Overview
The foot‑pound force (commonly written ft·lbf or ft·lb) is a unit used in systems based on the foot and the pound‑force. It serves in two related roles: as a measure of energy (work) and, by common practice, as a measure of torque (rotational force). The numerical value and context determine which physical quantity is intended.
Definition and conversion
By definition one foot‑pound force equals one pound‑force acting through a distance of one foot. In practical terms, lifting a one‑pound weight a distance of one foot requires one foot‑pound of work. In SI units this is approximately 1.356 joules. For more on the unit as a measure of energy and its conversion to SI, see the referenced explanations.
Usage and examples
The foot‑pound appears often in engineering and everyday measurements in countries that use Imperial or US customary units. Typical uses include:
- Describing the muzzle energy of firearms in foot‑pounds.
- Specifying torque for bolts, engines, and hand tools (often written as ft·lb or lb·ft).
- Estimating small amounts of mechanical work in workshops and maintenance tasks.
Distinctions and notation
Although the same unit form applies, energy and torque are different physical concepts: energy is a scalar describing work, while torque is a vector (moment) describing rotational tendency. Notational care helps avoid confusion—some writers prefer ft·lbf for work and lb·ft or ft·lb for torque. The foot‑pound should also not be confused with the foot‑poundal, a distinct unit based on the poundal force.
History and context
The unit arises from the English system of measures and has long use in British and American engineering practice. Over time, international standards have encouraged SI units such as the newton‑metre and the joule, but the foot‑pound remains common in legacy documentation and in sectors like firearms, automotive maintenance and construction. For further discussion of torque terminology and conventions see torque resources and the succinct definition summaries.