Foot-pound-second System - Conversions

Conversions

Together with the fact that the term "weight" is used for the gravitational force in some technical contexts (physics, engineering) and for mass in others (commerce, law), and that the distinction often does not matter in practice, the coexistence of variants of the FPS system causes confusion over the nature of the unit "pound". Its relation to international, metric units is expressed in kilograms, not newtons, though, and in earlier times it was defined by means of a mass prototype to be compared with a two-pan balance which is agnostic of local gravitational differences.

In July 1959 the various national foot and avoirdupois pound standards were replaced by the international foot of precisely 0.3048 m and the international pound of precisely 0.45359237 kg, making conversion between the systems a matter of simple mathematics. The conversion for the poundal is given by 1 pdl = 1 lb·ft/s2 = 0.138254954376 N (precisely).

To convert between the absolute and gravitational FPS systems one needs to fix the standard acceleration g which connects the pound and the pound-force.

While g strictly depends on one's location on the Earth surface, since 1901 in most contexts it is fixed conventionally at precisely g₀ = 9.80665 m/s2 ≈ 32.17405 ft/s2. Therefore the slug is about 32.17405 lb_m or 14.593903 kg.