In short
- FPS is speed, joules are energy. A speed only means something next to a BB weight.
- Great Britain: 1.3 J and 2.5 J. 1.3 joules for a gun that can fire automatically, 2.5 for any other: about 374 and 518 fps with a 0.20 g BB.
- Northern Ireland and Ireland: 1 joule. About 328 fps with a 0.20 g BB.
- 350 fps is a site rule, not the law. Sites set their own limits per kind of gun.
- Heavier BBs can raise the energy. That is joule creep. Test with the BB you actually play with.
FPS and joules in plain English
FPS stands for feet per second: how fast the BB leaves the barrel. A chronograph, chrono for short, measures it. Joules measure energy: how hard the BB lands. Energy depends on the speed and on the weight of the BB together.
Think of a table tennis ball and a golf ball thrown at the same speed: very different energy. That is why "350 fps" on its own says little, and why British sites add "with a 0.20 g BB".
Airsoft FPS calculator and chart per BB weight
Enter the speed from the chrono and the weight of the BB that was used, and the calculator gives the energy in joules.
Joule and FPS calculator
Enter the speed and the BB weight to see the energy. Below is the speed that matches the same energy with other BBs.
Energy
J
The same energy in FPS per BB weight
- 0,20 g
- 350
- 0,23 g
- 326
- 0,25 g
- 313
- 0,28 g
- 296
- 0,30 g
- 286
- 0,32 g
- 277
- 0,36 g
- 261
- 0,40 g
- 247
- 0,45 g
- 233
Calculated with E = ½ × m × v². A site measures with a chrono and sets its own limits; that reading decides.
Airsoft FPS chart: one energy, four BB weights
| FPS with 0.20 g | Joules | 0.25 g | 0.30 g | 0.40 g |
|---|---|---|---|---|
| 300 | 0.84 | 268 | 244 | 212 |
| 328 | 1.00 | 293 | 267 | 231 |
| 350 | 1.14 | 313 | 285 | 247 |
| 374 | 1.30 | 334 | 305 | 264 |
| 400 | 1.49 | 357 | 326 | 282 |
| 425 | 1.68 | 380 | 347 | 300 |
| 450 | 1.88 | 402 | 367 | 318 |
| 500 | 2.32 | 447 | 408 | 353 |
| 518 | 2.50 | 463 | 423 | 366 |
FPS to joules by hand
-
Turn FPS into metres per second
Multiply by 0.3048. A reading of 350 fps is 106.68 metres per second.
-
Square it
106.68 times 106.68 is about 11,381.
-
Multiply by the BB weight in kilograms
A 0.20 g BB is 0.0002 kg: 11,381 times 0.0002 is 2.28.
-
Halve it
Half of 2.28 is 1.14 joules.
The legal ceiling: 1.3 J automatic, 2.5 J in any other case
Three mix-ups about the law
Myth
350 fps is the legal limit.
Fact
It is a site rule. The law draws its lines in joules, at 1.3 and 2.5 in Great Britain.
Myth
A semi-automatic gun falls under 1.3 joules.
Fact
The 1.3 joule line is for guns that can fire automatically. Any other gun, semi-automatic included, falls under 2.5 joules.
Myth
Only the reading with 0.20 g counts.
Fact
The law asks what the gun can do with any BB. If a heavier BB lifts it over the line, it is over the line.
Typical UK site limits for AEG, DMR and sniper
No law tells a site which limit to use. Each site sets its own, usually per kind of gun, in FPS with a 0.20 g BB. The players' association UKAPU says it too: the legal figures "do not represent limits that are in place" at sites. The figures below are ones you will often see, not a standard.
Site limits per kind of gun
Electric rifle (AEG)
The standard class, for an AEG (a battery-powered gun) and anything else that can fire automatically. A figure you often see is 350 fps with a 0.20 g BB: 1.14 joules, under the legal 1.3.
DMR
A designated marksman rifle: a rifle locked to single shots and allowed more power. Around 425 fps with 0.20 g is often seen: 1.68 joules. At that energy the gun must not be able to fire automatically, or it is over the legal 1.3 joule line.
Bolt-action sniper
One shot per pull of the bolt. Around 500 fps with 0.20 g is often the top figure: 2.32 joules, under the legal 2.5. It comes with a minimum engagement distance.
Indoors (CQB)
Sites for close-quarters play inside buildings (CQB) often set one lower limit for every gun, and many allow single shots only. Ask before you travel.
Minimum engagement distances
How a minimum engagement distance (MED) works
-
What it is. The shortest distance from which a gun in a higher power class may be fired at another player.
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Who sets it. The site, per class. The figure is in its rules and in the safety brief.
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Inside that distance. You change to a pistol or another gun in the standard class; many sites require you to carry one.
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Judging it. Nobody measures it for you in a game, so pace it out once beforehand.
Joule creep with heavy BBs and HPA
Joule creep means that the same gun gives more energy with a heavier BB. A heavy BB moves down the barrel more slowly, so the air or gas behind it pushes for longer. Gas guns and HPA guns, which run on compressed air from a tank, do this most.
The trap: a gun passes the chrono with 0.20 g and is then played with heavier BBs at a higher energy. That is why sites may test with the BB you actually use.
BBs in the webshop
The weight is printed on every bag or bottle.
