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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) and is deprecated for most uses. The kilogram-force is equal to the magnitude of the force exerted on one kilogram of mass in a 9.80665 m/s gravitational field (standard gravity, a conventional value approximating the average magnitude of gravity on Earth). That is, it is the weight of a kilogram under standard gravity. One kilogram-force is defined as 9.80665 N. Similarly, a gram-force is 9.80665 mN, and a milligram-force is 9.80665 μN.

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/s for this purpose in 1901, 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. Instead, the newton was proposed in 1913 and accepted in 1948. 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, draw weight of bows in archery, and tensile strength of electronics bond wire, 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. In addition, the kilogram force was the standard unit used for Vickers hardness testing.

In 1940s, Germany, the thrust of a rocket engine was measured in kilograms-force, in the Soviet Union it remained the primary unit for thrust in the Russian space program until at least the late 1980s. 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.

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.

See also

• Metrology
• Avoirdupois

References

1. NIST Guide to the SI, Chapter 5: Units Outside the SI
2. The international system of units (SI) Archived 2016-06-03 at the Wayback Machine – United States Department of Commerce, NIST Special Publication 330, 2008, p. 52
3. ^ NIST Guide for the Use of the International System of Units (SI) Special Publication 811, (1995) page 51
4. BIPM SI brochure Archived 2004-06-15 at the Wayback Machine, chapter 2.2.2.
5. Resolution of the 3rd CGPM (1901)
6. Proceedings of the 3rd General Conference on Weights and Measures, 1901, pages 62–64 and 68, (french)
7. Proceedings of the 5th General Conference on Weights and Measures, 1913, pages 51 and 56, (french)
8. "Resolution 7 of the 9th meeting of the CGPM (1948)". Archived from the original on 2020-06-22. Retrieved 2021-03-02.
9. "Balancing wheel tension with the TM-1 Spoke Tension Metre". Cyclingnews. Retrieved 2013-09-03. The recommended tension for spokes in bicycle wheels can be as low as 80 Kilograms force (Kfg) and as high as 230 Kilograms force. Author=Park Tool
10. Harman, George G. (2010). Wire Bonding in Microelectronics (3rd ed.). New York: McGraw-Hill. p. 408. ISBN 978-0-07-164265-1. OCLC 609421363. Breaking load (BL): The strength of a wire and its actual force (usually given in grams, grams-force, mN, etc.) required to break a particular wire in a tensile pull. It is not tensile strength, which by definition is the force per unit area.
11. Callister, William D. Jr. (2010). Materials Science and Engineering: An Introduction. David G. Rethwisch (8th ed.). Hoboken, NJ: John Wiley & Sons, Inc. ISBN 978-0-470-41997-7. OCLC 401168960. In the past the units for Vickers hardness were kg/mm2; in Table 12.6 we use the SI units of GPa.
12. Comings, E. W. (1940). "English Engineering Units and Their Dimensions". Industrial & Engineering Chemistry. 32 (7): 984–987. doi:10.1021/ie50367a028.
13. Klinkenberg, Adrian (1969). "The American Engineering System of Units and Its Dimensional Constant g_c". Industrial & Engineering Chemistry. 61 (4): 53–59. doi:10.1021/ie50712a010.
14. European Economic Community, Council Directive of 18 October 1971 on the approximation of the laws of the Member States relating to units of measurement

• Units of force
• Non-SI metric units