Air rifle muzzle energy calculator: UK 12 ft lb check

The UK air weapon limits

An air rifle that can produce more than 12 ft lb of muzzle energy, or an air pistol over 6 ft lb, is classed as "specially dangerous" under the Firearms (Dangerous Air Weapons) Rules 1969. Owning one needs a firearm certificate from the police. Below those limits, an air rifle needs no certificate in England and Wales. Scotland has required an air weapon certificate since the Air Weapons and Licensing (Scotland) Act 2015, and Northern Ireland has its own licensing rules, so check with Police Scotland or the PSNI.

How this is calculated

Pellet mass: grains × 0.0648 = grams.
Speed: feet per second × 0.3048 = metres per second.
Energy in joules = ½ × mass in kg × speed in m/s × speed in m/s.
Foot-pounds = joules ÷ 1.3558.

The calculator uses the exact conversion factors, so it matches the "grains × fps² ÷ 450,240" shortcut on airgun forums to within a fraction of a percent.

Worked example

An 8.44 grain .177 pellet leaves the barrel at 790 fps.

Mass = 8.44 × 0.0648 = 0.547g. Speed = 790 × 0.3048 = 240.8 m/s. Energy = 0.5 × 0.000547 × 240.8² = 15.9 joules, or 11.7 ft lb. That's under the 12 ft lb limit, with 0.3 ft lb to spare. At 820 fps the same pellet would carry 12.6 ft lb, over the limit.

Reading your chronograph

Shot-to-shot speeds vary, especially on springers. Take a string of shots and look at the highest reading, not the average, because the law is about what the gun can do. A rifle that reads close to 12 ft lb with one pellet can go over with another, so test each pellet you use. If a gun reads over the limit, stop using it and get it checked by a registered firearms dealer.

This page explains the numbers. It is not legal advice. If you are unsure whether a gun is legal, ask your police firearms licensing team.

Questions people ask

What is the legal limit for an air rifle in the UK?

12 foot-pounds (ft lb) of muzzle energy for an air rifle and 6 ft lb for an air pistol. Under the Firearms (Dangerous Air Weapons) Rules 1969, an air weapon that can go over these limits is "specially dangerous" and needs a firearm certificate.

How do I work out muzzle energy?

Muzzle energy is half the pellet's mass times its speed squared. You need the pellet weight (printed on the tin, usually in grains) and the speed from a chronograph. An 8.44 grain pellet at 790 feet per second carries about 11.7 ft lb.

Does a heavier pellet change the power?

It can. The same rifle often gives a different energy with different pellets, so a gun that is legal with one pellet may go over 12 ft lb with another. Chrono your rifle with each pellet you use.

Do I need a licence for an air rifle in Scotland?

Yes. Under the Air Weapons and Licensing (Scotland) Act 2015 you need an air weapon certificate for an air rifle up to 12 ft lb or an air pistol up to 6 ft lb. Air weapons at or below 1 joule (about 0.74 ft lb) are exempt.