Kilogram-force

"kgf" redirects here. For other uses, see KGF (disambiguation). "Kilopond" redirects here and is not to be confused with Kilopound.

The kilogram-force (kgf or kg_F), or kilopond (kp, from Latin: pondus, lit. 'weight'), is a non-standard gravitational metric unit of force. It is not accepted for use with the International System of Units (SI)^[1] and is deprecated for most uses.^{[citation needed]} The kilogram-force is equal to the magnitude of the force exerted on one kilogram of mass in a 9.80665 m/s² gravitational field (standard gravity, a conventional value approximating the average magnitude of gravity on Earth).^[2] That is, it is the weight of a kilogram under standard gravity. One kilogram-force is defined as 9.80665 N.^[3][4] Similarly, a gram-force is 9.80665 mN, and a milligram-force is 9.80665 μN.

kilogram-force
Unit system	Gravitational metric system
Unit of	Force
Symbol	kgf
Conversions
1 kgf in ...	... is equal to ...
SI units	9.806650 N
CGS units	980,665.0 dyn
British Gravitational units	2.204623 lbf
Absolute English units	70.93164 pdl

History

The gram-force and kilogram-force were never well-defined units until the CGPM adopted a standard acceleration of gravity of 9.80665 m/s² for this purpose in 1901,^[5] though they had been used in low-precision measurements of force before that time. Even then, the proposal to define kilogram-force as a standard unit of force was explicitly rejected.^[6] Instead, the newton was proposed in 1913^[7] and accepted in 1948.^[8] The kilogram-force has never been a part of the International System of Units (SI), which was introduced in 1960. The SI unit of force is the newton.

Prior to this, the units were widely used in much of the world. They are still in use for some purposes; for example, they are used to specify tension of bicycle spokes,^[9] draw weight of bows in archery, and tensile strength of electronics bond wire,^[10] for informal references to pressure (as the technically incorrect kilogram per square centimetre, omitting -force, the kilogram-force per square centimetre being the technical atmosphere, the value of which is very near those of both the bar and the standard atmosphere), and to define the "metric horsepower" (PS) as 75 metre-kiloponds per second.^[3] In addition, the kilogram force was the standard unit used for Vickers hardness testing.^[11]

Three approaches to metric units of mass and force or weight^[12][13]

v t e Base	Force	Weight	Mass
2nd law of motion	m = ⁠F/a⁠	F = ⁠W ⋅ a/g⁠	F = m ⋅ a
System	GM	M	CGS	MTS	SI
Acceleration (a)	m/s2	m/s2	Gal	m/s2	m/s2
Mass (m)	hyl	kilogram	gram	tonne	kilogram
Force (F), weight (W)	kilopond	kilopond	dyne	sthène	newton
Pressure (p)	technical atmosphere	standard atmosphere	barye	pieze	pascal

In 1940s, Germany, the thrust of a rocket engine was measured in kilograms-force,^{[citation needed]} in the Soviet Union it remained the primary unit for thrust in the Russian space program until at least the late 1980s.^{[citation needed]} Dividing the thrust in kilograms-force on the mass of an engine or a rocket in kilograms conveniently gives the thrust to weight ratio, dividing the thrust on propellant consumption rate (mass flow rate) in kilograms per second gives the specific impulse in seconds.

The term "kilopond" has been declared obsolete.^[14]

Related units

The tonne-force, metric ton-force, megagram-force, and megapond (Mp) are each 1000 kilograms-force.

The decanewton or dekanewton (daN), exactly 10 N, is used in some fields as an approximation to the kilogram-force, because it is close to the 9.80665 N of 1 kgf.

The gram-force is 1⁄1000 of a kilogram-force.

Units of force

v t e	newton	dyne	kilogram-force, kilopond	pound-force	poundal
1 N	≡ 1 kg⋅m/s2	= 105 dyn	≈ 0.10197 kp	≈ 0.22481 lbF	≈ 7.2330 pdl
1 dyn	= 10−5 N	≡ 1 g⋅cm/s2	≈ 1.0197×10−6 kp	≈ 2.2481×10−6 lbF	≈ 7.2330×10−5 pdl
1 kp	= 9.80665 N	= 980665 dyn	≡ gn × 1 kg	≈ 2.2046 lbF	≈ 70.932 pdl
1 lbF	≈ 4.448222 N	≈ 444822 dyn	≈ 0.45359 kp	≡ gn × 1 lb	≈ 32.174 pdl
1 pdl	≈ 0.138255 N	≈ 13825 dyn	≈ 0.014098 kp	≈ 0.031081 lbF	≡ 1 lb⋅ft/s2
The value of gn (9.80665 m/s2) as used in the official definition of the kilogram-force is used here for all gravitational units.

See also

• Metrology
• Avoirdupois