Car Crash Calculator returns the average impact force on your body from car speed, your mass and seat belt use, plus deceleration in g, stopping time and an equivalent fall height.
Enter the speed and your mass. For the force on the car itself, enter the car mass and a custom crush distance.
Additional Variables
Step-by-Step Working
- Known values: v = 30 km/h = 8.33333 m/s, m = 70 kg, d = 20 cm = 0.2 m
- F = m × v² ÷ (2 × d) = 70 × 8.33333² ÷ (2 × 0.2) = 12152.8 N
- Stopping time: t = 2 × d ÷ v = 2 × 0.2 ÷ 8.33333 = 0.048 s
- Deceleration: a = F ÷ m = 173.611 m/s², and a ÷ 9.80665 = 17.7034 g
- Result: average impact force F = 12.15 kN
Impact Force at Other Speeds
| Speed (v) | Force (F) | Deceleration | Fall Height |
|---|---|---|---|
| 30.00 km/h | 12.15 kN | 17.70 g | 3.54 m |
| 50.00 km/h | 33.76 kN | 49.18 g | 9.84 m |
| 70.00 km/h | 66.17 kN | 96.39 g | 19.28 m |
| 90.00 km/h | 109.38 kN | 159.33 g | 31.87 m |
| 110.00 km/h | 163.39 kN | 238.01 g | 47.60 m |
| 130.00 km/h | 228.20 kN | 332.43 g | 66.49 m |
What the Car Crash Calculator Works Out
The Car Crash Calculator estimates the average force that stops your body when a car hits something and comes to rest. You enter the car speed and your mass, then say whether you wear a seat belt. The result is the average impact force in kilonewtons or pounds-force.
The seat belt answer sets the stopping distance, which is how far your body travels while it slows to zero. A belt stretches and lets the body move about 20 cm. Without a belt, a windshield or steering wheel gives only about 4 cm.
Both distances are rough estimates, and the Additional Variables section accepts your own figure in cm, m, inches or feet. Georgia State University’s HyperPhysics crash example uses 1 ft for a belted driver and 0.2 ft for an unbelted one. Either value can be typed in as a custom stop.
Car Crash Impact Force Formula
The tool works as a car crash impact force calculator built on the work-energy principle. The force that stops the body, multiplied by the stopping distance, must equal the kinetic energy the body carried into the crash.
$$F = \frac{m v^2}{2 d}$$
F is the average impact force in newtons, m is the mass in kilograms, v is the speed in meters per second and d is the stopping distance in meters. The Car Crash Calculator converts mph, km/h, ft/s, pounds, inches and feet to these units before it runs the formula.
Some crash data gives the duration of the stop instead of the distance. Set Stop Is Known By to stopping time, and the force comes from the change in momentum. That is the same relation used in the Impulse and Momentum Calculator.
$$F = \frac{m v}{t}$$
The two forms agree whenever the deceleration is constant, because the time and the distance of the stop are tied together. Switching between them carries your entry across, so a 20 cm stop at 30 km/h becomes 48 ms.
$$t = \frac{2 d}{v}$$
G-Force in a Car Crash
The first result card shows the deceleration as a multiple of standard gravity, which is 9.80665 meters per second squared. The deceleration comes from the speed and the stopping distance alone.
$$a = \frac{v^2}{2 d}$$
Mass does not appear in this formula. Two people in the same crash with the same stopping distance get the same g figure, and only the force in newtons or pounds grows with body mass. The Acceleration Calculator covers the general case, where speed changes over any time or distance.
The same card shows the result as a share of 60 g. Federal Motor Vehicle Safety Standard No. 208 limits chest acceleration on a crash test dummy to 60 g, apart from intervals that add up to 3 milliseconds or less. NHTSA restates that rule in an interpretation letter on chest g’s.
That limit applies to the peak measured on a dummy. The Car Crash Calculator gives one average value for the whole stop, and real peaks run higher than the average. A result under 60 g is a point of comparison and not a prediction of injury.
