Airsoft FPS to joules calculator
Work out the muzzle energy of your airsoft gun from its FPS and BB weight, or the FPS that a joule limit allows.
FPS limits by BB weight
| BB weight | 1.14 J | 1.30 J | 1.88 J | 2.32 J | 2.50 J |
|---|---|---|---|---|---|
| 0.20 g | 350 | 374 | 449 | 499 | 518 |
| 0.23 g | 326 | 348 | 419 | 465 | 483 |
| 0.25 g | 313 | 334 | 402 | 446 | 463 |
| 0.28 g | 296 | 316 | 380 | 422 | 438 |
| 0.30 g | 286 | 305 | 367 | 408 | 423 |
| 0.32 g | 276 | 295 | 355 | 395 | 410 |
| 0.36 g | 261 | 278 | 335 | 372 | 386 |
| 0.40 g | 247 | 264 | 318 | 353 | 366 |
| 0.43 g | 238 | 255 | 306 | 340 | 353 |
| 0.45 g | 233 | 249 | 299 | 333 | 345 |
| 0.48 g | 226 | 241 | 290 | 322 | 334 |
What the numbers mean
FPS is how fast a BB leaves the barrel, in feet per second. Joules measure the energy it carries, and that depends on weight as well as speed: half the BB's mass times its speed squared, with the speed in metres per second. A foot is exactly 0.3048 metres.
The same speed carries more energy in a heavier BB. A 0.20 g BB at 350 fps carries 1.14 J (1.139 J, rounded up to three decimal places), and a 0.40 g BB at 350 fps carries twice that. To stay at 1.14 J, the 0.40 g BB has to leave the barrel at 247 fps.
The calculator rounds in one direction each way: joules up and fps down. A result never reads lower in energy than the gun delivers. That is why 450 fps on a 0.20 g BB (1.882 J) is not in the 1.88 J column of the chart, which stops at 449 fps.
Why sites measure in joules or at a set BB weight
A chronograph measures speed, so it can't tell you the energy on its own. You need the BB weight to compare one gun with another. A limit in fps only means something next to a BB weight, and a limit in joules needs a conversion before you can read it off a chrono result.
Ask which weight your site chronos with, and use the calculator to convert your own reading: enter the fps your chrono showed and the weight of the BB you fired.
Site limits are site rules
Each site sets its own limits. They are rules of that site, not law, and a site can set them below what the law allows. Limits differ by gun type.
Many UK sites cap full-auto guns at 1.14 J, which is 350 fps on a 0.20 g BB. Semi-auto-locked DMRs are often allowed about 1.88 J (450 fps), with a minimum engagement distance. Bolt-action rifles are often allowed 2.32 J (500 fps), also with a minimum engagement distance.
Figures vary from site to site. One site we checked sets DMRs at 1.67 J, so read your site's own rules before you play, and ask the marshals if anything is unclear.
On the chart, 1.14 J, 1.88 J and 2.32 J are common site limits. The 1.3 J and 2.5 J columns are the legal thresholds described below.
What the law says in Great Britain
Section 57A of the Firearms Act 1968 says an airsoft gun is not a firearm for the purposes of the Act. The section covers England, Wales and Scotland. Northern Ireland is not covered, and this page doesn't cover its law.
The definition
An airsoft gun is a barrelled weapon designed to discharge only a small plastic missile: made wholly or partly of plastic, spherical, and no more than 8 mm across. It must also not be capable of discharging a missile with muzzle energy that exceeds the permitted level.
The permitted level is 1.3 J for a weapon capable of discharging two or more missiles in succession without repeated pressure on the trigger. In any other case it is 2.5 J. A gun at exactly 1.30 J or 2.50 J is within the level, because the test is whether energy exceeds it.
Read the source
This is a summary, not legal advice. Read section 57A on legislation.gov.uk for the exact wording.
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