One g is exactly 9.80665 m/s², defined by the International Bureau of Weights and Measures (BIPM). It represents the standard acceleration due to gravity at sea level and is widely used in aviation, motorsport, and space to express forces experienced by people and structures.

The Gal (symbol: Gal) equals 1 cm/s² and is named after Galileo Galilei. It is used mainly in geodesy and gravimetry to measure variations in the Earth's gravitational field. The milliGal (mGal = 0.001 Gal) is typical in field surveys. 1 Gal = 0.01 m/s².

Velocity measures how fast position changes (m/s), while acceleration measures how fast velocity changes (m/s²). A car going 60 km/h at a constant speed has zero acceleration. When it brakes from 60 km/h to a stop in 5 s, its acceleration is roughly −3.3 m/s² (about −0.34 g).

The value 9.80665 m/s² was adopted in 1901 by the 3rd General Conference on Weights and Measures (CGPM) as a conventional reference. Actual gravitational acceleration varies from about 9.764 m/s² at the equator to 9.863 m/s² at the poles due to Earth's oblateness and rotation, so a single round number cannot capture the variation.