Car Crash Force Calculator (Impact G-Force on an Occupant)

How it works

average deceleration a = v² ÷ (2 × d) · g-force = a ÷ 9.81 · average force F = m × a · stopping time = 2 × d ÷ v · equivalent fall height = v² ÷ (2 × g) · v in m/s, d in m, m in kg

In a crash the occupant's kinetic energy has to be absorbed over a distance; the shorter that distance, the larger the force. Crumple zones, seat belts that stretch and airbags all work by lengthening the distance over which the body stops, turning a lethal spike into a survivable average. This calculator uses a constant-deceleration model, so peak forces in a real crash are higher than the average shown — often 1.5 to 2 times — and the numbers are for understanding, not for crash-test or legal analysis.

Worked example

A 75 kg occupant in a 50 km/h (13.9 m/s) frontal crash who stops over 0.6 m (crumple zone plus belt stretch) has an average deceleration of 13.9² ÷ (2 × 0.6) = 161 m/s², or 16.4 g. The average force is 75 × 161 = 12.1 kN — like a load of 1,229 kg — and the crash is the same as falling 9.8 m.

Sources: NHTSA — Seat belts · HyperPhysics — Impact force from a fall or collision

Frequently asked questions