Standard acceleration of gravity

The 3rd CGPM adopted 9.806 65 m/s² in 1901 as a conventional standard, originally tied to sea level at 45° latitude. It is exact by agreement, which is what makes the kilogram-force, the pound-force, the psi-to-head conversion and the "g" of acceleration well-defined units rather than fuzzy ones. Aviation, structural codes and instrument calibration all quote it.

Actual gravity is not 9.806 65 anywhere in particular. Earth's rotation and equatorial bulge give about 9.780 m/s² at the equator and 9.832 m/s² at the poles — a 0.5 % spread — and altitude removes a further 3 × 10⁻⁶ m/s² per metre of elevation, with local geology adding milligal wiggles that gravimetric surveys hunt for. Use g_n for defined units and legal metrology; use a measured local g when weighing to parts per million.