Kinetic Energy Calculator

Calculate the kinetic energy and momentum of a moving object from its mass and speed using KE = 1/2 x m x v squared.

This kinetic energy calculator is for physics and engineering students, teachers, and anyone who needs the energy or momentum of a moving object. It is useful for homework on mechanics, for rough estimates of impact energy in safety and design work, and for getting an intuitive feel for how energy grows with speed. Enter a mass and a speed and it returns the kinetic energy in joules and kilojoules, plus the momentum, so you can compare how hard something is to stop with how much energy it carries.

Kinetic energy is the energy an object has because it is moving. The formula is KE = one half times mass times speed squared, written KE = 1/2 x m x v squared, with mass in kilograms and speed in metres per second giving energy in joules. The squared term is the important part: doubling the speed quadruples the energy. The calculator multiplies half the mass by the square of the speed for the energy, divides by a thousand to also show kilojoules, and separately multiplies mass by speed to give momentum in kilogram-metres per second. Momentum grows in step with speed, while energy grows with its square, which is why high speed is so dangerous.

Picture a 1,000 kilogram car moving at 20 metres per second, roughly 72 kilometres per hour. Half of 1,000 is 500, and 20 squared is 400, so the kinetic energy is 500 times 400, which is 200,000 joules, or 200 kilojoules. The momentum is 1,000 times 20, or 20,000 kilogram-metres per second. Now double the speed to 40 metres per second: the momentum doubles to 40,000, but the energy quadruples to 800 kilojoules. That fourfold jump in energy is why stopping distances and crash forces rise so sharply with speed.

Physics does not change from place to place, so the result is the same anywhere. The only regional difference is units. This calculator uses SI units - kilograms, metres per second, and joules - which are standard in science worldwide. If you are used to miles per hour, pounds, or foot-pounds, convert to kilograms and metres per second first. As a guide, one metre per second is about 3.6 kilometres per hour or 2.24 miles per hour, and one kilogram is about 2.2 pounds.

• Forgetting to square the speed. The speed term is squared, so leaving it out underestimates the energy badly.
• Using the wrong units. Mass must be in kilograms and speed in metres per second to get joules; mixing in grams or kilometres per hour gives a wrong answer.
• Confusing energy with momentum. Momentum rises with speed, but energy rises with the square of speed, so they are not interchangeable.
• Applying this near the speed of light, where the classical formula breaks down and a relativistic version is needed.
• Treating kinetic energy as a vector. Energy is a single number with no direction, unlike momentum and velocity.

There is no deadline in mechanics, but there is a safety lesson worth keeping in mind: because energy grows with the square of speed, modest increases in speed produce large increases in impact energy and braking distance. In design and safety work, that square relationship is the reason speed limits and crumple zones matter so much. When estimating impact energy, use realistic masses and speeds, and remember the calculated figure is the energy that must be absorbed somewhere when the object stops.

• Kinetic energy - Reference (retrieved 2026-06-11)