Pound (force)
The pound (force) (symbol: lbf) is a unit of force in some systems of measurement, including the English Engineering Units and the British Gravitational System. It is defined as the gravitational force exerted on a mass of one avoirdupois pound on Earth's surface. Because the acceleration due to gravity varies by as much as 0.5% at different locations on Earth, for metrological purposes, a standard or conventional value of gravitational acceleration (gn) is typically used. The standard value is defined as 9.80665 m/s2 (approximately 32.17405 ft/s2). Thus, one pound-force is defined as:
1 lbf = gn × 1 lb = 9.80665 m/s2 × 0.45359237 kg ≈ 4.44822 N
where:
- gn is the standard acceleration due to gravity
- lb is the avoirdupois pound (mass)
- kg is the kilogram
- N is the Newton (SI unit of force)
The pound-force should not be confused with the pound (mass), which is a unit of mass. While they are numerically equal at standard gravity, they represent different physical quantities.
The pound-force per square inch (psi or lbf/in2) is a common unit of pressure derived from the pound-force.
Historically, the pound-force was used in engineering calculations, particularly in the United States. However, the SI unit of force, the Newton, is preferred in scientific and engineering applications globally. The pound-force is still encountered in some contexts, especially in older equipment and literature.