Torque

Torque is the rotational equivalent of force — the tendency of a force to cause or change the rotational motion of an object. In practical terms, torque is what you apply when you turn a wrench to tighten a bolt.

How it works

Torque is calculated as force multiplied by the perpendicular distance from the pivot point: Torque = Force × Distance. A longer wrench gives you more torque for the same hand force because the distance from the pivot (the fastener) is greater. This is why a breaker bar (a long, non-ratcheting handle) can loosen a stubborn bolt that a short wrench cannot.

Units

• Newton-metres (N·m): The SI unit. One newton-metre is the torque produced by one newton of force applied one metre from the pivot.
• Foot-pounds (ft·lb): The imperial unit. One foot-pound is the torque produced by one pound of force applied one foot from the pivot.
• Inch-pounds (in·lb): Used for smaller fasteners — one twelfth of a foot-pound.

Why torque matters

Every bolted joint has a specified torque value — the amount of tightening force needed for the joint to hold without overstressing the bolt. Too little torque and the bolt loosens; too much and the bolt stretches, strips, or snaps. A torque wrench measures the applied torque and signals when the target is reached, ensuring consistent, safe fastening.

Common torque values

• Oil drain plug: 25–35 N·m (18–25 ft·lb)
• Wheel lug nuts: 100–140 N·m (75–100 ft·lb)
• Cylinder head bolts: 80–120 N·m (varies by engine)
• Spark plugs: 15–25 N·m (11–18 ft·lb)