Kinetic Energy Calculator
Work out the kinetic energy of anything that moves. Enter mass (kg or lb) and speed (m/s, mph, or km/h) to get energy in joules and foot-pounds — with a sense of how much that actually is.
Example: with Mass 1500 · Mass unit kg (kilograms) · Speed 65 · Speed unit mph → Kinetic energy: 633.3 kJ (633,258 J).
- In foot-pounds467,067 ft-lb
- Comparable toA car crash — hundreds of times a rifle shot
Computed by the calculator below using its default values. Change any input to see your own numbers.
KE = ½mv². Doubling speed quadruples energy — that is why highway crashes are so much more destructive than city ones.
What kinetic energy tells you
Kinetic energy is the work an object can do by virtue of its motion — the energy invested to get it moving, and the energy released when it stops. The formula KE = ½mv² ties that energy to mass and speed, but the two do not count equally: energy grows in direct proportion to mass yet with the square of speed.
That squared speed term is why crash severity and stopping distance rise so steeply with velocity. A car at 60 mph carries four times the energy it had at 30 mph, and every joule must be absorbed by brakes, tires, or whatever it strikes.
Why speed beats mass
Because velocity is squared and mass is not, doubling speed quadruples energy while doubling mass merely doubles it. A light, fast object can out-energize a heavy, slow one — a few grams of bullet carry more energy than a gently tossed brick.
Engineers exploit this directly. Crumple zones, run-off areas, and airbags all reduce harm by spreading the same kinetic energy over more distance and time, which lowers the peak force felt at any instant.
How it’s calculated
KE = ½ × m × v², with mass in kilograms and speed in meters per second (SI). Pounds are converted at 1 lb = 0.45359237 kg; 1 mph = 0.44704 m/s; 1 km/h = 1/3.6 m/s. Result in joules; foot-pounds at 1 J = 0.73756 ft-lb.
Point-mass translation only — rotation, deformation, and drag are ignored. Real crash energy dissipates over distance and time, which is what crumple zones exploit.
Kinetic energy of familiar things
| Object | Speed | Kinetic energy |
|---|---|---|
| Baseball fastball | 95 mph | ≈ 120 J |
| 9mm bullet | 1,200 ft/s | ≈ 500 J |
| Compact car (1,300 kg) | 30 mph | ≈ 117 kJ |
| Semi truck (36,000 kg) | 65 mph | ≈ 15 MJ |
Computed with KE = ½mv² from typical published masses and speeds; rounded for comparison.
Common mistakes
- Mixing units — 65 in mph is 29 m/s; forgetting the conversion inflates energy 5×.
- Doubling speed and expecting double energy: it is 4× (the v is squared).
- Using weight in pounds-force as mass without converting.
Frequently asked questions
What is the kinetic energy formula?
KE = ½mv²: half the mass (kg) times velocity (m/s) squared. The result is in joules — one joule is the energy to lift a small apple one meter.
Why does speed matter more than mass?
Velocity is squared and mass is not. A car at 60 mph carries 4× the energy it has at 30 mph, but a car twice as heavy at the same speed carries only 2×.
How do I convert joules to foot-pounds?
Multiply joules by 0.73756. One foot-pound is the energy to lift one pound one foot; one joule is about three-quarters of that.
Does this work for rotating objects?
No — spinning things also carry rotational energy (½Iω²). This tool covers straight-line motion only, which dominates for vehicles, projectiles, and falling objects.