What Is a Pound-Force (lbf)?

The pound-force is the force of standard gravity, exactly 9.80665 m/s², acting on a mass of one avoirdupois pound, exactly 0.45359237 kg. Multiplying the two gives an exact value:

• 1 lbf = 0.45359237 kg × 9.80665 m/s² = 4.4482216152605 N

In everyday US speech, "pound" is used for both mass and force. In engineering they are kept apart: lb (or lbm) is mass and lbf is force.

On Earth a mass of 1 lb weighs about 1 lbf, which is why the two are easy to confuse. They are still different quantities: on the Moon, where gravity is about one sixth of Earth's, the same 1 lb mass weighs only about one sixth of a pound-force.

Some older texts write the pound-force as "lb" or "pound" without the f, and the pound of mass as "lbm". When both appear in one formula, as in some US thermodynamics books, a conversion constant called gc (about 32.174 lbm·ft/(lbf·s²)) links them. With SI units no such constant is needed.

• 1 kip = 1000 lbf
• 1 ozf = 0.0625 lbf
• 1 lbf = 0.45359237 kgf (exact, because both are defined with standard gravity)
• 1 pdl ≈ 0.031081 lbf

Pound-force is standard in US engineering: jet and rocket engine thrust, spring rates (lbf/in), torque (lbf·ft) and pressure (psi is pounds-force per square inch). International standards and science use newtons instead.

• A force of 50 lbf is 222.411 N: multiply by 4.4482216152605.
• A force of 200 N is 44.9618 lbf: divide by 4.4482216152605.
• One kip is 1000 lbf, which is 4448.2216 N.

The pound-force is part of two very common US units. Pressure in psi is pounds-force per square inch, and torque in "foot-pounds" is pounds-force multiplied by feet (lbf·ft).

• 1 psi ≈ 6.89476 kPa
• 1 lbf·ft ≈ 1.35582 N·m
• 1 lbf·in ≈ 0.112985 N·m

When a value is labeled only "lb" in a pressure or torque specification, it nearly always means pound-force. Check the document if the unit is unclear.

• Converting a force in lbf with the mass factor 0.45359237. That factor turns pounds of mass into kilograms; for force, use 4.4482216152605 N per lbf.
• Mixing foot-pounds of torque with foot-pounds of energy. Both are written lbf·ft but measure different things.
• Rounding 4.4482216152605 to 4.4 early in a calculation, which gives an error of about one percent.