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The slug is a unit of mass associated with British Imperial or United States customary units. It is a mass that accelerates by 1 ft/s2 when a force of one pound-force (lbF) is exerted on it.

With standard gravity gc = 9.80665 m/s2, the international foot of 0.3048 m and the avoirdupois pound of 0.45359237 kg, one slug therefore has a mass of approximately 32.1740lbm or 14.593903 kg.[1] At the surface of the Earth, an object with a mass of 1 slug exerts a force of about 32.17 lbF or 143 N.[2][3]


The slug is part of a subset of units known as the gravitational FPS system, one of several such specialized systems of mechanical units developed in the late 19th and the 20th century. Geepound was another name for this unit in early literature.[4]

The name "slug", as a unit of inertia, was coined before 1900 by British physicist Arthur Mason Worthington,[5] but it did not see any significant use until decades later. A 1928 textbook says:

No name has yet been given to the unit of mass and, in fact, as we have developed the theory of dynamics no name is necessary. Whenever the mass, m, appears in our formulae, we substitute the ratio of the convenient force-acceleration pair (w/g), and measure the mass in lbs. per ft./sec.2 or in grams per cm./sec.2.
Three approaches to mass and force units[6][7]

v · d · e


force, length, time weight, length, time mass, length, time
Force (F) F = m·a = w·a/g F = m·a/gc = w·a/g F = m·a = w·a/g
Weight (w) w = m·g w = m·g/gc ≈ m w = m·g
Acceleration (a) ft/s2 m/s2 ft/s2 m/s2 ft/s2 gal m/s2 m/s2
Mass (m) slug hyl, also called “metric slug” or “TME” lbm kg lb g t kg
Force (F) lb kp lbF kp pdl dyn sn N
Pressure (p) lb/in2 at PSI atm pdl/ft2 Ba pz Pa

The slug is listed in the "Regulations under the Weights and Measures (National Standards) Act, 1960". This regulation defines the units of weights and measures, both regular and metric, in Australia.

Similar units[]

The blob or slinch (a portmanteau of the words slug and inch[8]) is an inch version of the slug (1 slinch = 1 lbf·s2/in = 12 slugs).[9] Slang terms for the slinch include the slugette.[10][11]

Metric units include the "glug" in the centimeter-gram-second system, and the "mug", "par", or "MTE" in the meter-kilogram-second system.[12]


  1. Shigley, Joseph E. and Mischke, Charles R. Mechanical Engineering Design, Sixth ed, pp. 18–19. McGraw Hill, 2006. ISBN 0-07-365939-8.
  2. Shevell, R.S. Fundamentals of Flight, 2nd ed, p. xix. Prentice-Hall, 1989.
  3. Beckwith, Thomas G., Roy D. Marangoni, et al. Mechanical Measurements, Fifth ed, pp. 34-36. Addison-Wesley Publishing, 1993. ISBN 0-201-56947-7.
  4. [1]. unit2unit.eu
  5. Worthington, Arthur Mason (1900). Dynamics of Rotation: An Elementary Introduction to Rigid Dynamics (3rd ed.). Longmans, Green, and Co.. p. 9. http://www.archive.org/stream/dynamicsofrotati00wortuoft#page/8/mode/2up. 
  6. Lindeburg, Michael, Civil Engineering Reference Manual for the PE Exam 
  7. Wurbs, Ralph A, Fort Hood Review Sessions for Professional Engineering Exam, http://engineeringregistration.tamu.edu/tapedreviews/Fluids-PE/PDF/Fluids-PE.pdf, retrieved October 26, 2011 
  8. Miscellaneous
  9. Slug - DiracDelta Science & Engineering Encyclopedia
  10. Celmer, Robert. Notes to Accompany Vibrations II. Version 2.2. 2009.
  11. "1 blob". Wolfram Alpha Computational Knowledge Engine. http://www.wolframalpha.com/input/?i=1+blob&lk=1. Retrieved 27 October 2011. 
  12. Cardarelli, François (1999). Scientific Units, Weights and Measures. Springer. pp. 358, 377. ISBN 185233682X. 

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