What is FOC and how is it calculated?

FOC, or front of center, is how far forward the arrow's balance point sits as a percentage of its length. The formula is 100 times the balance point minus half the length, divided by length, with both measurements taken from the throat of the nock. Higher FOC improves stability and penetration.

What FOC should I aim for?

Target and field archers typically run 7 to 12 percent. Hunters often prefer 10 to 19 percent for better penetration and forgiveness. Extreme or Ashby setups exceed 19 percent for maximum penetration on large game, at the cost of a more arcing trajectory.

How is arrow kinetic energy calculated?

Kinetic energy in ft-lbs equals total arrow weight in grains times speed in fps squared, divided by 450,240. Momentum in slug-fps equals weight times speed divided by 225,218. Momentum often predicts penetration better than energy for heavy arrows.

What is grains per pound and why does it matter?

Grains per pound (gpp) is total arrow weight divided by draw weight. Shooting below about 5 gpp can damage a compound bow because too little of the stored energy is absorbed by the arrow. The tool flags any setup under 5 gpp as unsafe.

Where do I measure the balance point from?

Both the balance point and the arrow length are measured from the throat of the nock (where the string sits) toward the point. Balance the finished arrow on a straight edge to find its center of gravity, then measure that distance.

Bow Arrow Kinetic Energy & FOC Calculator

The bow arrow kinetic energy and FOC calculator gives bowhunters the three numbers that define an arrow’s terminal performance: how much energy it carries, how much momentum drives penetration, and how its weight is balanced toward the front.

How it works

You enter each component’s grain weight — point or broadhead, insert, shaft, fletching, and nock — and the tool sums them into the total arrow weight. From that it computes:

KE (ft-lbs) = (total_grains × speed_fps²) / 450240 Momentum (slug-fps) = (total_grains × speed_fps) / 225218

Front of center is computed from your measured balance point and arrow length:

FOC % = 100 × (balance_point − length / 2) / length

Both are measured from the throat of the nock. A balance point exactly at the middle gives 0 percent FOC; the farther forward it sits, the higher the FOC.

Tips and notes

A 420-grain arrow at 290 fps produces about 78 ft-lbs of energy — plenty for deer-sized game. The tool also reports grains per pound (total weight divided by draw weight); keep this above 5 gpp to avoid dry-fire-like stress on a compound bow. Higher FOC flies more stably and penetrates better but drops faster, so most hunters settle in the 10 to 15 percent range. As always, a sharp, well-tuned broadhead and correct shot placement matter more than any single number.