Calculate impact G-force from speed and stopping time or distance, convert an acceleration to g, and estimate average impact force in newtons or pounds.

Optional settings
Blank omits net force. Mass does not change the deceleration.

Impact G Force Formula

The following equation is used to calculate the Impact G Force.

G = (vᵢ² − vf²) / (2 d g₀), g₀ = 9.80665 m / s²
  • Where G is the magnitude of the distance-averaged net deceleration in standard-gravity units
  • d is the total distance traveled during impact (m)
  • v_i and v_f are initial and final speeds in m/s, with 0 ≤ v_f < v_i and no direction reversal

To find the distance-averaged deceleration, divide (v_i² − v_f²) by 2d and then by standard gravity 9.80665 m/s². Time mode instead uses (v_i − v_f)/(Δt g_0). The equivalent time or distance and the target mode assume constant deceleration. Optional mass gives average net force, not peak contact force. See constant-acceleration kinematics.

What is an Impact G Force?

Definition:

An impact G force is the number of units of acceleration experienced during an impact relative to the acceleration due to gravity on Earth.

It is net deceleration divided by the conventional standard gravity, 9.80665 m/s². Standard-gravity units are a reference scale and do not require the collision to be driven by Earth’s gravity.

How to Calculate Impact G Force?

Example Problem:

The following example outlines the steps and information needed to calculate the Impact G Force.

First, determine the velocity at impact. In this example, initial speed is 45 m/s and final speed is 0 m/s.

Next, determine the total distance of the impact. In this case, the impact distance was 1.25 m.

Finally, calculate the number of g forces during impact using the formula above:

G = (v_i² − v_f²)/(2 d g_0), g_0 = 9.80665 m/s²

G = (45² − 0²)/(2 × 1.25 × 9.80665)

G ≈ 82.5970 g₀ (distance-averaged net deceleration)

FAQ

What factors can affect the calculation of Impact G Force?

Endpoint speeds and the measured stopping time or distance determine the corresponding average deceleration. Mass changes net force, not G for those same speeds and stopping measurement. Rotation, rebound, deformation and a varying acceleration profile require more information; derived equivalent time/distance assumes constant deceleration.

How does the Impact G Force relate to the safety of vehicles in a crash?

The Impact G Force is a critical factor in assessing the safety of vehicles during a crash. Higher G forces typically indicate more severe impacts, which can lead to greater injury risk for occupants. Vehicle manufacturers design crumple zones, safety belts, and airbags to absorb and reduce the G forces experienced by occupants during a crash, enhancing safety.

Can the Impact G Force formula be used for any type of impact?

Use this model for straight-line slowing without direction reversal. It does not measure peak loads, body-segment acceleration or injury risk. The standard-g reference is the same on other planets; local gravity and other forces must be included separately when determining contact loads. Select a mode, enter the shown speeds and stopping measurement, then Calculate. A default final speed of zero means a complete stop.