Crash Impact Force Calculator
The average force and g-load in a crash from the speed, the mass of the vehicle or occupant, and the distance over which it stops — and the fall height it is equivalent to.
A moving object’s kinetic energy is ½mv², and stopping it over a distance d takes an average force of that energy divided by d.
How the crash impact force calculator works
A moving object’s kinetic energy is ½mv², and stopping it over a distance d takes an average force of that energy divided by d. The shorter the stop, the larger the force: a car’s crumple zone stretches the stop to half a metre or more, a seat belt lets the occupant ride it down, and hitting a wall unbelted stops you in a few centimetres. The same speed is also the speed you would reach falling from v² ÷ 2g.
Formula: F = m v² ÷ (2d); g-force = F ÷ (m g); equivalent fall height = v² ÷ (2g)
Worked examples
| Inputs | Average impact force | Note |
|---|---|---|
| 75 kg occupant at 50 km/h, belted, 0.6 m | 12.1 kN | 12.1 kN, 16 g — a fall of 9.8 m |
| Same crash unbelted, 5 cm | 144.7 kN | 145 kN, 197 g |
| A 1,500 kg car at 30 mph into a wall, 0.5 m crumple | 269.8 kN | 270 kN |
FAQFrequently asked questions
Why does the stopping distance matter so much?
The energy has to go somewhere, and force is energy divided by distance. Crumple zones, airbags and seat belts all work by making the stop longer — a belt that lets you move 30 cm cuts the force to a tenth of a 3 cm stop against the dashboard.
What g-force can a person survive?
Restrained and well supported, brief peaks of 40–60 g have been survived; sustained loads above about 20 g cause injury. Unrestrained, the head hits something at the full impact speed and the limit is far lower.
Is this the peak force?
No — the average over the stop. Real crashes have peaks two or three times the average, which is why the reads-as row is conservative.
Why is a 50 km/h crash like a fall from 10 metres?
Because a body falling 9.8 m reaches 50 km/h. The fall comparison is the fastest way to feel what a speed means: 100 km/h is a fall from a 12-storey building.
Where these figures come from
- ISO 4000 / ETRTO — tyre designation and dimensions — the width / aspect ratio / rim marking this page decodes
- UK Vehicle Certification Agency — the official UK fuel and emissions data
Last checked: September 2026. Tyre dimensions follow the ISO metric marking standard; engine formulas are the standard textbook forms.