Muzzle energy is the number printed on ammunition boxes and used in sport shooting, hunting regulations and ballistics tables. It’s just kinetic energy, calculated from the weight and speed of the projectile.
Grains or grams, feet or metres per second: it takes either and answers in both joules and foot-pounds.
How to use the Bullet Energy Calculator#
- Enter the weight. Choose grains or grams and type the projectile weight.
- Enter the velocity. Choose feet or metres per second.
- Read the energy. The result shows joules and foot-pounds.
The formula#
A 150-grain bullet weighs 9.72 grams, or 0.00972 kg. At 2,800 feet per second, which is 853.4 metres per second, its energy is ½ × 0.00972 × 853.4² = about 3,540 joules, or around 2,610 foot-pounds. Doubling the speed quadruples the energy, while doubling the weight only doubles it.
Energy and momentum#
Energy and momentum are different measures. Energy depends on the square of velocity, and momentum depends on velocity alone. Hunters and sports shooters use both, along with bullet design, when they compare cartridges. Many hunting rules set a minimum energy for a species, so check your local regulations.
Units#
One joule equals 0.7376 foot-pounds, and one grain is 0.0648 grams. A power factor, which is weight in grains multiplied by velocity in feet per second and divided by 1,000, is used in some shooting sports to classify loads. This tool is for physics and sport calculations and doesn’t replace safe handling practices or local laws.
Two worked examples#
A 9 mm round with a 124-grain bullet leaves the barrel at 1,150 feet per second. In SI units the mass is 8.04 g, or 0.00804 kg, and the speed is 350.5 m/s. Kinetic energy is ½ × 0.00804 × 350.5² = 494 J, or 364 ft-lb. Momentum is 2.82 kg·m/s.
A 5.56 mm rifle round with a 55-grain bullet at 3,200 ft/s has a mass of 3.56 g and a speed of 975 m/s, giving 1,695 J (1,250 ft-lb). The rifle bullet weighs less than half as much but carries more than three times the energy, because energy rises with the square of speed.
Why speed counts for more than weight#
The formula KE = ½mv² means that doubling the mass doubles the energy, but doubling the speed quadruples it. Momentum, p = mv, behaves differently, doubling when either one doubles. This is why a heavy, slow bullet and a light, fast one can have similar momentum but very different energy. Two numbers are therefore needed to describe a load, and the calculator shows both.
| Change | Effect on energy | Effect on momentum |
|---|---|---|
| Mass × 2 | × 2 | × 2 |
| Speed × 2 | × 4 | × 2 |
| Speed × 1.1 | × 1.21 | × 1.1 |
| Mass × 0.5, speed × 1.41 | × 1 | × 0.71 |
Power factor and legal limits#
The calculator also gives the power factor used in practical shooting sports, which is the bullet weight in grains times the velocity in feet per second, divided by 1,000. In USPSA, handgun ammunition needs a power factor of at least 125 to score as minor and 165 for major. The 124-grain load above works out to about 142.6. Many countries set legal limits on air guns by energy. In the UK, an air rifle producing more than 12 ft-lb (16.3 J) needs a firearm certificate. These limits are written in energy because it is the measurable quantity. Always check the law and the rules of your range, and measure velocity with a chronograph.
Frequently asked questions#
Half the mass times the velocity squared. Use kilograms and metres per second for joules.
One joule is 0.7376 foot-pounds.
Because energy depends on the square of the velocity.
No. Stopping power depends on bullet design and many other factors, not just energy.
A grain is a unit of mass used for bullets, powder and arrows. One grain is 0.0648 grams, so a 124-grain bullet weighs about 8.04 grams. There are 7,000 grains in a pound. The unit comes from the weight of a grain of cereal, and it is still the standard in ballistics.
Yes. Air resistance slows the bullet, and energy falls with the square of the speed. A bullet that loses 5 percent of its speed has lost about 9.75 percent of its energy, since 0.95² = 0.9025. Heavier, streamlined bullets keep their speed better. The calculator gives muzzle values from the speed you enter, so use the speed at the distance you care about for an energy estimate there.