Kinetic energy calculator
Half the mass times the speed squared. The square is what makes this worth a page: get the speed unit wrong by a factor of 3.6 and the energy is wrong by 12.96, which is nearly thirteenfold and looks perfectly plausible. Nothing you type here leaves your browser — it is worked out on your own device, so there is nothing to send and nothing to log.
The unit error gets squared too
½mv² wants the speed in metres per second. 108 km/h is 30 m/s — a factor of 3.6. But the formula squares the speed, so it squares the mistake:
| 1500 kg at 30 m/s | 675 kJ |
|---|---|
| 1500 kg at 108 km/h | 675 kJ — the same speed |
| 108 put straight into ½mv² | 8,748 kJ |
Out by 12.96, which is 3.6 squared. Neither number looks obviously wrong: both are in the range you would expect for a car, and the expression is identical. This page converts first and tells you what the unconverted version would have given.
And in metres per second there is nothing to get wrong. Every worked example in every textbook uses m/s, because that is the unit the formula is written for — which is exactly why the habit never gets tested.
Twice the speed is four times the energy
This is the part that matters outside a physics class. Energy grows as the square of speed, so:
| 20 mph to 30 mph | 2.25 times the energy |
|---|---|
| 30 mph to 60 mph | 4 times the energy |
| 60 mph to 70 mph | 1.36 times the energy |
A 20 mph limit is not a third gentler than 30; it carries 44% of the energy. Braking distance follows the same square, which is why the stopping distances in the Highway Code are not evenly spaced and why the gap between 60 and 70 costs more road than it looks.
What this is not
This is the energy of the whole object moving in one direction. It is not the energy in its heat, which is the same molecules moving in every direction at once, and it is not the energy you would need to accelerate it — that depends on how much you lose to drag and friction on the way, and both of those depend on things this page does not ask about.
At speeds approaching light, ½mv² is wrong for a different reason and relativity takes over. The crossover is not sharp; the formula is out by about 1% at 10% of light speed, which is 30,000 km/s, and this page will happily accept a larger number without warning you because nothing you are likely to be weighing goes that fast.