gc (engineering)

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In engineering, gc is a unit conversion factor used to convert mass to force or vice versa.[1] It is defined as

In unit systems where force is a derived unit, like in SI units, gc is equal to 1 and is not needed. In unit systems where force is a primary unit, like in English units, gc is not equal to 1, and is required to obtain correct results.[2]


According to Newton's law of motion, the force F is proportional to mass m and acceleration a

or in general

If F = 1 lbf, m = 1 lb, and a = 32.2 ft/s2, then

Leading to

gc is defined as the reciprocal of the constant K

or equivalently, as

Specific systems of units[edit]

International System English System British Customary System
gc = 1 (kg·m)/(N·s2) gc = 32.2 (lb·ft)/(lbf·s2) gc = 1 (slug·ft)/(lbf·s2)


  1. ^ Janna, William (2015). Introduction to Fluid Mechanics (5th ed.). CRC Press. p. 5. ISBN 9781482211610.
  2. ^ Cengel, Yunus; Boles, Michael (2014). Thermodynamics: An Engineering Approach (8th ed.). McGraw-Hill Education. p. 6. ISBN 9780073398174.