Arrow FOC Calculator

How to use the Arrow FOC Calculator

What this calculator does

This calculator converts two physical measurements into an arrow's front-of-center percentage. FOC expresses how far the finished arrow's balance point lies ahead of the midpoint, relative to the arrow's measured length. It is useful for recording and comparing completed arrow builds, checking whether component changes moved balance forward or rearward, and documenting a repeatable setup. It does not determine whether an arrow is safe for a particular bow, correctly spined, fully tuned, or guaranteed to group well; those questions also depend on draw weight, arrow mass, dynamic spine, release quality, broadhead behavior, and other variables. Easton's explanation of how FOC affects arrow flight provides additional context for the metric.

When to use it

Use the calculator when assembling a new arrow, comparing point or insert weights, switching between vanes and feathers, checking consistency across a matched set, or recording a hunting or target configuration after tuning. It is especially practical when a small hardware change shifts the balance point but the overall arrow length remains unchanged.

How to calculate

1. The calculator opens with a ready-to-use demonstration: an Arrow length (A) of 30 cm and a Length to balance point (B) of 18 cm. The result and a validated Excel workbook are available immediately.
2. Select the Length unit you used. Switching between centimeters and inches converts both measurements automatically. Always measure A and B from the throat of the nock, and use the same assembled arrow for both values.
3. Replace the demonstration values with your measurements. Results update live. Read the primary Front-of-center (FOC) result, then use Geometric center, Forward offset, Balance position, and Hunting reference to audit and interpret it.
4. Select Download Excel to export the current typed inputs, canonical measurements, outputs, formula notes, and validation checkpoints. Select Reset to clear the demonstration and all calculated content. Reset may disable the export until both required measurements are entered again.

Input guide

Arrow length (A) is required. Enter a positive decimal measured from the throat of the nock to the end of the shaft, excluding the point when following the common AMO-style convention. Plain numbers and U.S.-style thousands separators are accepted; scientific notation and decimal commas are rejected. A realistic example is 30 cm or about 11.81 in. Increasing A while B stays fixed generally lowers FOC because the midpoint moves forward and the offset is divided by a longer arrow. A common mistake is measuring from the back of the nock instead of the nock throat.

Length to balance point (B) is required. Enter the distance from the same nock-throat origin to the point where the fully assembled arrow balances. A realistic example is 18 cm. B must be between zero and A. Increasing B while A stays fixed raises FOC. Measure the finished arrow with the point or broadhead, insert, nock, wraps, and fletching installed; omitting a component can materially move the balance point.

Length unit is required and accepts centimeters or inches. It changes the displayed linear values and workbook units, but not the percentage because the ratio is unitless. Do not manually convert only one input; use the selector so A and B remain in the same unit.

Output guide and worked example

Front-of-center (FOC) is the exact percentage identity produced by the entered measurements. Positive values indicate a balance point ahead of center; zero means the arrow balances at its midpoint; negative values mean rear-of-center balance. Geometric center is A ÷ 2. Forward offset is B – A ÷ 2 and can be negative. Balance position states front-balanced, centered, or rear-balanced. Hunting reference compares the result with the commonly cited 10 – 15% starting range; it is a comparison, not a guarantee or individualized recommendation. The header pills repeat the current FOC, offset, and balance direction, while the measurement-audit table shows the same canonical calculation values and each row's role.

For the startup example, A = 30 cm and B = 18 cm. The midpoint is 30 ÷ 2 = 15 cm. The balance point is therefore 18 – 15 = 3 cm ahead of center. Dividing 3 by 30 and multiplying by 100 gives 10.00%. The calculator accordingly displays a 15.00 cm geometric center, a 3.00 cm forward offset, a front-balanced position, and a result within the 10 – 15% hunting reference range.