Arrow FOC calculator
This arrow FOC calculator finds how far an arrow balances ahead of its midpoint, expressed as a percentage of arrow length. Enter the nock-to-shaft-end length and the distance from the nock to the balance point of the assembled arrow. FOC describes weight distribution; it does not tell you whether the arrow is correctly tuned to your bow.
Throat of the nock to the end of the shaft, without the point.
From the throat of the nock, with the point installed.
Front of center
8.6%
What FOC measures
FOC means front of center. An arrow has positive FOC when its balance point lies ahead of the halfway point along the measured arrow length. At zero FOC, it balances at that midpoint. A larger percentage means the balance point is farther forward relative to the arrow's length.
The percentage is not the share of total arrow weight contained in the point. A 100 grain point on a 400 grain arrow does not automatically create 25 percent FOC. The position of every component contributes to balance. For more on the term, see the FOC definition.
Measure the actual arrow before deciding that its balance needs changing. Component lists can estimate total mass, but they do not locate the finished balance point without accounting for where that mass sits. A wrap or different nock can move it even when the point stays the same.
How to measure arrow length and balance point
- Assemble the arrow in the configuration you want to evaluate, including its point, insert, nock, fletching, and any wrap. Handle sharp points safely.
- Measure arrow length from the throat of the nock, where the string seats, to the end of the shaft. Exclude the point projecting beyond the shaft.
- Rest the arrow horizontally across a narrow, blunt support. Move it along the support until it balances without tipping toward either end.
- Mark that balance location lightly, then measure from the same nock throat to the mark. Keep the point installed for this measurement.
- Enter both distances using the same unit. Repeat the balance check to make sure the mark is repeatable before comparing small differences.
The nock throat is inside the string groove. Starting a tape at the rear tips of the nock changes the reference. You can keep the point installed while measuring length; simply stop at the shaft end. Removing the point for the balance test would describe an unfinished arrow.
The formula, with a 30 inch example
FOC % = (balance point - arrow length / 2) / arrow length x 100. First find the midpoint of the measured arrow length. Subtract that midpoint distance from the balance-point distance, then divide by arrow length and multiply by 100.
| Step | Calculation | Result |
|---|---|---|
| Find the midpoint | 30 / 2 | 15 in |
| Find the forward offset | 18 - 15 | 3 in |
| Express the offset as a percentage | 3 / 30 x 100 | 10% FOC |
The point remains part of the assembled arrow during balancing, even though its projecting length is excluded from the 30 in length measurement. This convention keeps the measurement consistent when points have different shapes. If another chart specifies a different length convention, follow that chart and avoid comparing unlike measurements.
The ratio works in inches or millimeters, provided both entries use the same unit. Converting only one input gives a meaningless result. If the answer looks extreme or negative for an ordinary complete arrow, check your units and reference points before changing any components.
What FOC range should you aim for?
Target arrows commonly sit around 7 to 15 percent FOC. Many hunters build higher. Treat that range as a reference while checking a build. It does not require you to change an arrow that already tunes and groups well with your bow.
For target shooting, a repeatable tune and the groups you shoot are more useful than chasing a particular percentage. If your arrows group well, an FOC number outside a familiar range is a reason to check the measurement and equipment specifications. It is not enough evidence by itself to rebuild the set.
For a hunting build, adding point or insert mass may raise FOC, but it also changes total weight and how the shaft behaves during the shot. There is no single FOC percentage in this calculation that proves adequate penetration. Test the finished arrow configuration and keep it within the equipment manufacturers' recommendations.
How component changes move the balance point
| Change | Usual effect on FOC | What else to check |
|---|---|---|
| Heavier point or front insert | Increases FOC | Total mass and shaft suitability |
| Heavier nock or rear wrap | Decreases FOC | Nock fit and total mass |
| Heavier fletching at the rear | Decreases FOC | Clearance and arrow flight |
A lighter shaft with the same components can also alter FOC, but shaft selection has to account for diameter, construction, and stiffness. Read the manufacturer's selection chart before replacing a shaft just to move the balance point. Arrow spine describes stiffness and remains a separate equipment decision.
Use the arrow weight calculator to estimate the mass change before buying components. Then assemble one suitable test arrow and measure its balance again. Adding 25 grains at the point moves the balance differently on a light arrow than on a heavy one, so measure the new balance point rather than guessing the percentage.
Record enough detail to repeat the test
Write down arrow length, balance-point distance, component weights, and the measurement convention beside the FOC result. When comparing fletching options, include the vane or feather model and count. The number alone will not explain why a later arrow balances differently.
Check several arrows from the set using the same support and ruler. If one balances differently, inspect it for a missing component or a mismatched point before drawing conclusions about the whole build. Keep the measured lengths in your notes so you can recalculate the percentage without having to balance every arrow again.
Questions
Should the point be installed when measuring FOC?
Yes. Balance the complete arrow with the point and all other shooting components installed. Measure arrow length from the nock throat to the shaft end, excluding the projecting point. Measure the balance-point distance from that same nock throat. Those two measurements are different, even though they share a starting reference.
Does adding a heavier point always improve an arrow?
A heavier point generally moves the balance forward, but a higher FOC percentage is not an automatic improvement. Point weight also changes total mass and the shaft's response during the shot. Check shaft recommendations and evaluate flight and groups after the change before building the rest of your arrows.
Can I calculate FOC from component weights alone?
Component weights give you total arrow mass, but FOC also depends on where each part's mass is located. A detailed model could estimate balance from those positions. For a finished arrow, finding its physical balance point is more direct and accounts for installed parts such as glue and wraps.
Why do two FOC calculators give different answers?
Check whether they use the same arrow-length definition and whether both inputs start at the nock throat. Including the point length, measuring from the rear of the nock, or mixing units changes the result. With identical measurements and the same formula, the answers should match apart from rounding.