Use this free airsoft FPS and joule calculator to convert FPS to joules, work out your muzzle energy from any BB weight, switch between FPS and metres per second, and check whether you are inside the UK legal limits. There is also a second tool that shows how your FPS changes across different BB weights, because muzzle energy stays the same for a given gun while the speed does not.
| BB weight | FPS | Energy | Auto / single |
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The UK legal limits for an airsoft gun are 1.3 joules for fully automatic and 2.5 joules for single-shot or semi-automatic (Policing and Crime Act 2017). Individual skirmish sites set their own, usually stricter, FPS limits, and these are normally measured on a 0.20 g BB. Muzzle energy stays the same across BB weights for a given gun, so a heavier BB leaves the barrel slower. Always check your site rules and chrono on the day.
What is muzzle energy, and why does it matter?
Muzzle energy is the kinetic energy a BB carries as it leaves the barrel, measured in joules. It is the number that actually matters, because both UK law and every skirmish site set their limits in energy, even when a site quotes a limit in FPS. FPS on its own is meaningless without the BB weight it was measured on, since a heavier BB at the same energy travels slower. That is why a chrono reading is always tied to a specific BB weight, almost always 0.20 g.
The maths is simple. Energy in joules equals half the BB mass in kilograms multiplied by the velocity in metres per second squared. The calculator above does it for you, converts FPS to metres per second, and flags the result against the two UK legal thresholds so you know exactly where you stand.
The UK airsoft power limits
UK law sets two hard limits on the muzzle energy of an airsoft gun, under the Policing and Crime Act 2017. Go above these and the gun is no longer legally an airsoft gun, it becomes a firearm:
- 1.3 joules for a gun that can fire fully automatic (two or more BBs from one pull of the trigger). That is about 374 FPS on a 0.20 g BB.
- 2.5 joules for a single-shot or semi-automatic gun, such as a bolt-action sniper rifle. That is about 519 FPS on a 0.20 g BB.
Those are the legal ceilings, not the site rules. Individual skirmish sites almost always set their own limits well below the legal maximum, and they vary from site to site. As a rough guide you will often see an AEG limit around 350 FPS (roughly 1.14 joules) on 0.20 g, higher limits for semi-only support weapons and DMRs with a minimum engagement distance, and separate rules for bolt-action snipers. Always read your site rules and chrono your gun on the day, because the site limit is the one that gets you sent home if you are over it. For the full picture on limits, two-tone rules and buying legally, see our guide to airsoft FPS limits in the UK.
Why heavier BBs give a lower FPS
This is where a lot of players trip up. Your gun puts out a fixed amount of energy, set by its spring or gas system. When you load a heavier BB, that same energy has to move more mass, so the BB leaves the barrel slower. The energy stays the same, the FPS drops. That is exactly what the “FPS by BB weight” tab of the calculator shows: enter your chrono reading and the weight you measured on, and it works out your FPS and energy on every common BB weight.
This matters for two reasons. First, it lets you check you are still site-legal if you switch BB weights, since a gun that passes the chrono on 0.20 g carries the same energy on 0.28 g or 0.32 g even though the FPS looks much lower. Second, heavier BBs are more stable in flight and less affected by wind, which is why many players run 0.28 g, 0.30 g or heavier for better range and accuracy. Choosing the right BB is a topic in itself, covered in our airsoft BBs guide.
Getting set up in Kent
If your gun is reading over your site limit, or you want it tuned to a specific energy, our in-house workshop in Rochester can help. We can chrono your gun, adjust the power where the platform allows, and make sure you are set up legally and for your site before your next game. If you are buying your first gun, we can point you to something that arrives site-legal out of the box. Browse the airsoft guns we stock, or read our pick of the best airsoft guns for 2026 to get started. New to the sport and not sure about two-tone or UKARA? Start with our guides to two-tone airsoft and UKARA registration.
Frequently asked questions
How do you convert FPS to joules?
Multiply the FPS by 0.3048 to get metres per second, then work out half the BB mass in kilograms times that speed squared. For example, 350 FPS on a 0.20 g BB is 106.7 metres per second, which is about 1.14 joules. The calculator above does this instantly for any FPS and BB weight.
What FPS is 1 joule on a 0.20 g BB?
About 328 FPS on a 0.20 g BB is 1 joule. The common 350 FPS site limit works out at roughly 1.14 joules, and the UK legal auto limit of 1.3 joules is about 374 FPS on 0.20 g.
Does BB weight change the joules?
No. For a given gun the muzzle energy is fixed, so the joules stay the same whatever BB weight you use. A heavier BB simply leaves the barrel slower, so the FPS drops while the energy holds. That is why site and legal limits are really energy limits, even when quoted in FPS on a 0.20 g BB.
What is the maximum legal FPS for airsoft in the UK?
The law is set in energy, not FPS. An airsoft gun must not exceed 1.3 joules if it can fire fully automatic, or 2.5 joules if it is single-shot or semi-automatic. On a 0.20 g BB that is roughly 374 FPS and 519 FPS. Skirmish sites set their own, usually lower, limits.
Why does my gun chrono lower with heavier BBs?
Because the energy is fixed and a heavier BB has more mass to move, so it comes out slower. This is normal and expected. Use the “FPS by BB weight” tab above to see your exact FPS on any weight from a single chrono reading.