An object accelerates toward the Earth at a rate of 32 feet per second per second, or 32 ft/s², irrespective of its mass. Scientists refer to this as the acceleration due to gravity. The concept of G’s, or “G-forces,” refers to multiples of the acceleration due to gravity and the concept applies to acceleration in any direction, not just toward the Earth. Scientists sometimes use G-forces to express the forces on the human body during acceleration. A person’s “weight” represent the downward force that results from the force of gravity acting on his body mass. This force bears a directly proportional relationship to acceleration. Thus, if you experience an acceleration of 64 ft/s², or twice the acceleration due to gravity,your weight doubles compared to weight at rest.
Convert all units of speed, distance and time to feet and seconds using an online calculator, such as that provided in the Resources. An object moving at 60 miles per hour, for example, moves at 88 feet per second, or 88 ft/s.
Calculate the acceleration of a person or object by dividing the change in velocity by the time over which the change took place. For example, consider a race car that starts from rest, or zero mph, and accelerates to a final velocity of 155 mph in 6.1 seconds. A velocity of 155 mph converts to 227 ft/s. The car’s average acceleration is therefore (227 - 0 ft/s) / 6.1 s = 37.2 ft/s².
Determine the G-forces on the object by dividing its average acceleration by the acceleration due to gravity: 32 ft/s². A car accelerating at 37.2 ft/s² experiences 37.2 / 32 = 1.16 G’s.
TL;DR (Too Long; Didn't Read)
If you prefer to work in the metric or SI system of units, the metric equivalent of the acceleration constant is 9.81 meters per second per second, or 9.81 m/s².