Archery Arrow FOC Calculator
Created by: Ethan Brooks
Last updated:
Calculate arrow front-of-center from measured length and balance point, with a measurement diagram and signed balance offset.
Archery Arrow FOC Calculator
ArcheryMeasured balance • one shared reference point
What does this calculator measure?
An arrow FOC calculator expresses the balance point's displacement from the arrow's midpoint as a percentage of its reference length. FOC means front-of-center. Enter length L from the nock throat to the shaft end and balance distance B from that same nock throat. The calculator subtracts half of L from B, divides by L and multiplies by 100. A positive result places the balance point ahead of the midpoint.
The measurement convention matters as much as the arithmetic. This tool follows the reference points described by Easton's FOC calculator: the point stays installed while balancing, but its projecting length is excluded from L. Both measurements start at the throat of the nock. A tip-to-nock overall measurement or a bare-shaft cut length uses different endpoints and can produce a different percentage for the same physical arrow.
FOC is a normalized distance, not the percentage of the arrow's weight in its front half. A 10% result means the balance point lies one tenth of the reference length ahead of the midpoint. It does not mean that only 10% of the mass is forward. This distinction also explains why the calculator needs two distances and does not ask for shaft GPI, total grains or component weights.
The result provides a signed percentage, signed offset, geometric midpoint and measured balance distance. It retains a negative percentage so that a rearward balance point remains visible. No universal preferred range is applied, and the result is not a flight prediction. Use it to document a measured setup alongside arrow mass and equipment details, then compare observations made with the same reference points and assembly state.
How the calculation works
Let L be the positive reference length and B the nonnegative measured balance distance, both in the same unit. The geometric midpoint is L divided by two, and the signed offset is B minus that midpoint. Dividing offset by L removes the unit, making FOC dimensionless. Multiply by 100 to express the ratio as a percentage. Converting both distances by the same factor leaves their ratio unchanged, so inches and millimeters give identical FOC.
This implementation supports balance points from the nock throat through the shaft end. A point outside that interval receives an unsupported result and a request to recheck measurements, rather than an invented correction. Midpoint balance produces zero and a point behind it produces a negative value. The table lists the entered distances and computed offset so the reference geometry remains visible alongside the percentage. Arithmetic uses the input precision before display rounding.
Formula and symbols
Midpoint = L / 2; offset = B − L / 2; FOC (%) = 100 × (B − L / 2) / L.
- L: Nock throat to shaft end, excluding point (inches or millimeters)
- B: Nock throat to assembled-arrow balance point (same unit as L)
How to use the calculator
- Measure arrow length. With the arrow assembled, measure from the nock throat to the shaft end, excluding the point's projecting length.
- Find the balance point. Balance the complete arrow with all components installed, then measure from the same nock throat to that point.
- Enter matching units. Enter both distances in inches or both in millimeters. Switching the unit selector converts existing entries.
- Read the signed result. Calculate FOC and compare the balance point with the midpoint. Retain the measurement convention with your record.
Worked examples
A forward balance point: 10% FOC
An assembled arrow has a reference length of 30 inches, measured from nock throat to shaft end. Its balance point is 18 inches from the same origin. The midpoint is 15 inches, leaving an offset of 3 inches toward the point. Divide 3 by 30 and multiply by 100 to get 10% FOC. This is a description of the measured geometry, not a recommendation to achieve that number. Record the actual assembly and measurement convention with the result.
The same arrow measured in millimeters
Convert 30 inches to 762 millimeters and 18 inches to 457.2 millimeters. The midpoint becomes 381 millimeters and the forward offset becomes 76.2 millimeters. Dividing 76.2 by 762 still gives 0.1, or 10%. A unit switch should therefore change the displayed distances without changing the percentage. If two manually entered unit versions disagree substantially, inspect the conversions and reference endpoints before interpreting that disagreement as a change in the arrow.
Keeping a negative result visible
For a 30-inch reference length and a 14-inch balance distance, the midpoint remains 15 inches. The signed offset is −1 inch, giving approximately −3.333% FOC. The calculator retains that value and suggests checking the measurement. Entering 15 inches instead produces exactly zero: the balance point and geometric midpoint coincide. Neither result is silently replaced with a preferred range.
Practical applications
- Record the balance geometry of an assembled arrow. A saved result containing L, B and the calculated offset is more informative than an isolated percentage, because someone reviewing it can check the measurement convention and reproduce the arithmetic.
