Torque

Torque is the rotational analogue of force: a force applied at a distance from an axis. The SI unit is the newton-metre (N·m), and the US customary unit is the pound-foot (lbf·ft), with 1 lbf·ft equal to exactly 1.3558179483 N·m. Despite sharing the same numerical conversion factor, pound-foot (torque) and foot-pound (energy) describe distinct physical quantities. Torque is a vector with direction and sign; energy is a scalar. The two just happen to share units because force times distance is algebraically identical whether the distance is the lever arm or the displacement.

Automotive specifications quote engine torque in N·m or lbf·ft. A typical compact car produces 150 to 200 N·m (110 to 150 lbf·ft); a large pickup engine can exceed 900 N·m (660 lbf·ft). Wheel lug nuts on passenger cars usually require 100 to 140 N·m (74 to 103 lbf·ft), with exact values in the vehicle service manual. Low-torque work on bicycles, electronics, and small fasteners uses pound-inches (lbf·in) or newton-centimetres (N·cm), where 1 lbf·ft equals 12 lbf·in.

The kilogram-force metre (kgf·m), once common in European and Japanese engineering, equals 9.80665 N·m by virtue of the standard gravity definition. It still appears on older Japanese motorcycle service manuals and on some torque wrenches sold outside North America. The cgs dyne-centimetre survives in physics textbooks and in measurements of very small torques, such as those in micromechanical devices.

Accurate torque matters because fasteners have narrow tolerance windows. Over-tightening stretches the bolt past its yield point, so future load cycles stretch it further until it breaks. Under-tightening leaves the joint free to loosen under vibration. Fastener manufacturers and ISO 16047 publish torque specifications and test methods so that installed bolts achieve the correct preload. This converter supports nine torque units, with all factors traceable to NIST and CGPM definitions.