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Guesstimate is an informal English portmanteau of guess and estimate, first used by American statisticians in 1934[1] or 1935.[2] It is defined as an estimate made without using adequate or complete information,[3][4] or, more strongly, as an estimate arrived at by guesswork or conjecture.[2][5][6] Like the words estimate and guess, guesstimate may be used as a verb or a noun (with the same change in pronunciation as estimate). A guesstimate may be a first rough approximation pending a more accurate estimate, or it may be an educated guess at something for which no better information will become available.

The word may be used in a pejorative sense if information for a better estimate is available but ignored.[7][8]

Guesstimation techniques are used:

  • in physics, where the use of guesstimation techniques to solve Fermi problems is taught as a useful skill to science students.[9]
  • in cosmology, where the Drake equation is a well-known guesstimation method.[10]
  • in economics, where economic forecasts and statistics are often based on guesstimates.[11]
  • in software engineering, where new development of features and release timelines are based on effort guesstimates of tasks.

Lawrence Weinstein and John Adam's book Guesstimation: Solving the World's Problems on the Back of a Cocktail Napkin, based on the course "Physics on the Back of an Envelope" at Old Dominion University, promotes guesstimation techniques as a useful life skill. It includes many worked examples of guesstimation, including the following problems:

  • How many total miles do Americans drive in a year?
Answer: about 2 trillion (2x1012).[12]
  • How much high-level nuclear waste does a 1 GW nuclear power plant produce in a year?
Answer: about 60 tons.[13]

See also[edit]


  1. ^ guess Online Etymological Dictionary
  2. ^ a b guesstimate Unabridged (v 1.1)
  3. ^ guesstimate Merriam-Webster On-line Dictionary
  4. ^ guesstimate MSN Encarta Dictionary. Archived 2009-10-31.
  5. ^ guesstimate Archived 2008-03-16 at the Wayback Machine American Heritage Dictionary
  6. ^ Compact Oxford English Dictionary guesstimate
  7. ^ "Guesstimate with confidence using confidence intervals" from back cover of Statistics for Dummies
  8. ^ Guesstimate; Grades 4-6 NTTI Lesson Plan
  9. ^ Guesstimation: Solving the World's Problems on the Back of a Cocktail Napkin, Tony Mann, Times Higher Education Supplement
  10. ^ The Drake Equation Archived 2009-09-26 at the Wayback Machine
  11. ^ Economic outlooks often rely on guesstimation, M. Ray Perryman, San Antonio Business Journal
  12. ^ Weinstein & Adam (2008) Problem 5.1
  13. ^ Weinstein & Adam (2008) Problem 10.5


  • Weinstein, Lawrence; Adam, John A. (2008). Guesstimation: Solving the World's Problems on the Back of a Cocktail Napkin. Princeton University Press. ISBN 978-0-691-12949-5.