Force units: overview and conversion factors
Newton, kilopond, pound-force and dyne – physics, statics and mechanical engineering
Force is a physical quantity that accelerates, deforms, or holds a body in equilibrium. Newton's second law describes force as the product of mass and acceleration: F = m × a. The SI base unit of force is the newton (N), named after Sir Isaac Newton. One newton is the force that gives a mass of 1 kilogram an acceleration of 1 m/s². The weight of a bar of chocolate (100 g) on Earth is about 0.981 N, and the weight of a litre of water (1 kg) is about 9.807 N.
In construction and structural engineering, kilonewtons (kN = 1,000 N) and meganewtons (MN = 1,000,000 N) are common. Bridges must bear loads of thousands of kilonewtons. The permitted load capacity of a 630 kg elevator corresponds to about 6.18 kN. Wind loads on buildings are given in kN/m². In mechanical engineering, bolt tightening torques and material test forces are measured in newtons or kilonewtons.
The kilopond (kp) was a force unit formerly common in German-speaking countries, replaced by the newton in 1978. 1 kp corresponds to the weight force of 1 kilogram at sea level: 1 kp = 9.80665 N. The kilopond was practical because mass and force were numerically equal – a 1 kg mass had a weight force of 1 kp. This intuitive relationship explains why kp is still used colloquially today. Technical pressures used to be given in kp/cm² (technical atmosphere, at).
In the English-speaking system, force is measured in pound-force (lbf). 1 lbf = 4.448222 N is the weight force of one pound (about 454 g). Pound-force is used in US aerospace, mechanical engineering, and for engine thrust ratings. Aircraft thrust is often measured in lbf: a typical jet engine produces 15,000–30,000 lbf of thrust (67–134 kN).
The dyne is the force unit in the outdated CGS system. 1 dyne = 10⁻⁵ N = 0.00001 N – a very small unit. In the CGS system, surface tension is given in mN/m (millinewtons per metre), which used to be expressed in dyn/cm: 1 dyn/cm = 1 mN/m. Biological and chemical applications still sometimes use dynes today, e.g. when measuring cell forces in biophysics. Our force converter covers all common units.