15 percent for hunting and accuracy, and many target archers run roughly 9 to 12 percent. Higher FOC is more stable but drops faster, while lower FOC is flatter but can fly erratically.

How do I change my arrow's FOC?

Add front weight with a heavier point, broadhead, or insert to raise it, or add rear weight with heavier vanes or nock to lower it. A large point-weight change may need a stiffer arrow spine.

References

1. Easton Archery, What Is F.O.C. and How Does It Affect Arrow Flight? https://eastonarchery.com/2014/06/foc/