Car Crash Calculator
This car crash calculator estimates the forces at work when a vehicle hits something and stops abruptly. When a car crashes it goes from its travelling speed to a standstill over a short distance, the crumple of the bodywork and the barrier, and the shorter that distance the harder the stop. You enter the mass of the vehicle, the speed it was travelling and the distance over which it comes to rest, and the calculator works out the average deceleration, then the average force from mass times that deceleration. It reports the force in kilonewtons, the peak deceleration expressed as a multiple of gravity, the impact speed converted to metres per second, and the kinetic energy that has to be absorbed in the collision. The physics is the standard work-energy and constant-deceleration model taught in first-year mechanics, which treats the stop as steady over the crumple distance to give a clear average rather than the jagged real-world peak. It helps explain why crumple zones, longer stopping distances and lower speeds make crashes so much more survivable, since the force falls as the stopping distance grows and rises steeply with speed. Use it for study, road-safety demonstrations or curiosity. It is a simplified estimate of average forces, not a substitute for crash engineering or accident reconstruction, which model the changing force through the impact in far more detail.
The stopping distance is how far the car travels while coming to rest, roughly the depth of the crumple plus any give in the object struck. A head-on into a rigid wall is around 0.4 to 0.6 m; a collision with another car or a barrier is usually more.
This is the average force over the crumple distance using a constant-deceleration model. Real crashes have a changing force that peaks higher. Estimate for education only.
How it works
First the impact speed is converted from kilometres per hour to metres per second by dividing by 3.6. The average deceleration is that speed squared divided by twice the stopping distance, which comes straight from the constant-acceleration equations of motion. The average force is then the vehicle mass multiplied by the deceleration, following Newton's second law, and it is shown in kilonewtons. Dividing the deceleration by the acceleration of gravity, 9.81 metres per second squared, expresses it as a number of g. The kinetic energy that must be absorbed is one half of the mass times the speed squared, and it is this energy that the crumple zone turns into deformation and heat as the car stops.
Worked example
A 1,500 kg car hits a solid wall at 50 km/h and crumples to a stop over 0.5 m. Fifty kilometres per hour is 13.9 m/s. The deceleration is 13.9 squared divided by two times 0.5, which is about 193 m/s squared. The average force is 1,500 kg times that, which is roughly 289,000 newtons, or 289 kN. Dividing 193 by 9.81 gives about 19.7 g. The kinetic energy the car carries into the crash is one half times 1,500 times 13.9 squared, which is about 144.7 kilojoules, all of which the crumple zone has to absorb in a fraction of a second.
Related calculators
- G-Force Calculator: g-force from speed and radius.
- Kinetic Energy Calculator: energy of a moving object.
- Momentum Calculator: mass times velocity.
- Acceleration Calculator: change in speed over time.
- Drag Force Calculator: aerodynamic drag.