standard gravity
Accelerationexact by definition
Standard gravity, symbol g or gₙ, is defined as exactly 9.80665 m/s². It is a conventional reference value adopted by the CGPM in 1901, not a measurement of gravity at any particular place. Accelerations expressed in "g" are multiples of this fixed number.
Watch out: Actual local gravity varies from about 9.78 m/s² at the equator to about 9.83 m/s² at the poles, plus altitude and geology effects. Standard gravity is exact by definition, but it is not the gravity where you are standing, and precision weighing instruments must be calibrated for local g.
| 1 g | 9.80665 m/s² |
The value was chosen to represent gravitational acceleration at about 45° latitude at sea level, and it was fixed so that quantities depending on it, notably the kilogram-force and units like psi that descend from weight, would have unambiguous definitions.
About the standard gravity
Expressing acceleration in g is a way of asking "how does this compare to falling". At 1 g you feel your normal weight; at 2 g you feel twice it. A brisk car pulling 0 to 60 mph in three seconds averages about 0.9 g, a passenger aircraft in a normal turn holds about 1.3 g, and a fighter pilot in a hard turn takes 7 to 9 g with a pressure suit to keep blood in the brain. Roller coasters are designed around sustained g limits and the rate at which they change, since a sudden onset is harder on a body than a steady load.
The number 9.80665 is a committee decision, not a measurement, which is why it carries six digits with no uncertainty attached. Fixing it mattered because a whole family of units hangs off it: the kilogram-force is the force standard gravity exerts on one kilogram, and pressure units built on weight inherit the same constant. Real gravity is not that number anywhere in particular. It falls with altitude by roughly 3 μm/s² per metre of elevation, varies with latitude because the Earth is flattened and spinning, and shifts measurably over dense or light rock, which is what gravity surveys in mineral exploration are looking for.