Foot–pound–second system of units
A system of units based on foot, pound, and second.
The foot–pound–second system (FPS system) is a system of units built on three fundamental units: the foot for length, the avoirdupois pound for either mass or force, 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.
- fundamental_units
- foot, pound, second
- mass_unit_variant
- pound (lb) or slug
- force_unit_variant
- pound-force (lbf) or poundal (pdl)
- coherent_subsystem
- Absolute English System (pound as mass, poundal as force)
- gravitational_subsystem
- British gravitational system (pound-force as force, slug as mass)
Lore & Background
The FPS system has several variants. When the pound is used as a unit of mass, the coherent system is similar to the International System of Units (SI) and the centimetre–gram–second system (CGS), and is sometimes called the Absolute English System. In this sub-system, the unit of force is the poundal, defined as 1 lb·ft/s².
Reader's Guide
The FPS system's significance lies in its historical dominance in English-language technical publications until the mid-20th century. Its coexistence of variants—using the pound as mass or force, and derived units like the poundal or slug—causes confusion over the nature of the unit 'pound'. The system's relation to international metric units is expressed in kilograms, not newtons, and was defined by a mass prototype compared with a two-pan balance. Errors can be avoided by labeling all physical quantities consistently with their units, a practice sometimes known as the Stroud system.
Did You Know?
- The electrostatic and electromagnetic FPS systems are derived from units of length and force.
More in Metrology 1-24
Elsewhere in the Metrology universe
Spotted an error? Know more?
This is a living reference — every entry is fact-audited, and reader corrections feed straight into our audit queue. Suggest an edit · See this site's audit record