Worked Example at 30 mph
HyperPhysics works a case with a 160 lb driver in a car that stops in 1 ft from 30 mph. To repeat it in the Car Crash Calculator, set the speed to 30 mph and the mass to 160 lb. Then choose Custom stop and enter 1 ft.
The speed converts to 13.4112 m/s, the mass to 72.575 kg and the distance to 0.3048 m. Those three values go straight into the distance form of the impact force formula.
$$F = \frac{72.575 \times 13.4112^2}{2 \times 0.3048} = 21413 \text{ N}$$
That is 21.41 kN, or 4,813.82 lbf, with a deceleration of 30.09 g over 45.45 ms. HyperPhysics lists 21,412 N and 30 g for the same case, so the two results agree to rounding.
With the built-in seat belt stop of 20 cm, the same driver at 30 mph gets 32.63 kN and 45.85 g. With the 4 cm no-belt stop the force is five times higher at 163.17 kN, and the deceleration reaches 229.26 g.
25 mph and 35 mph Car Crash Force
Force grows with the square of the speed when the stopping distance stays fixed. A 35 mph crash carries 1.96 times the force of a 25 mph crash, even though the speed is only 40% higher.
The table gives the average force the Car Crash Calculator returns for a 160 lb person at six road speeds. It uses the two built-in stopping distances, and the last column is the fall that reaches the same speed at the ground.
| Speed | Force, Seat Belt | G-Force, Seat Belt | Force, No Belt | G-Force, No Belt | Fall Height |
|---|---|---|---|---|---|
| 15 mph | 1,834.06 lbf | 11.46 g | 9,170.32 lbf | 57.31 g | 7.52 ft |
| 25 mph | 5,094.62 lbf | 31.84 g | 25,473.12 lbf | 159.21 g | 20.89 ft |
| 35 mph | 9,985.46 lbf | 62.41 g | 49,927.31 lbf | 312.05 g | 40.95 ft |
| 45 mph | 16,506.58 lbf | 103.17 g | 82,532.90 lbf | 515.83 g | 67.69 ft |
| 55 mph | 24,657.98 lbf | 154.11 g | 123,289.89 lbf | 770.56 g | 101.12 ft |
| 70 mph | 39,941.85 lbf | 249.64 g | 199,709.24 lbf | 1,248.18 g | 163.80 ft |
The 25 mph car crash force on a belted 160 lb person is 5,094.62 lbf, and the 35 mph car crash force is 9,985.46 lbf. The Car Crash Calculator’s Impact Force at Other Speeds chart builds the same rows for your own mass and stopping distance.
The 35 mph row matters because NHTSA runs its frontal crash rating test at 35 mph into a fixed barrier. At that speed the 20 cm seat belt stop averages 62.41 g, which sits just above the 60 g chest limit.
The faster rows assume the body still stops in 20 cm or 4 cm. In a real frontal crash the front of the car also crushes, and a belted occupant slows with the cabin over that extra distance. Add the crush length to the stop under Custom stop to model it.
Average Force of a Car Crash on the Car
The Car Crash Calculator also gives the average force of a car crash on the vehicle itself. Enter the car’s mass in place of your own, choose Custom stop, and enter how far the front end crushed.
A 3,300 lb car that hits a rigid barrier at 35 mph and crushes 2 ft sees an average force of 300.56 kN, or 67,568.90 lbf. The cabin slows at 20.48 g over 77.92 ms, and a firmly belted occupant shares that gentler stop.
Crush depth matters as much as speed here, because doubling the crush distance halves the average force. A stiff object that barely gives, such as a large tree or a concrete wall, keeps the distance short and the force high.
More Car Crash Calculator Results
The second card turns the force into a mass by dividing it by standard gravity. At the default 30 km/h with a 70 kg person and a seat belt, the 12.15 kN force equals the weight of 1.24 metric tons resting on you. The rows under it show the 48 ms stopping time and the 2.43 kJ of kinetic energy.
The third card reruns the crash with the opposite seat belt choice. In the default case the force without a belt is 60.76 kN at 88.52 g. That is five times the belted figure, because 20 cm is five times 4 cm.
