Car Crash Calculator
Enter your vehicle speed and your body weight to see the average impact force, the G-force, the deceleration, and the stopping time during a collision. Toggle the seatbelt to compare the dramatic difference it makes. Switch between metric and imperial, and choose between a stopping-distance or stopping-time calculation. All results update instantly as you type.
How to calculate car crash impact force
The physics comes from the law of conservation of energy. Before impact, the passenger has kinetic energy equal to half the mass times the velocity squared: KE = 0.5 x m x v^2. During the collision, this energy is absorbed over a short stopping distance (or stopping time). The average impact force using stopping distance is F = m x v^2 / (2 x d), and using stopping time it is F = m x v / t. Both equations show the same principle: the longer the stopping distance or stopping time, the lower the average force. This is why crumple zones, seatbelts, and airbags save lives - they increase d and t, spreading the same energy absorption over more space and time.
Seatbelt and airbag effect on impact force
Without a seatbelt the effective stopping distance of the human body is roughly 4 cm - the distance from the resting position to the steering wheel or dashboard. With a seatbelt the belt itself stretches and the airbag adds cushioning, extending the stopping distance to about 20 cm. Plugging those numbers into F = m x v^2 / (2 x d) at 30 km/h gives roughly 60 kN (89 g) without a seatbelt versus about 12 kN (18 g) with one: a 5-fold reduction. According to NHTSA data, seatbelts reduce crash fatality rates by 45% and the risk of serious injury by 50%.
G-force and what it means
G-force is the deceleration expressed as a multiple of gravitational acceleration (9.80665 m/s^2). One g is what you feel standing still - gravity pulling you down. During a moderate hard-braking event a driver might experience 0.5 to 1 g. Rollercoasters hit 3 to 5 g. Fighter pilots in tight turns can sustain 7 to 9 g with a pressure suit. In a crash at highway speed without a seatbelt, instantaneous G-force can exceed 100 g, which is why the loads are almost always injurious. The NHTSA Federal Motor Vehicle Safety Standard 208 sets a maximum chest deceleration of 60 g for durations longer than 3 milliseconds as a pass/fail test threshold for new vehicles.
Speed, kinetic energy and crash severity
Kinetic energy grows with the square of speed, not linearly. Doubling the speed from 50 km/h to 100 km/h quadruples the kinetic energy that must be absorbed in the crash. At 30 km/h with a seatbelt the impact force is already about 1-2 tonnes-force. At 60 km/h it is four times higher. This non-linear relationship is why even modest speed reductions - such as urban 30 km/h limits - produce large reductions in crash severity. Use the chart below to see how G-force rises with speed with and without a seatbelt.
G-force effects on the human body
| G-force | Duration / context | Effect |
|---|---|---|
| 1 g | Constant (gravity) | Normal standing weight |
| 2-3 g | Hard braking | Noticeable chest pressure |
| 4-6 g | Fighter pilot maneuver | Greyout (reduced vision) if sustained |
| 9 g | NHTSA car seat limit | Threshold for serious internal injury risk |
| 10-18 g | Crash with seatbelt (30 km/h) | Typical restrained crash force |
| 30-50 g | Moderate crash, no belt | High risk of fractures and internal injury |
| 60+ g | NHTSA chest limit (>3 ms) | Potentially fatal chest loading |
| 89+ g | Hard surface, no belt | Extreme: near-certain serious injury |
Approximate effects of sustained or peak deceleration on an unrestrained adult. Crash forces are transient but can peak much higher.
Frequently asked questions
What is impact force in a car crash?
Impact force is the average force acting on the passenger during the deceleration phase of a collision. It equals the kinetic energy of the moving body divided by the stopping distance: F = m x v^2 / (2 x d). Because the passenger is brought from their pre-crash speed to zero over a very short distance, even moderate speeds produce forces many times the body weight.
Why does a seatbelt reduce crash force so much?
A seatbelt increases the effective stopping distance of your body from about 4 cm (hitting the steering wheel) to about 20 cm (belt stretch plus airbag). Because force is inversely proportional to stopping distance, this five-fold increase in distance reduces the average impact force by roughly the same factor. The belt also distributes that force across the chest, pelvis, and shoulder - the strongest parts of the skeleton - instead of concentrating it on the head or face.
What G-force can a human survive?
Humans regularly survive transient crash G-forces well above 60 g when restrained by a proper seatbelt and harness, because the duration is so short (a few milliseconds). Racing drivers have survived 100 g impacts. However, sustained G-forces above about 5 g cause greyout, and above 9 g cause loss of consciousness even in trained pilots with G-suits. In a road crash context, the NHTSA limits peak chest deceleration to 60 g for periods longer than 3 ms in safety tests, because beyond that threshold rib fractures and aortic rupture become likely.
How do I use the stopping-time mode?
Switch "Calculate using" to "Stopping time" and enter how long deceleration lasts in milliseconds. The formula then becomes F = m x v / t. A typical belted crash lasts 10-20 ms; without a seatbelt the body stops within 3-5 ms when it strikes the vehicle interior. If you are not sure which to use, the stopping-distance mode (default) is more reliable because crumple-zone depths and seatbelt stretch are more easily estimated than precise crash durations.
Does the calculator account for the mass of the whole vehicle?
No. This calculator models only the deceleration of a passenger, using the passenger's own mass. The vehicle's behavior depends on its mass, the other vehicle, and the crumple-zone design, which vary enormously. If you want to estimate vehicle-level forces, use the vehicle's own mass in the "Passenger weight" field and a crumple-zone depth in the stopping-distance field.
What is the "equivalent mass" output?
It shows the equivalent static weight that would press on you if the impact force were applied vertically in normal gravity. For example, a 5000 N impact force is equivalent to having a 510 kg (1124 lb) mass resting on your chest. This framing makes very large Newton values more intuitive.