Foot-pound (energy)

Not to be confused with Pound-foot (torque) or Foot-poundal.

Foot-pound
Unit system	English engineering units and British gravitational system
Unit of	Energy
Symbol	ft⋅lbf, ft⋅lb
Conversions
1 ft⋅lbf in ...	... is equal to ...
SI units	1.355818 J
CGS units	13,558,180 erg

The foot-pound force (symbol: ft⋅lbf, ^[1] ft⋅lb_f,^[2] or ft⋅lb ^[3]) is a unit of work or energy in the engineering and gravitational systems in United States customary and imperial units of measure. It is the energy transferred upon applying a force of one pound-force (lbf) through a linear displacement of one foot. The corresponding SI unit is the joule, though in terms of energy, one joule is not equal to one foot-pound.

Usage

The term foot-pound is also used as a unit of torque (see pound-foot (torque)). In the United States this is often used to specify, for example, the tightness of a fastener (such as screws and nuts) or the output of an engine. Although they are dimensionally equivalent, energy (a scalar) and torque (a Euclidean vector) are distinct physical quantities. Both energy and torque can be expressed as a product of a force vector with a displacement vector (hence pounds and feet); energy is the scalar product of the two, and torque is the vector product.

Although calling the torque unit "pound-foot" has been academically suggested, both are still commonly called "foot-pound" in colloquial usage. To avoid confusion, it is not uncommon for people to specify each as "foot-pound of energy" or "foot-pound of torque" respectively.

In small arms ballistics and particularly in the United States, the foot-pound is often used to specify the muzzle energy of a bullet.

Conversion factors

Energy

1 foot pound-force is equivalent to:

• 1.355 817 948 331 400 4 joules
• 13558179.483314004 ergs
• about 1.285×10⁻³ British thermal units
• 0.323832 calories
• 8.462238×10⁺¹⁸ eV = 8.462238 EeV = 8.462238×10⁺⁹ GeV

Power

1 foot pound-force per second is equivalent to:

• 1.3558179483314 watts
• 1.818×10⁻³ horsepower

Related conversions:

• 1 watt ≈ 44.25372896 ft⋅lbf/min = 0.737562149333 ft⋅lbf/s
• 1 horsepower (mechanical) = 33,000 ft⋅lbf/min = 550 ft⋅lbf/s

See also

• Conversion of units
• Pound-foot (torque)
• Poundal
• Slug (unit)
• Units of energy

References

1. IEEE Std 260.1™-2004, IEEE Standard Letter Symbols for Units of Measurement (SI Units, Customary Inch-Pound Units, and Certain Other Units)
2. Fletcher, Leroy S.; Shoup, Terry E. (1978), Introduction to Engineering, Prentice-Hall, ISBN 978-0135018583, LCCN 77024142.^: 257
3. Budynas, Richard G.; Nisbett, J. Keith (2014-01-27). Mechanical Engineering Design. McGraw Hill Education. ISBN 978-0073529288. Archived from the original on 2018-08-27. Retrieved 2016-12-20.