What this calculator does
Muzzle energy is the kinetic energy a projectile carries the instant it leaves the barrel, and it is the standard way ammunition and cartridges are compared for stopping power or hunting suitability. This muzzle energy calculator takes a projectile weight and muzzle velocity and returns the energy in both foot-pounds and joules, the two units most commonly quoted on ammunition boxes and ballistics tables.
The formula is the same kinetic energy equation used throughout physics, E = 1/2 x mass x velocity squared, with the projectile’s weight and speed as the only two variables. Because velocity is squared, a modest increase in muzzle velocity increases energy far more than the same percentage increase in weight, which is why lighter, faster projectiles can carry as much or more energy than heavier, slower ones.
The formula
The projectile weight is converted to kilograms (grains x 0.00006479891, or grams / 1000) and the velocity to metres per second (feet per second x 0.3048). Kinetic energy in joules is then 1/2 x mass x velocity squared, and the foot-pound figure is the joule figure multiplied by 0.737562149.
| Term | Meaning |
|---|---|
| Grain | A traditional unit of mass used for bullet and arrow weights: 7,000 grains to the pound. |
| Muzzle velocity | The speed of the projectile the instant it leaves the barrel, before air resistance begins slowing it. |
| Foot-pound (ft-lb) | A unit of energy commonly used for ammunition in countries that still quote velocity in feet per second. |
The inputs explained
| Field | What to enter |
|---|---|
| Projectile weight | The projectile’s weight, as printed on the ammunition box or measured directly. |
| Weight unit | Whether the weight above is in grains (the usual unit for bullets) or grams. |
| Velocity unit | Whether the velocity above is in feet per second or metres per second. |
| Muzzle velocity | The muzzle velocity, usually printed on ammunition packaging or measured with a chronograph. |
When to use it
Comparing two cartridges or loads
Ammunition boxes quote weight and velocity separately, which makes them hard to compare directly. Converting both to a single muzzle energy figure gives a fair, apples-to-apples comparison.
Checking a chronographed load against the box figure
A chronograph measures the actual velocity a particular firearm and load produces, which can differ from the advertised figure. Recalculating the energy from the measured velocity shows what that specific combination is actually delivering.
Working out energy for hunting or reloading purposes
Many hunting regulations and guidelines reference a minimum muzzle energy for a given game species, so knowing the figure for a chosen load matters before deciding whether it is suitable.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does muzzle energy change with velocity, at a fixed 150 grain weight?
A 150 grain projectile at a range of muzzle velocities.
| Muzzle velocity | Muzzle energy | Muzzle energy (joules) |
|---|---|---|
| 2000 fps | 1,332 ft-lbs | 1,806 J |
| 2400 fps | 1,918 ft-lbs | 2,601 J |
| 2800 fps | 2,611 ft-lbs | 3,540 J |
| 3200 fps | 3,410 ft-lbs | 4,623 J |
| 3600 fps | 4,316 ft-lbs | 5,851 J |
Questions
Why does energy increase so much faster than velocity?
The kinetic energy formula squares the velocity term, so a given percentage increase in speed produces roughly double that percentage increase in energy. A 20% faster projectile carries about 44% more energy at the same weight.
Is muzzle energy the same as stopping power?
No single number fully captures stopping power, which also depends on projectile design, expansion and penetration. Muzzle energy is one widely used input into that broader picture, not a complete measure on its own.
Why do ammunition boxes quote foot-pounds in some countries and joules in others?
It follows the same convention as the velocity units used locally: countries that measure velocity in feet per second typically quote energy in foot-pounds, while those using metres per second typically quote joules.
Does this account for energy lost to air resistance downrange?
No. This calculates energy at the muzzle only, the instant the projectile leaves the barrel. Energy decreases with distance as air resistance slows the projectile, and that drop-off depends on the specific projectile’s shape and ballistic coefficient.
For the equivalent kinetic energy calculation applied to arrows rather than bullets, see the arrow kinetic energy calculator.