In stock, ships right away
BB's Xtreme Precision Bio 0.25 Earth
In stock, ships right away
0.30g Bio BB High Performance 3335rds — White
In stock, ships right away
0.20g Bio BB High Performance 5000rds — White
In stock, ships right away
0.32g Bio BB High Performance 3125rds — White
Before you walk up to the chrono
0 of 4 ticked
Within the line or not?
Four cases to work out with the calculator above.
-
1. A rifle that can fire automatically reads 350 fps with a 0.20 g BB. Where does it stand in Great Britain?
Correct. Not quite.
It is 1.14 joules, under the 1.3 for a gun that can fire automatically. Your site may still set a lower figure.
- AUnder the legal line: about 1.14 joules
- BOver the legal line
Show the answer
A. Under the legal line: about 1.14 joules
It is 1.14 joules, under the 1.3 for a gun that can fire automatically. Your site may still set a lower figure.
-
2. A bolt-action rifle reads 430 fps with a 0.30 g BB. Which side of 2.5 joules is it?
Correct. Not quite.
The 518 fps figure only holds for a 0.20 g BB. A gun that can do 2.58 joules is a firearm in law in Great Britain.
- AUnder: 430 is less than 518
- BOver: about 2.58 joules
Show the answer
B. Over: about 2.58 joules
The 518 fps figure only holds for a 0.20 g BB. A gun that can do 2.58 joules is a firearm in law in Great Britain.
-
3. A gas rifle that can fire automatically reads 345 fps with 0.20 g. With 0.32 g BBs the same gun reads 300 fps. What happened?
Correct. Not quite.
This is joule creep: slower but heavier, and over the 1.3 joule line.
- AThe energy fell with the speed
- BThe energy rose from about 1.11 to about 1.34 joules
Show the answer
B. The energy rose from about 1.11 to about 1.34 joules
This is joule creep: slower but heavier, and over the 1.3 joule line.
-
4. You live in Northern Ireland and your gun reads 340 fps with 0.20 g. Is that within the line there?
Correct. Not quite.
The 1.3 and 2.5 joule lines are for Great Britain only. In Northern Ireland the line is 1 joule.
- AYes, it is under 1.3 joules
- BNo: it is about 1.07 joules, and the line there is 1 joule
Show the answer
B. No: it is about 1.07 joules, and the line there is 1 joule
The 1.3 and 2.5 joule lines are for Great Britain only. In Northern Ireland the line is 1 joule.
Limits in Ireland and Europe at a glance
The energy line in the law of four other countries
| Country | Line in the law | Where it says so |
|---|---|---|
| France | Below 2 joules | Code de la sécurité intérieure, article R311-1 |
| Germany | 0.5 joule without a mark; up to 7.5 joules with the F mark, and then no automatic fire | Waffengesetz, Anlage 2 |
| Netherlands | 3.5 joules | Regeling wapens en munitie, article 1 |
| Italy | 1 joule, measured one metre from the muzzle | Legge 110/1975, article 5 |
Used airsoft guns on the marketplace
Ask the seller for the chrono reading and the BB weight it was taken with.
Frequently asked questions
How do I convert FPS to joules?
Multiply the FPS by 0.3048 to get metres per second, square that, multiply by the BB weight in kilograms and halve the result. For 350 fps with a 0.20 g BB that gives 1.14 joules. The calculator on this page does it for you.
What is the FPS limit for airsoft in the UK?
The law sets none in FPS: it works in joules. In Great Britain the lines are 1.3 joules for a gun that can fire automatically, about 374 fps with a 0.20 g BB, and 2.5 joules for any other gun, about 518 fps. In Northern Ireland the line is 1 joule.
Why do UK sites chrono on 0.20 g BBs?
Because a speed means nothing without a weight, and one reference weight lets a site put a single number on the board. A test with 0.20 g alone can miss joule creep, so sites may also test with the BBs you play with.
What is joule creep?
The same gun giving more energy with a heavier BB. The heavy BB stays in the barrel longer, so the air or gas pushes it for longer. It shows most in gas and HPA guns.
What are the usual limits for DMRs and snipers?
There is no national standard. Figures often seen on British site boards are around 425 fps for a DMR, a rifle locked to single shots, and around 500 fps for a bolt-action sniper rifle, both with a 0.20 g BB: 1.68 and 2.32 joules. Your own site's figure is the one that counts.
Is 350 fps the legal limit?
No. It is a common site figure for guns that can fire automatically: about 1.14 joules with a 0.20 g BB. The law in Great Britain draws its lines at 1.3 and 2.5 joules.
Sources
- Firearms Act 1968, section 57A (airsoft guns) legislation.gov.uk, 12 November 2026
- Firearms (Northern Ireland) Order 2004, Schedule 1 legislation.gov.uk, 12 November 2026
- Criminal Justice Act 2006, section 26 (amending the Firearms Act 1925) Irish Statute Book, 12 November 2026
- What are the limits on muzzle energy? UKAPU, United Kingdom Airsoft Players Union, 12 November 2026
5 more sources
- Site and game rules Gunman Airsoft, 12 November 2026
- Code de la sécurité intérieure, article R311-1 Légifrance, 12 November 2026
- Waffengesetz, Anlage 2 Gesetze im Internet, 12 November 2026
- Regeling wapens en munitie wetten.overheid.nl, 12 November 2026
- Legge 18 aprile 1975, n. 110, article 5 Normattiva, 12 November 2026
