Kinetic Energy Calculator

Frequently asked questions

What is the formula for kinetic energy?

KE = ½mv², with m in kilograms and v in meters per second, gives joules. For 10 kg at 20 m/s: ½ × 10 × 20² = 2,000 J, which the calculator shows as 2 kJ.

What happens to kinetic energy when speed doubles?

It goes up four times, because speed is squared. 1 kg at 2 m/s has 2 J and at 4 m/s has 8 J; a 1,500 kg car has 134.9 kJ at 30 mph and 539.6 kJ at 60 mph.

How do I find kinetic energy with pounds and mph?

Pick lb and mph next to the fields. The calculator converts with the exact definitions 1 lb = 0.45359237 kg and 1 mph = 0.44704 m/s. 0.145 kg at 100 mph is 144.9 J, and 12 lb at 30 km/h is 189 J. Ounces, US short tons (2,000 lb), ft/s and knots work the same way: a 1 ton load at 60 mph carries 326.3 kJ, and 1 tonne at 10 knots 13.23 kJ.

Why does it show mJ, kJ or MJ instead of joules?

The headline uses the prefix that puts the number between 1 and 1,000, with 4 significant digits, and the line below gives plain joules to 6 significant digits. 1 g at 1 m/s reads 500 µJ, which is 0.0005 J. The value is rounded to 4 digits before the prefix is picked, so 999.95 J reads 1 kJ rather than 1000 J. A row under the result repeats the energy in Wh, kWh, kcal, BTU and ft-lbf, also to 4 significant digits: the 1,500 kg car at 30 m/s is 187.5 Wh, 639.8 BTU and 497,900 ft-lbf.

Can velocity be negative?

Yes, but it gives the same energy as the positive speed, since only v² counts: 5 kg at -3 m/s is 22.5 J. Mass must be 0 or more.

How do I find mass or velocity from kinetic energy?

Pick Mass or Velocity at the top, then enter the energy in J, kJ or MJ and the other quantity. Mass is m = 2 × KE ÷ v², so 2,000 J at 20 m/s is 10 kg. Velocity is v = √(2 × KE ÷ m), so 2,000 J in 10 kg is 20 m/s, and 675 kJ in 1,500 kg is 30 m/s (67.1081 mph). Velocity must not be 0 when solving for mass, and mass must be above 0 when solving for velocity.