Newton vs Pound-Force (N vs lbf)

Both units measure force. The newton belongs to the SI and is used in science and by most countries. The pound-force belongs to the US customary system and is common in US engineering and aerospace.

One pound-force is exactly 4.4482216152605 N, so one newton is about 0.224809 lbf.

To estimate in your head, multiply pounds-force by 4.5 to get newtons, or divide newtons by 4.5 to get pounds-force. The shortcut is off by just over 1%. Use the converter for exact values.

Use the unit your drawing, standard or customer uses, and convert at the end, not in the middle of a calculation. International standards (ISO, EN) use newtons and kilonewtons. Many US documents, especially in aerospace, use pounds-force and kips.

If a document mixes both, convert all values to one unit before adding or comparing them. Keep the full factor 4.4482216152605 in the calculation and round only the final answer to the precision you need, for example three or four significant digits for most engineering work.

For larger forces the same factor applies: 1 kip (1000 lbf) is about 4.44822 kN.

• 22.4809 lbf is 100 N.
• 444.822 N is 100 lbf.
• 224.809 lbf is 1 kN.

• Mixing up the pound (lb), a unit of mass, with the pound-force (lbf), a unit of force. The pound is exactly 0.45359237 kg; the pound-force is the force of standard gravity on that mass.
• Reading a number as newtons when it is in lbf. One lbf is about 4.45 N, so the numbers differ by more than a factor of four.
• Rounding the factor too early. Use 4.4482216152605 for the full precision, and round only the final result.

Units built from force inherit the difference between the two systems. Pressure in SI is newtons per square meter (pascals); in US customary it is pounds-force per square inch (psi). Torque is newton meters in SI and pound-force feet in US customary.