From tightening a bolt with a wrench to a diesel engine's torque: how to convert moment of force without the 10% error.
Torque, also called moment of force, is the physical quantity that describes the tendency of a force to rotate an object around an axis. In the International System of Units (SI), the standard unit of torque is the newton-metre (N·m), defined as a force of one newton applied to a lever arm of one metre. The concept is intuitive: when tightening a screw with a wrench, the longer the wrench and the more force you apply, the greater the torque — hence the formula torque = force × distance.
Although the newton-metre is the official unit, several other torque units coexist in practice, especially in automotive and mechanical engineering contexts. The kilogram-force metre (kgf·m), equivalent to 9.80665 N·m, is widely used in Brazil and in metric-influenced countries to specify engine torque and bolt tightening. The pound-force foot (lbf·ft or lb·ft), equivalent to approximately 1.35582 N·m, is the standard unit in the United States and appears in American vehicle spec sheets. The pound-force inch (lbf·in), equivalent to 0.112985 N·m, is used for smaller torques, in electronics and precision instruments.
An important note: the newton-metre (N·m) as a unit of torque has the same dimensional form as the joule (J) as a unit of energy — both are force × distance. However, they are not interchangeable: torque measures a rotational effect, while energy measures work done. By convention, N·m is used for torque and J for energy, and torque should never be written in joules, to avoid confusion. This distinction is taught in all physics and engineering courses and is formalized in BIPM guidelines.
The conversion formulas between torque units are: 1 kgf·m = 9.80665 N·m; 1 lbf·ft = 1.35582 N·m; 1 lbf·in = 0.112985 N·m; 1 kgf·m = 7.23301 lbf·ft. A common mistake is to treat kgf·m and N·m as equal — the difference is almost 10% (1 kgf·m ≈ 9.81 N·m), which in torque specifications for cylinder head bolts or suspension components can mean an insufficient or excessive tightening, with risk of mechanical failure.
A practical example: an assembly manual specifies the torque of a wheel bolt at 120 N·m. Converting to kgf·m: 120 / 9.80665 = 12.237 kgf·m, or approximately 12.2 kgf·m. Converting to lbf·ft: 120 / 1.35582 = 88.51 lbf·ft. In the reverse direction, an American torque wrench set to 80 lbf·ft equals 80 × 1.35582 = 108.47 N·m. These calculations are routine in shops that handle imported vehicles and manuals in different unit systems.
In the automotive industry, torque is one of the most important engine specifications, often as relevant as power. A diesel truck engine may have a torque of 800 N·m at 1,500 rpm, while a gasoline sports car engine may have 400 N·m at 4,000 rpm — the first prioritizes low-rev force (for load), the second high-rev performance. The maximum torque and the engine speed at which it occurs define the vehicle's drivability character, and manufacturers always publish these figures in spec sheets.
Care with precision is critical in mechanical assembly. Cylinder head, connecting rod, and suspension component bolts have tightening torques specified by the manufacturer with narrow tolerances — tightening below specification can allow play and loosening in use, and tightening above can strip the thread or break the bolt. For this reason, professional shops use calibrated torque wrenches and follow the exact manual values. In technical work, use exact conversion factors and round only the final result; for general use, one decimal place in N·m or kgf·m is usually sufficient.
Real applications of torque conversion go beyond automotive: mechanical engineers calculate torque in reducers, gears, and industrial transmission systems, power tool designers specify the torque of screwdrivers and drills, and the aerospace industry defines tightening torques for aircraft components with strict tolerances. The TudoConversor torque converter calculates between N·m, kgf·m, lbf·ft, lbf·in, and other units with official factors and double precision, ensuring reliable results in any direction.