Airsoft FPS Calculator

Many airsoft players view FPS as important. Feet Per Second  measures the muzzle velocity, or the speed your BBs travel when you fire your airsoft gun. A decent rate of fire can make the difference between victory and defeat, so it’s essential to know what exactly how fast you’re firing.

To get the highest FPS for their airsoft guns, some players stick to the lighter BBs. Lighter BBs means a higher FPS. Heavier BBs require more energy to travel through the air, so their FPS is lower.

Countries that use the Metric System measure muzzle velocity in Metres Per Second (MPS). You may find MPS on imported airsoft guns. As a general rule, 1FPS = 0.3048MPS.

Another useful measurement for airsoft gun firing quality is the kinetic energy transferred to the target. Basically this means how hard you hit. In airsoft, you aren’t actually trying to hurt anyone, but the impact needs to be strong enough for the target to register the hit regardless of what they’re wearing.

The factors that determine what your FPS will be include the weight of your BBs in combination with your airsoft gun power and FPS limit on your make of gun. But FPS is only half of the story when it comes to the power of your airsoft gun.

Airsoft sites differ in what is an acceptable FPS and Site Limits should always remain under the legal recommendations. Fully automatic airsoft guns operating at 1.3 joules or less, and single shot (or semi automatic) airsoft guns operating at 2.5 joules or less do not engage the lethality threshold under the Firearms Act.

Joules is the important measure for airsofters. The energy expended upon firing your airsoft gun determines whether, in the eyes of the law, your airsoft gun is considered a dangerous firearm.

The Policing & Crime Act introduced an exception from the classification for airsoft guns as firearms, so long as they meet certain criteria.

Airsoft guns, regardless of how they are powered, are not considered firearms if they are designed to fire a spherical plastic projection that is no bigger than 6 mm in diameter and they have a muzzle energy that is no greater than 1.3 joules if they can fire successive shots or 2.5 joules if they can only fire individual shots. So, 1.3 joules for auto-fire or burst fire airsoft guns and 2.5 joules for single shot or semi-auto airsoft guns.

If you’re customising your airsoft gun or pick up heavier BBs, you need to make sure you’re still within the limits of acceptable FPS. FPS is relative to the BB weight you are using but the kinetic energy it produces is always the same.

If your airsoft gun is shooting 400 FPS with a .20g BB and you switch to a .45g BB, your gun will be shooting 270 FPS. There’s a big difference in FPS, but the kinetic energy it carries on flight will be the same, 1.52 Joules, regardless of what BB weight you use.

This is where it can get confusing. To calculate kinetic energy in Joules, you must use this formula:

J = 0.5 x m x v²

Joules (J) = 0.5 x the mass (m) of the bb (kg) x the speed squared (v²)

To help you figure out this confusing, but essential aspect of airsoft gun quality, we’ve created this handy chart. Just line up your current FPS with the BBs you plan to use.