- Compare repeated measurements on the same arrow. Keep the assembly and reference origin unchanged, then repeat the balance procedure. Differences between results help identify how consistently you can locate the balance point; the calculator does not manufacture an uncertainty estimate.
- Document a component change after reassembly. Measure the new balance point rather than assuming that a known change in component mass predicts a specific new FOC. Save before-and-after values with notes identifying what changed and what stayed the same.
- Translate a measurement workflow between inches and millimeters. The percentage is independent of length units when both inputs share the same unit and origin. This makes it easier to compare records from different rulers without changing the definition of arrow length.
- Explain balance geometry during club practice or equipment discussions. The diagram separates nock throat, shaft end, midpoint and balance point, while the detail table connects those landmarks to the formula. It is an illustration of reference points rather than a physical scale drawing.
- Keep weight and balance records separate. Two arrows can have the same total weight but different balance points. Pair a completed-arrow mass record with measured FOC to describe those distinct properties without suggesting that either number alone establishes equipment suitability.
Tips for consistent measurements
Use the same origin for both distances, and write down what you mean by arrow length before taking readings. Install the components that belong to the measured setup. Locate the balance point, mark it and measure back to the nock throat. Repeat the procedure to check consistency. If you change components, remeasure rather than editing the previous balance distance based on a guess about the expected effect.
Keep the number of decimals realistic for your ruler and balance procedure. For a fixed 30-inch length, a 0.1-inch difference in B changes FOC by about 0.333 percentage points, so small reading differences can be visible. Do not confuse percentage points with percent change when comparing results. A move from 10% to 11% FOC is an increase of one percentage point. Save the actual distances with the result for later review.
Frequently asked questions
Where do I measure arrow length for FOC?
Use the distance from the throat of the nock to the end of the shaft, excluding the point's projecting length. This calculator follows Easton's stated convention. Keep the point installed when finding the completed arrow's balance point, but do not extend L to its tip. The diagram labels the shared origin and both measurement endpoints.
Does 10% FOC mean 10% of the weight is in front?
No. It means the balance point is ahead of the geometric midpoint by a distance equal to 10% of the reference length. For a 30-inch arrow, that is a 3-inch forward offset. FOC is a ratio of distances; the calculation does not divide the arrow into front and rear masses or measure their separate weights.
Can FOC be negative?
Yes, the formula is negative when the measured balance point lies behind the midpoint. For example, B of 14 inches and L of 30 inches produces approximately −3.333%. The calculator preserves the sign and asks you to recheck the shared origin and balance reading. It does not automatically clamp a negative result to zero or label it a preferred setup.
What is the best FOC percentage?
This tool does not select a best percentage. It reports your measured balance geometry without applying a universal target band. Equipment configuration, the intended discipline and the applicable manufacturer's guidance belong in a separate assessment. Keep the result with your setup notes and compare measurements and practical observations using consistent methods rather than treating one percentage as a guarantee.
Why is the balance point outside the supported range?
The supported interval runs from the nock throat through the shaft end. A balance distance larger than L is not evaluated by this tool. Recheck whether one measurement includes the point or starts at the opposite end, and confirm that both fields use the selected units. The calculator explains that boundary instead of silently changing your measurement.
Can I calculate FOC from component weights alone?
Not with this measured-balance method. Knowing total component masses does not tell the calculator their positions or mass distribution along the arrow. Measure the completed arrow's balance point and use the specified length convention. The separate Arrow Weight Calculator can total the component masses, but its result is not a substitute for the two FOC measurements.
Sources and measurement scope
- Easton Archery: FOC calculator — length and balance measurements — Undated online calculator; Arrow length and balance point input instructions. Accessed 2026-10-01. Nock-throat origin, shaft-end length excluding point, fully assembled balance measurement.
- Easton Archery: Calculating front-of-center — Article URL dated June 2014; Calculating F.O.C, steps 1–5. Accessed 2026-10-01. Subtract midpoint from measured balance distance and divide by reference length. No recommended FOC band is applied here.
- NIST: Guide to the SI, Appendix B.9 — conversion factors — SP 811 online Appendix B.9; Length; Mass and Moment of Inertia. Accessed 2026-10-01. Inch-to-meter and grain-to-kilogram conversion factors; no archery selection guidance.
Worked examples use illustrative measurements. These tools do not select equipment or prescribe a preferred setup. Any optional manufacturer comparison uses the reference entered by the user.