Kinetic Energy Calculator

The formula

KE = ½ × mass (kg) × velocity² (m/s)², giving energy in joules. Kinetic energy grows with the square of velocity, so doubling speed quadruples the kinetic energy for the same mass.

Worked example

A 1,000 kg car travelling at 20 m/s (about 72 km/h) has a kinetic energy of ½ × 1,000 × 20² = ½ × 1,000 × 400 = 200,000 J (200 kJ).

Why speed matters more than weight for impact energy

Because velocity is squared in the formula, a relatively small increase in speed has a much bigger effect on kinetic energy than the same percentage increase in mass — this is why stopping distances and impact forces increase so sharply at higher speeds.

Inputs and output format

• Mass is in kilograms and velocity in meters per second. There is no unit selector, so convert km/h to m/s by dividing by 3.6.
• Energy is shown in joules with digit grouping and up to two decimals, and again in kilojoules to three decimals.
• Mass must be above zero. Velocity can be zero or negative, because squaring removes the sign: 75 kg at −3 m/s returns 337.5 J.
• A velocity of 0 is accepted and returns 0 J. An empty box gives a prompt to enter both values.

More worked examples

The same 1,000 kg car at 40 m/s has 800,000 J, four times the 200,000 J at 20 m/s, since doubling the speed quadruples the energy. A 0.145 kg ball thrown at 40 m/s carries ½ × 0.145 × 1,600 = 116 J.

A 1,500 kg car at 100 km/h: 100 ÷ 3.6 is 27.78 m/s, and the tool returns 578,796.3 J, or 578.796 kJ.

Limits

Very small energies round off: 0.001 kg at 1 m/s is 0.0005 J, which is displayed as 0 J and 0.000 kJ. The calculator uses the classical formula at any speed and does not limit velocity, so it is not meant for speeds near the speed of light. Rotational energy and potential energy are not included.