The unit of mass in the British gravitational system of units, approximately 14.59390 kilograms. Symbol¹, slug. One slug is a mass such that a 1-pound force acting on it will produce an acceleration of 1 foot per second per second. Also called the geepound. This unit, rather than the pound (as a unit of mass), is used as the system's unit of mass in order to make the system coherent.

The slug was never used much outside of textbooks, which in one respect was unfortunate because using either it or the poundal would have helped to dispel the confusion between the pound as mass and the pound as force. One slug = 32.1740 pounds (as mass).

1. IEEE Std 260.1-2004.
IEEE Standard Letter Symbols for Units of Measurement (SI Units, Customary Inch-Pound Units, and Certain Other Units).
Approved 22 July 2004, American National Standard Institute; approved 25 March 2004, IEEE-SA Standards Board.
New York: IEEE, 24 September 2004.


Consider first the British engineering system of units, in which forces are expressed in pounds and acceleration in ft/sec2. In this system, the mass of a body is expressed in lb/(ft/sec²). It is often convenient to introduce a single term for the combination of units in which a physical quantity is expressed, and the unit above, 1 lb/(ft/sec²), is called one slug. (This term arose from the concept of mass as inertia or sluggishness.)

Francis Weston Sears and Mark W. Zemansky.
University Physics. Complete Edition. 2nd ed.
Reading, MA: Addison-Wesley, 1955.
Page 76.

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