Speargun rubber length calculator
This is a rough guide on band stretch however the ultimate determining factor when measuring rubbers and band stretch is determined by the manufacturer and the compounds used in the rubber itself.
Short answer: to size a band, measure the loaded distance from your muzzle band anchor to the shaft notch, subtract your wishbone length, then divide by your chosen stretch ratio. Use around 3.1 for an easy load with less power, 3.3 for a balanced setup, and 3.5 for maximum power that is harder to load. The relaxed band you cut will always be much shorter than the gun, because the rubber does its work stretched. Here is the full method, a worked example and a quick chart so you can get it right the first time.
The one number that matters: stretch ratio
A speargun band only stores energy when it is stretched, and there is a sweet spot for how far. Stretch it too little and the gun is weak and the shaft drops off at range. Stretch it too much and the load becomes a fight, the rubber wears out fast, and you are into the danger zone where a band can let go while you load. Most latex speargun rubbers are designed to run between 3.1 and 3.5 times their relaxed length. That multiplier is your stretch ratio, and it is the single choice that sets both your power and how hard the gun is to load.
- 3.1x – easiest load, least power. Good for newer divers, long sessions and smaller fish.
- 3.3x – the balanced middle. A sensible default for most reef guns.
- 3.5x – most power, hardest load. For strong loaders and bigger fish.
The method, step by step
Step 1. Measure the loaded distance. With the shaft sitting in the loaded position at the trigger, measure the straight-line distance from where the band attaches at the muzzle to the shaft notch. This is shorter than the barrel, because the anchor sits back from the muzzle tip and the notch sits forward of the butt. Call this number L.
Step 2. Subtract the wishbone. The wishbone is the dyneema loop or metal fitting that seats in the notch. It does not stretch, so it does not count as rubber. Subtract its length, usually 4 to 8 cm, from L. Call the result the stretched band span.
Step 3. Divide by your stretch ratio. Take the stretched band span and divide by 3.1, 3.3 or 3.5. The answer is the relaxed length of rubber you need between the muzzle anchor and the wishbone, on one side.
Relaxed band length = (loaded distance L − wishbone length) ÷ stretch ratio
Step 4. Work out the tubing to cut. If your band is a single piece of tubing threaded through the muzzle so both legs return to one wishbone, cut roughly twice the relaxed length plus a small allowance for the wishbone knots. If each side anchors separately, cut the relaxed length per side. Always cut a touch long on the first attempt and trim in small steps.
[collection-carousel="accessories"]
Worked example
Say you are banding a 100 cm railgun. You measure the loaded distance from the muzzle anchor to the notch at 92 cm, and your dyneema wishbone is 5 cm. That gives a stretched band span of 87 cm. For a balanced 3.3x load, 87 divided by 3.3 is about 26.4 cm, so you want roughly a 26 to 27 cm relaxed band on each side. Threaded as a loop through the muzzle, you would cut close to 55 cm of tubing plus your wishbone allowance. Want it punchier for mackerel? Drop to 3.5x, which gives 87 divided by 3.5, or about 24.9 cm, a shorter and harder-loading band.
Quick reference chart
Relaxed band length per side, in centimetres, for a range of stretched band spans. Find your span from Step 2, then read across.
| Stretched span (L − wishbone) | 3.1x (easy) | 3.3x (balanced) | 3.5x (powerful) |
|---|---|---|---|
| 70 cm | 22.6 cm | 21.2 cm | 20.0 cm |
| 80 cm | 25.8 cm | 24.2 cm | 22.9 cm |
| 90 cm | 29.0 cm | 27.3 cm | 25.7 cm |
| 100 cm | 32.3 cm | 30.3 cm | 28.6 cm |
| 110 cm | 35.5 cm | 33.3 cm | 31.4 cm |
[tip]Always err longer on your first cut. A band that is slightly too long loads easily and you can trim it shorter in 1 cm steps until the load feels right. A band cut too short is wasted rubber and dangerous to load.[/tip]
A few things that change the numbers
Band diameter does not change the length maths, but it changes how the load feels, so pick your diameter first from the rubber size chart and then size the length. Roller guns use a different geometry again, since the band runs over the muzzle roller, so measure the actual band path on your roller rather than treating it like a railgun. Cold water stiffens latex slightly, and rubber relaxes a little as it beds in over the first few sessions, so a band that feels perfect in the shed may want a small trim after a couple of dives.
Common sizing mistakes
A few errors account for most badly sized bands. The first is measuring the barrel instead of the loaded distance to the notch, which leaves the band far too long, slack and weak. The second is forgetting to subtract the wishbone, which quietly adds several centimetres of slack you did not account for. The third is chasing maximum power by cutting to a 3.5x stretch on a gun you cannot comfortably load, which wears the rubber out quickly and makes the gun harder to shoot accurately. Measure the real loaded distance, take off the wishbone, choose a stretch ratio you can actually load in the water, and re-check the feel after the bands have bedded in over a couple of sessions.
Prefer to skip the maths?
Cutting your own bands is worth learning, but you do not have to. Our pre-made bands are sized to suit each Aimrite gun, so you can fit and go. If you are running a custom shaft, an unusual length or a roller setup and want a hand with the numbers, our team dive these waters themselves and will help you dial it in. Once your bands are sized, read the safe loading guide so you fit and load them without hurting yourself or the gun.
[button url="/collections/accessories"]Shop pre-cut bands[/button]