Arrow FOC and Weight Calculator: Formula and Examples

An arrow FOC and weight calculator estimates an arrow’s front-of-center (FOC) percentage from its balance point and overall length, and can total its component weights. Enter measurements in one consistent unit system; the results show FOC and, if you provide the parts, total arrow weight. FOC describes balance location, not whether an arrow is safe or suitable for a particular bow.

Formula

FOC is calculated by comparing the balance point’s distance ahead of the arrow’s midpoint with the arrow’s measured length:

FOC (%) = [(balance point − arrow length ÷ 2) ÷ arrow length] × 100

• Balance point is measured from the nock groove to the point where the completed arrow balances, in inches or centimeters.
• Arrow length is measured from the nock groove to the end of the shaft, using the same unit as the balance measurement.
• Arrow midpoint is half the arrow length.

The calculator’s total-weight formula is shaft weight + point and insert weight + fletching weight + other component weight. All those inputs are in grains. One grain is 1/7000 of a pound; approximately 15.43 grains equal one gram. The calculator does not convert units automatically, so use inches for both FOC measurements and grains for every weight input.

Worked Example

Using the calculator’s defaults, the balance point is 20 inches from the nock groove and arrow length is 30 inches.

1. Find the midpoint: 30 ÷ 2 = 15 inches.
2. Find how far forward the balance point lies: 20 − 15 = 5 inches.
3. Divide by arrow length and convert to a percentage: (5 ÷ 30) × 100 = 16.67% FOC.
4. Add the component weights: 300 + 100 + 18 + 22 = 440 grains.

The displayed results are 16.67% FOC and 440.0 grains total. The FOC is rounded to two decimal places and the weight to one, as specified in the calculator block.

How to Measure FOC

FOC should be measured on the finished arrow, with the components installed. A bare shaft’s balance point can differ from that of the completed arrow because the point, insert, fletching, nock, wrap, and other items affect its mass distribution.

1. Assemble the arrow in the configuration you want to evaluate.
2. Measure arrow length from the nock groove—the point where the bowstring seats—to the end of the shaft. Do not measure to the tip of the point for the formula used here.
3. Balance the arrow on a narrow, stable edge or a purpose-made balance tool. Mark the balance point, then measure from the nock groove to that mark.
4. Enter both measurements in the same units. A balance point beyond the midpoint produces positive FOC; one behind the midpoint produces negative FOC.

For example, if a 30-inch arrow balances 18 inches from its nock groove, its FOC is [(18 − 15) ÷ 30] × 100 = 10%. The distance is measured from the same reference end in both measurements; changing the reference can invalidate the result.

What FOC and Arrow Weight Tell You

FOC expresses how far forward the balance point is as a share of arrow length. It is not the same as the arrow’s total mass: two arrows can weigh the same but have different FOC if their weight is distributed differently. Conversely, arrows with similar FOC can have different total weights.

Total arrow weight is useful when comparing a finished arrow with equipment requirements or calculating mass per unit of length. For grains per inch (GPI), divide the relevant arrow weight by arrow length in inches. Be clear about what the shaft manufacturer’s GPI figure includes: it commonly describes the shaft rather than the fully assembled arrow, and component weights must be added separately for a finished-arrow total.

Neither FOC nor total weight alone tells you whether an arrow matches a particular bow setup. Shaft selection also depends on factors such as draw weight, draw length, shaft design, point weight, and the manufacturer’s selection guidance. Follow the bow and arrow makers’ specifications and have a qualified archery shop check uncertain setups.

Common Measurement and Calculation Mistakes

• Mixing units: Do not enter a balance point in centimeters and length in inches. Convert one measurement first, or measure both in the same unit.
• Measuring the wrong length: This formula uses nock groove to shaft end. If a different definition is used, the calculated FOC will not be directly comparable.
• Leaving components out: Measure balance on the completed arrow. For total mass, include every component you want counted; an omitted wrap or insert makes the sum too low.
• Confusing FOC with a recommendation: A calculated percentage is a measurement, not a universal target. Suitable values depend on the intended setup and the relevant manufacturer or tuning guidance.
• Over-reading decimal places: A balance mark and scale have finite precision. Extra displayed digits do not make approximate measurements exact.

A common misconception is that adding a heavier point always makes the arrow’s FOC rise by a fixed amount. It generally moves the balance point forward, but the resulting percentage depends on the arrow’s length, starting balance, and the added mass and its position. Re-measure the assembled arrow rather than relying on a guessed change.

Limits and Safe Use

This calculator performs arithmetic only. It does not assess shaft spine, bow compatibility, arrow integrity, broadhead suitability, or safe minimum arrow weight. Those questions require the relevant bow and shaft specifications and, when needed, advice from a knowledgeable archery professional. Do not shoot a damaged arrow, and inspect arrows and components before use. A correct FOC result cannot make an unsuitable or damaged setup safe.

Small measurement differences can noticeably affect FOC, especially on shorter arrows. Keep the arrow steady while finding its balance, use a consistent length reference, and record whether measurements were taken on the completed configuration. When comparing arrows, use the same measurement method for each.

Frequently Asked Questions

How do you calculate arrow FOC?
Subtract half the arrow length from the balance-point distance, divide the difference by the full arrow length, then multiply by 100. Measure both distances from the nock groove using the same units.

Where do I measure arrow length for FOC?
For the formula on this page, measure from the nock groove—the string-seating point—to the end of the shaft. Use the same length convention consistently when comparing results.

Should I measure FOC with the point installed?
Yes. Measure the completed arrow in the configuration you intend to assess. The point and other fitted components affect the balance location.

How do I calculate total arrow weight?
Add the weights of the shaft, point and insert, fletching, nock, wrap, and any other included parts. Keep all component weights in the same unit; this calculator uses grains.

Can arrow FOC be negative?
Yes. If the balance point is behind the arrow’s midpoint, the formula returns a negative percentage. Check the reference point and measurements if that result was unexpected.

Does a higher FOC mean an arrow is better?
Not by itself. FOC describes balance distribution, not overall suitability. Consider the complete bow-and-arrow setup and follow applicable equipment guidance.

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in balance | Balance point from nock end | in | 20 | 0
in length | Arrow length from nock groove to shaft end | in | 30 | 0
in shaft | Shaft weight | grains | 300 | 0
in point | Point and insert weight | grains | 100 | 0
in fletch | Fletching weight | grains | 18 | 0
in other | Nock, wrap, and other component weight | grains | 22 | 0
out foc | FOC | % | (balance - length / 2) / length * 100 | 2
out total | Total arrow weight | grains | shaft + point + fletch + other | 1