Foot–pound–second system

The foot–pound–second system or FPS system is a system of units built on three fundamental units: the foot for length, the (avoirdupois) pound for either mass or force (see below), and the second for time.

Collectively, the variants of the FPS system were the most common system in technical publications in English until the middle of the 20th century.

Errors can be avoided and translation between the systems facilitated by labelling all physical quantities consistently with their units. Especially in the context of the FPS system this is sometimes known as the Stroud system after William Stroud, who popularized it.

When the pound is used as a unit of mass (often denoted as "pound-mass" and abbreviated as "lbm" to avoid confusion), the core of the coherent system is similar and functionally equivalent to the corresponding subsets of the International System of Units (SI), using metre, kilogram and second (MKS), and the earlier centimetre–gram–second system of units (CGS).

In this sub-system, the unit of force is a derived unit known as the poundal.

The international standard symbol for the pound as unit of mass rather than force is lb.

Everett (1861) proposed the metric dyne and erg as the units of force and energy in the FPS system.

Latimer Clark's (1891) "Dictionary of Measures" contains celo (acceleration), vel or velo (velocity) and pulse (momentum) as proposed names for FPS absolute units.

The technical or gravitational FPS system, is a coherent variant of the FPS system that is most common among engineers in the United States. It takes the pound-force as a fundamental unit of force instead of the pound as a fundamental unit of mass.

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