Speargun Band Length Calculator

Estimate band length from a measured pull distance, wishbone length and your stretch ratio.

For conventional circular (loop) bands only. This tool performs a geometric calculation from your measurements — it does not recommend a stretch factor, and it does not estimate power, velocity, penetration or range.

Measurement guidance

Speargun band measurement diagram Schematic showing the muzzle, the pull distance D from the muzzle to the shaft fin, the muzzle path allowance M around the muzzle, and the wishbone effective length W at the shaft fin. Muzzle M D — pull distance Shaft fin / notch W
Original SpearMetric schematic (not to scale). D is measured from the muzzle to the shaft fin/notch; M is the extra rubber path around/through the muzzle; W is the wishbone effective length at the shaft end.
  • Measure D from the band hole / rubber path at the muzzle to the shaft fin or notch where the wishbone engages.
  • Measure M as the additional rubber path caused by the muzzle's geometry or thickness — use 0 if your D measurement already includes this path.
  • Measure W as the effective distance between the two rubber connection points once the wishbone is assembled, including any cones or bushings that contribute to that distance.

Calculator

Units
Measure from the band hole / rubber path at the muzzle to the shaft fin or notch where the wishbone will engage.
Measure the additional path the band must travel around/through the muzzle. Use 0 for configurations where your pull-distance measurement already includes this path. This is not automatically the full outside width of every muzzle.
Effective distance between the two rubber connection points when the wishbone is assembled, including cones/bushings where they contribute to this distance. Optional, defaults to 0.
Select the stretch factor specified for your rubber or equipment.
Enter a custom stretch factor greater than 1.

Result

Estimated relaxed rubber length: —

Pull distance
Muzzle path allowance
Wishbone length
Stretch factor
Units

How this works

For a conventional circular band, the relaxed rubber length is calculated as: 

L = ((2 × D) + M - W) / R
  • L — relaxed rubber length, measured between rubber connection / tying points (the result)
  • D — band pull distance
  • M — muzzle path allowance
  • W — wishbone effective length
  • R — stretch factor (loaded rubber length ÷ relaxed rubber length)

Deterministic example (for verifying the arithmetic only, not a recommendation): D = 90 cm, M = 2 cm, W = 4 cm, R = 3.2 → L = ((2 × 90) + 2 - 4) / 3.2 = 55.6 cm.

Working band length vs. raw rubber cut length

The result represents the working band length — the relaxed rubber length between the connection / tying points. If you are cutting bulk rubber, extra material may be needed outside those points for knots, inserts, cones or your assembly technique. Working band length and raw rubber cut length are not always the same.

Double circular bands

For a speargun with two circular bands, calculate each band independently using its own pull distance — D1 to its loading fin/notch and D2 to its own loading fin/notch. Do not assume both bands have the same length. This page calculates one band at a time; run it again with each band's own measurements.

Need help collecting measurements? Read How to Measure Speargun Band Length before entering values.

Reference tool only. This calculator does not determine an appropriate stretch factor — always follow your band or equipment manufacturer's specifications. SpearMetric does not guarantee compatibility.