What This Calculator Does
Most FOC calculators only work forward: you balance a finished arrow on your finger and measure the balance point. This tool does the reverse: given your shaft's GPI (grains per inch), cut length, insert, nock, vane, and wrap weights, it calculates what point weight you need to hit any target FOC percentage — before you buy or glue anything. It also shows your current-point FOC, total arrow weight, kinetic energy, momentum, and a grains-per-pound (GPP) safety check.
How to Use It
- Shaft length: measure from the throat of the nock groove to the end of the cut shaft (not including the point/broadhead).
- GPI: find this on the arrow's label or the manufacturer's spec sheet. Multiply GPI × cut length to get shaft weight.
- Insert weight: aluminium standard inserts ≈ 12–20 gr; brass inserts ≈ 30–60+ gr; outserts vary by brand.
- Nock weight: standard press-in nocks ≈ 7–10 gr; lighted nocks ≈ 15–30 gr.
- Vane weight: enter the weight per vane, then vane count. Plastic vanes ≈ 5–10 gr each; feathers ≈ 8–15 gr each.
- Target FOC: set your desired FOC%. The tool solves for the required point weight.
- Current point: enter your actual point to see current FOC and all ballistic outputs at that weight.
- Arrow speed: use a chronograph for accuracy. Kinetic energy and momentum outputs use this value.
The Formula — Verified Method
This calculator uses the moment-arm (centre-of-mass) method, referenced in the AMO (Archery Manufacturers Organization) standard and described in detail by Easton Archery and the DO Archery Ballistics Lab. Each component's weight is multiplied by its distance from the nock end (position 0):
The 2-inch vane moment arm is a standard practical approximation for typical fletching setups; a more exact calculation would need the precise fletch offset from the nock groove. For the reverse solve, the calculator rearranges this algebra to isolate W_point.
Reverse Solve — Finding Required Point Weight
Source: AMO-standard FOC formula; moment-arm formulation consistent with Easton Archery and DO Archery Ballistics Math.
FOC Ranges by Application
- 7–9% — Target archery (recurve, Olympic, barebow): flatter trajectory, minimal wind drift, maximum consistency at standard distances.
- 10–15% — Bowhunting standard: Easton Archery's recommended range for most compound hunting setups. Good balance of penetration and accuracy.
- 15–20% — High-FOC hunting: favoured for tough game (elk, bear, wild boar). Dr Ed Ashby's penetration research supports this range for maximising pass-throughs.
- 20%+ — Extreme FOC (e.g. "Ranch Fairy" / Ashby-style): requires stiffer spine, tuning patience; maximises bone-breaking penetration on large dangerous game.
Kinetic Energy & Momentum Reference
Formula (Easton / industry standard): KE = (weight_gr × fps²) / 450,240
Momentum: p = weight_gr × fps / 225,400
- Small game: up to 25 ft·lb KE
- Deer / antelope / medium game: 25–41 ft·lb
- Elk / black bear / boar: 42–65 ft·lb
- Toughest game (brown bear, large African game): 65+ ft·lb
Note: these are estimates for planning only — not professional archery or hunting advice. Verify with a grain scale, chronograph, and physical tuning.
Frequently Asked Questions
Use the reverse-FOC formula derived from the moment-arm method (see above). Given your shaft GPI, cut length, insert, nock, and vane weights, you can solve algebraically for the point weight. This calculator does that instantly — you enter a target FOC% and it returns the exact point weight needed. Useful for planning a build before purchasing components.
For target archery, 7–9% FOC gives the flattest trajectory and best wind-bucking at competition distances. For bowhunting deer, 10–15% is the Easton standard. For elk, bear, and tough game, many hunters use 15–20%+ based on Dr Ed Ashby's penetration research. Target archers often deliberately keep FOC lower than hunters to preserve trajectory at distance.
Most compound bow manufacturers recommend at least 5 grains per pound (GPP) of draw weight. For a 65 lb bow, that means a minimum of 325 grains total arrow weight. Shooting lighter risks dry-fire-equivalent stress on limbs and cams. Most bowhunters shoot 7–8 GPP; target archers may use 5–6 GPP for speed.
Because inserts sit at the front end of the shaft (same position as the point in the moment-arm calculation). Any weight at the front end contributes directly to a higher CG and therefore higher FOC. Brass inserts (30–60+ gr) are a common way to raise FOC without changing broadhead or field point selection.
Yes, but in a minor way. Vanes sit near the nock end (~2" from the nock groove), so they pull the CG backward. Heavier vanes (feathers, large plastic vanes) slightly decrease FOC compared to lighter microvanes. If you're trying to maximise FOC, lighter vanes or fewer vanes help slightly, though the effect is much smaller than changing point or insert weight.
Kinetic energy favours speed (velocity is squared), so a faster, lighter arrow can have similar KE to a slower, heavier one. Momentum gives equal weight to mass and velocity, so heavier arrows retain it better over distance. For penetration on tough game — especially at longer ranges — momentum is generally considered more predictive. KE is more relevant for initial impact force. Many experienced bowhunters prioritise momentum over raw KE.