Convert your airsoft gun’s fps to joules, or joules to fps, for any BB weight. Type in either box and the other updates, then see how your gun compares with typical UK site limits, minimum engagement distances and the legal definition of an airsoft gun.
How fps to joules works
Fps (feet per second) tells you how fast a BB leaves the barrel, but it depends on the BB’s weight: the same gun shoots a light BB faster than a heavy one. Joules measure the BB’s energy, which stays roughly the same whatever weight you use. That’s why joules are the fairest way to compare guns, and why the law and a growing number of sites use them.
Worked example: a gun chrono’d at 350 fps with a 0.20g BB. 350 × 0.3048 = 106.68 m/s. 0.5 × 0.0002 × 106.68 × 106.68 = 1.138 joules.
FPS to joules chart
Joules for common chrono readings at each BB weight. Find your fps in the left column and read across.
| FPS | 0.20g | 0.25g | 0.28g | 0.30g | 0.36g | 0.40g |
|---|---|---|---|---|---|---|
| 250 fps | 0.58 J | 0.73 J | 0.81 J | 0.87 J | 1.05 J | 1.16 J |
| 280 fps | 0.73 J | 0.91 J | 1.02 J | 1.09 J | 1.31 J | 1.46 J |
| 300 fps | 0.84 J | 1.05 J | 1.17 J | 1.25 J | 1.51 J | 1.67 J |
| 320 fps | 0.95 J | 1.19 J | 1.33 J | 1.43 J | 1.71 J | 1.90 J |
| 328 fps | 1.00 J | 1.25 J | 1.40 J | 1.50 J | 1.80 J | 2.00 J |
| 350 fps | 1.14 J | 1.42 J | 1.59 J | 1.71 J | 2.05 J | 2.28 J |
| 370 fps | 1.27 J | 1.59 J | 1.78 J | 1.91 J | 2.29 J | 2.54 J |
| 400 fps | 1.49 J | 1.86 J | 2.08 J | 2.23 J | 2.68 J | 2.97 J |
| 420 fps | 1.64 J | 2.05 J | 2.29 J | 2.46 J | 2.95 J | 3.28 J |
| 450 fps | 1.88 J | 2.35 J | 2.63 J | 2.82 J | 3.39 J | 3.76 J |
| 470 fps | 2.05 J | 2.57 J | 2.87 J | 3.08 J | 3.69 J | 4.10 J |
| 500 fps | 2.32 J | 2.90 J | 3.25 J | 3.48 J | 4.18 J | 4.65 J |
| 520 fps | 2.51 J | 3.14 J | 3.52 J | 3.77 J | 4.52 J | 5.02 J |
| 550 fps | 2.81 J | 3.51 J | 3.93 J | 4.22 J | 5.06 J | 5.62 J |
Typical UK airsoft fps and joule limits
There’s no single national limit for game sites, but most UK sites use limits close to these. Speeds are shown for each BB weight at the same energy.
| Class | Joules | 0.20g | 0.25g | 0.28g | 0.30g | Typical MED |
|---|---|---|---|---|---|---|
| Indoor / CQB (some sites) | 1.00 J | 328 fps | 293 fps | 277 fps | 268 fps | None at most sites |
| Rifle, full-auto (AEG / GBBR) | 1.14 J | 350 fps | 313 fps | 296 fps | 286 fps | None at most sites |
| DMR, locked to semi-auto | 1.88 J | 450 fps | 402 fps | 380 fps | 367 fps | Usually 20 m |
| Bolt-action sniper | 2.32 J | 500 fps | 447 fps | 422 fps | 408 fps | Usually 30 m |
The legal side. In UK law an airsoft gun is one that fires a small plastic BB at no more than 1.3 joules for single-shot guns, or 2.5 joules for guns capable of automatic fire (about 374 and 519 fps with a 0.20g BB). Above these figures a gun may be treated as an air weapon, which has its own rules. Buying a realistic imitation firearm (RIF) also needs a valid defence such as UKARA registration. Our guide to UKARA, RIFs and UK airsoft law explains it all.
Getting an accurate chrono reading
- Use the BB weight your site uses, usually 0.20g, and good-quality BBs. Our guide to airsoft BBs explains why cheap BBs give poor readings.
- Take several shots and look at the highest reading, not the average. That’s what a marshal will judge you on.
- Set the hop-up as you play. Hop-up changes the reading slightly. See our hop-up guide.
- Gas guns change with temperature. GBBs and gas pistols read higher on a warm day, so chrono them in the conditions you’ll play in.
Changing BB weight? Use our BB Weight FPS Calculator to see what your gun will read on heavier BBs.
Frequently asked questions
How do you convert fps to joules?
Use the kinetic energy formula: joules = 0.5 × BB mass in kg × speed in m/s squared. Convert fps to m/s by multiplying by 0.3048, and grams to kg by dividing by 1,000. For example, 350 fps is 106.7 m/s, so with a 0.20g BB: 0.5 × 0.0002 × 106.7² = 1.14 joules.
What is 350 fps in joules?
With a 0.20g BB, 350 fps is 1.14 joules, which is the most common UK site limit for full-auto rifles. With a 0.25g BB the same gun would read about 313 fps.
What is 1 joule in fps?
1 joule is about 328 fps with a 0.20g BB, 293 fps with 0.25g and 277 fps with 0.28g. Some indoor and CQB sites use a 1 joule limit.
Why do sites chrono with 0.20g BBs?
0.20g is the traditional standard weight, so limits like "350 fps" assume a 0.20g BB. Energy is what really matters, which is why more sites now quote limits in joules. If your site chronos with a different weight, enter it above and compare the joules.
Is 400 fps legal in the UK?
400 fps with a 0.20g BB is 1.49 joules. That is above the 1.3 joule figure in the UK legal definition of an airsoft gun for single-shot guns, but within the 2.5 joule figure for guns capable of automatic fire. In practice, 400 fps is over most UK sites’ rifle limit, so it is normally only allowed for DMRs locked to semi-auto with a minimum engagement distance. Read our UK airsoft law guide for more.
What is a minimum engagement distance (MED)?
A minimum engagement distance is the closest you may shoot another player with a higher-powered gun. Typical UK MEDs are around 20 metres for DMRs and 30 metres for bolt-action snipers, and most sites expect you to carry a sidearm for closer targets. Each site sets its own MEDs, so check the rules before you play.
Protect your eyes
Whatever your gun shoots, full-seal eye protection is a must at every UK site.
More airsoft tools
BB Weight FPS Calculator
Chrono’d on 0.20g? See what your gun shoots with 0.25g, 0.28g, 0.30g and heavier BBs.
Open the calculator →Airsoft Range & BB Weight Calculator
Find the best BB weight for your gun’s power and how far it should reach with a good hop-up.
Open the calculator →UKARA Eligibility Checker
See when you can register for UKARA and buy a RIF, based on your game days so far.
Check your eligibility →Results are estimates for guidance. Always follow your site’s chrono and safety rules, and your gun maker’s advice.