Car Crash Force Calculator
Quick answer: a car that hits a solid object at 30 mph and crushes 0.6 m averages about 15 g; a belted occupant averages about 10 g, an unbelted one far more. The impact equals falling 9.2 m (30 ft), about three storeys. Formula: average deceleration = v² ÷ (2 × stopping distance); force = mass × deceleration.
Updated · formulas checked against the sources below
Default shown: 30 mph into a solid object, 1,500 kg car, 75 kg occupant with belt and airbag, 0.6 m of crumple. Change any value above.
Not a substitute for an accident reconstruction expert. These are textbook physics estimates for learning and rough checks. A real reconstruction measures the road, the vehicles, the marks and the event data recorder, and can give a different answer. Do not rely on this page for an insurance claim, a court case or a police report.
How crash force and G-force are worked out
When a car stops, its speed has to go to zero over some distance. The shorter that distance, the harder the stop. Assuming an even (constant) deceleration:
where v is the speed in m/s (mph × 0.447, or km/h ÷ 3.6), d the stopping distance in metres, m the mass in kg and t the time the stop takes. The "equivalent fall height" uses the same physics backwards: a fall from h = v² ÷ (2 × 9.81) metres ends at speed v.
Worked example
A 1,500 kg car hits a concrete wall at 30 mph = 13.4 m/s, and its front crushes 0.6 m.
The belted driver (75 kg) moves about 0.3 m further as the belt stretches and the airbag compresses, so stops over 0.9 m: about 10.2 g on average, a load of about 7.5 kN spread across the pelvis and chest. Without a belt the driver keeps going at 30 mph and stops against the wheel or dashboard in perhaps 5 cm: about 183 g on average. The physics is the same; only the stopping distance changes.
Average versus peak
Real crash pulses are not flat. If the deceleration rises and falls like a half sine wave, the peak is about π/2 ≈ 1.6 times the average, which is why the calculator also shows an estimated peak. Crash-test sensors record the full curve; this page cannot.
Crash speed, G-force and equivalent fall height
Solid-object crash; car crumple 0.6 m; belted occupant 0.9 m; car 1,500 kg. A storey is taken as 3 m.
| Speed | Fall height | Storeys | Car average | Belted occupant | Energy (1,500 kg) |
|---|---|---|---|---|---|
| 10 mph (16 km/h) | 1.0 m / 3 ft | 0.3 | 2 g | 1 g | 15 kJ |
| 20 mph (32 km/h) | 4.1 m / 13 ft | 1.4 | 7 g | 5 g | 60 kJ |
| 30 mph (48 km/h) | 9.2 m / 30 ft | 3.1 | 15 g | 10 g | 135 kJ |
| 40 mph (64 km/h) | 16.3 m / 53 ft | 5.4 | 27 g | 18 g | 240 kJ |
| 50 mph (80 km/h) | 25.5 m / 84 ft | 8.5 | 42 g | 28 g | 375 kJ |
| 60 mph (97 km/h) | 36.7 m / 120 ft | 12.2 | 61 g | 41 g | 540 kJ |
| 70 mph (113 km/h) | 49.9 m / 164 ft | 16.6 | 83 g | 55 g | 734 kJ |
| 80 mph (129 km/h) | 65.2 m / 214 ft | 21.7 | 109 g | 72 g | 959 kJ |
The same in km/h
| Speed | Fall height | Storeys | Car average | Belted occupant | Energy (1,500 kg) |
|---|---|---|---|---|---|
| 30 km/h (19 mph) | 3.5 m | 1.2 | 6 g | 4 g | 52 kJ |
| 50 km/h (31 mph) | 9.8 m | 3.3 | 16 g | 11 g | 145 kJ |
| 70 km/h (43 mph) | 19.3 m | 6.4 | 32 g | 21 g | 284 kJ |
| 90 km/h (56 mph) | 31.9 m | 10.6 | 53 g | 35 g | 469 kJ |
| 110 km/h (68 mph) | 47.6 m | 15.9 | 79 g | 53 g | 700 kJ |
| 130 km/h (81 mph) | 66.5 m | 22.2 | 111 g | 74 g | 978 kJ |
Doubling the speed quadruples the energy, the fall height and (for the same crumple distance) the G-force. Real crumple zones are tuned for test speeds, so at very high speeds the structure runs out of crush distance and the forces rise faster still.
Assumptions
- Solid object. The car stops completely, as in a crash into a wall or a large tree. Crashes into another car, a hedge or a guard rail usually involve a smaller change of speed and longer stops.
- Even deceleration. Real crashes have peaks; the page shows a half-sine peak estimate (about 1.6 × average).
- Distances are illustrations. 0.5–1 m of crumple is typical of a car front in a severe frontal crash; the extra 0.2–0.3 m for a belted occupant and the 5 cm for an unbelted one are round numbers chosen to show the effect, not measurements of any car.
- G-force is not injury. Whether a person is hurt depends on the direction of the force, how long it lasts, where on the body it lands and the person's age and health.
- Head-on crashes. Two equal cars meeting head-on at 50 mph each stop from 50 mph, much like hitting a solid wall at 50 mph, not 100 mph. A lighter car meeting a heavier one has a larger change of speed.
For context, official crash tests use speeds chosen to be survivable for a belted adult in a well-designed car: NHTSA's frontal test is run at 35 mph into a rigid barrier, and the IIHS frontal overlap tests at 40 mph. See the crash test ratings for any model.
Crash forces — FAQ
How many g's is a car crash?
How do you calculate the impact force of a car crash?
A car crash at 30 mph is like falling from how high?
Why does a seatbelt reduce the force so much?
Does a heavier car crash with more force?
Sources and method
- NASA Glenn Research Center: Newton's laws of motion — F = m·a; the stopping and fall-height formulas follow from constant-acceleration motion
- NHTSA: New Car Assessment Program (5-Star Safety Ratings) — frontal crash test at 35 mph
- IIHS: About our crash tests — frontal overlap tests at 40 mph
Method: a = v²/(2d), G = a/9.80665, F = m·a, t = 2d/v, h = v²/(2·9.80665), energy = ½·m·v²; peak ≈ π/2 × average for a half-sine pulse. Weights in lb are converted at 0.4536 kg. The calculator runs in your browser. Checked 2026-11-02.