Convert force

About the units

• Newtons (N) – The newton is the SI coherent unit of force: the force that gives a mass of one kilogram an acceleration of one metre per second squared. The International Committee for Weights and Measures put the definition in 1946, and the unit is legal throughout the EU and the one European structural and materials engineering counts in. A kilogram weighs 9.80665 newtons under standard gravity, so a hundred-gram apple weighs about one newton. The trap is that force and mass are different quantities.
• Kilonewtons (kN) – A kilonewton is 1,000 newtons and a legal unit throughout the EU. It measures large forces in engineering and safety equipment. Under EN 12275, climbing karabiners must have their strength marked on the body in kilonewtons, and a basic karabiner holds at least 20 kilonewtons along its major axis with the gate closed but only 7 with it open. One kilonewton is the weight of 102 kilograms under standard gravity, so 20 kilonewtons is just over two tonnes. The trap is that force and mass are different quantities.
• Kilograms-force (kgf) – A kilogram-force is the weight of one kilogram under standard gravity of 9.80665 metres per second squared, and therefore exactly 9.80665 newtons. That figure was fixed by the General Conference on Weights and Measures as early as 1901. The unit is now called obsolete, is absent from the EU units directive and is not legal in trade, but it survives on older machine plates and on gym equipment marked kp. Kilopond and kilogram-force are the same thing, and the number matches the mass.
• Pounds-force (lbf) – A pound-force is the weight of an avoirdupois pound under standard gravity: 0.45359237 times 9.80665, exactly 4.4482216152605 newtons. Both figures are exact, so the unit is too. It is absent from the EU units directive but is used in American and British engineering for tractive force, spring rates, bolted joints and rocket thrust in “pounds of thrust”. One newton is 0.2248 pound-force. The trap is that English writes both mass and force as “pound” – always write lbf when force is meant.
• Dynes (dyn) – The dyne is the CGS unit of force, giving one gram an acceleration of one centimetre per second squared: exactly 10⁻⁵ newtons. The EEC units directive of 1971 put the CGS units under review, and today’s EU directive omits it. The dyne lives on in surface tension: plastic film is tested for printing in dynes per centimetre, and ASTM D2578 gives 35 as acceptable for flexographic printing on certain polyethylene film. One dyne per centimetre is exactly one millinewton per metre, so the figure is the same in SI.