Kinetic Energy Calculator
Calculate the kinetic energy of a moving mass — ½mv² — in joules, kilojoules, kilowatt-hours and food calories.
The energy in a moving object; it grows with the square of the speed.
How the kinetic energy calculator works
Kinetic energy is ½ × mass × velocity². The square matters: doubling speed quadruples energy, which is why stopping distances balloon with speed. Enter the mass in kilograms and the speed in km/h (or m/s, mph in Detailed mode); the result is in joules, with kilojoules, kilowatt-hours and kilocalories alongside for scale.
A 1.5-tonne car at 100 km/h carries 579 kJ — roughly the energy in 140 food calories, or 0.16 kWh. All of it has to go somewhere when the car stops.
Formula: KE = ½ m v²
Worked examples
| Inputs | Kinetic energy | Note |
|---|---|---|
| 1,500 kg car at 100 km/h | 578,704 J | 579 kJ |
| 1,500 kg car at 50 km/h | 144,676 J | a quarter of the energy |
| 80 kg runner at 15 km/h | 694 J | 694 J |
FAQFrequently asked questions
Why does speed matter more than mass?
Energy scales with the square of speed but only linearly with mass. A 10% faster car has 21% more energy to dissipate in a crash.
How does kinetic energy relate to stopping distance?
Brakes remove energy at a roughly constant force, so distance is proportional to energy — and therefore to speed squared. Double the speed, four times the braking distance.
What is a joule?
The work done by a force of one newton over one metre. A kilowatt-hour is 3.6 million joules; a food calorie (kcal) is 4,184.
Does rotation count?
Rotating parts have their own kinetic energy (½Iω²). This calculator covers straight-line motion only.
Where these figures come from
- NIST — CODATA 2018 fundamental physical constants — G, g₀, R, c
- NIST Special Publication 811 — Guide for the use of the International System of Units — unit conversions
- The Engineering ToolBox — material properties — specific heats, expansion coefficients, densities
- National Physical Laboratory — the UK's national measurement institute
Last checked: September 2026. Constants are the CODATA 2018 values; formulas are the standard textbook forms.