The fourth card gives the equivalent fall height. A 30 km/h crash matches a 3.54 m fall, and a 35 mph crash matches a 40.95 ft fall. The Free Fall Calculator runs the same relation the other way, from a drop height to an impact speed.
$$h = \frac{v^2}{2 g}$$
Two rows sit under the fall height. One is the momentum, the same quantity the Momentum Calculator returns. The other is the speed that would halve the force, which is the current speed divided by 1.414 for a fixed stopping distance. For the default 30 km/h that speed is 21.21 km/h.
Bike Crashes and Other Uses
The tool also works as a bike crash calculator. Enter the rider’s mass and the riding speed, then choose Custom stop, because the seat belt distances do not apply on a bike. A 180 lb rider who stops from 20 mph over 4 inches takes 7,220.73 lbf at 40.12 g.
Hitting a barrier is often the end of a longer chain, such as a car leaving a curve too fast. The Angle of Banking Calculator relates speed, curve radius and bank angle. The other dynamics and statics calculators cover the forces that act before the impact.
This is a free car crash calculator that runs in any phone or desktop browser, so there is no car crash calculator app to install. People searching for a free online car accident simulation should note the difference. A simulation animates how vehicles move and deform, while this tool returns one average force from three numbers.
Crash Speed and G-Force FAQs
How many g’s is a 30 mph crash?
The answer depends on the stopping distance. With a 1 ft stop, a 30 mph crash averages 30.09 g. With the 20 cm seat belt stop in the Car Crash Calculator it averages 45.85 g, and with the 4 cm no-belt stop it averages 229.26 g.
The stopping time shrinks in the same order, from 45.45 ms at 1 ft to 29.83 ms at 20 cm and 5.97 ms at 4 cm. Body mass changes none of these figures, because mass cancels out of the deceleration formula.
How much force is a car crash at 60 mph?
For a 160 lb person at 60 mph, the 20 cm seat belt stop gives 130.53 kN, or 29,345.03 lbf, at 183.41 g. The 4 cm no-belt stop gives 652.67 kN, or 146,725.16 lbf, at 917.03 g. Both are four times the 30 mph values, since the speed has doubled.
A 60 mph crash matches a fall of 120.35 ft. Figures this large describe a body that stops within centimeters. A car that crushes several feet spreads the stop out, so enter the longer distance in the Car Crash Calculator to see the lower average.
At what speed does a crash reach 60 g?
The average deceleration reaches 60 g at one speed for each stopping distance. Rearranging the deceleration formula gives that speed in meters per second, with the distance d in meters.
$$v = \sqrt{2 \times d \times 60 \times 9.80665}$$
For the 20 cm seat belt stop the speed is 15.34 m/s, which is 34.32 mph or 55.23 km/h. For the 4 cm no-belt stop it is only 15.35 mph, or 24.70 km/h. Above those speeds the alert under the results turns red.
No single speed makes a crash fatal, and the Car Crash Calculator does not predict survival. Injury depends on the peak deceleration, how long it lasts, which part of the body takes the load and how the car deforms.
Is hitting a wall at 35 mph the same as a head-on crash?
NHTSA treats a 35 mph crash into a fixed barrier as equal to a head-on crash between two similar vehicles that are each moving at 35 mph. Each car stops in its own crush distance, just as it would against the wall.
For a head-on crash between similar cars, enter your own speed of 35 mph and not the combined 70 mph. A 70 mph entry would describe hitting a fixed barrier at 70 mph, which carries four times the force.
Does a heavier person feel more force in a crash?
The force rises in direct proportion to mass, so a heavier body needs a harder push to stop. At 35 mph with the 20 cm seat belt stop, a 120 lb person takes 7,489.10 lbf and a 200 lb person takes 12,481.83 lbf.
Both people slow at the same 62.41 g, because the deceleration depends only on the speed and the stopping distance. The heavier person needs a larger force from the belt to follow that same deceleration.