Calculators & Tools

Practical arrow calculations and technical tools for archers who measure the details.

This is where Precision Arrow Builder publishes practical arrow calculators and technical tools. Every calculation is only as useful as the measurements behind it, so this page summarizes the core arrow calculations and points to the reference material for measuring components.

Total arrow weight

Total arrow weight is the sum of every component on the finished arrow, usually expressed in grains:

• shaft;
• insert, outsert, or halfout;
• point;
• nock and any nock adapter;
• every vane or feather;
• any collars, wraps, or added weights.

Shaft weight from GPI

Shaft weight is commonly estimated from the shaft's grains per inch (GPI) and its cut length:

Shaft weight = GPI × cut length

With GPI in grains per inch and cut length in inches, the result is in grains.

For example, a shaft listed at 9.0 GPI and cut to 28.5 inches works out to about 256.5 grains. Published GPI and component weights are nominal values, and individual pieces can differ, so weigh components where you can.

Front-of-center (FOC)

FOC describes how far an arrow's balance point sits forward of its midpoint, expressed as a percentage of arrow length. A widely used formula is:

FOC (%) = 100 × (A − L / 2) / L

• L is the arrow length, measured from the throat of the nock to the end of the shaft, not including the point.
• A is the distance from the throat of the nock to the arrow's balance point.

For example, an arrow with L = 28.5 inches that balances 16.6 inches from the nock throat has an FOC of about 8.2%. Measurement methods vary between sources, so use the same method whenever you compare arrows.

The balance point depends on how much each component weighs and where that weight sits along the arrow. Recording component lengths and center-of-gravity locations consistently makes those effects easier to calculate and compare.

Supporting references