''Guesstimate'' is an informal English

portmanteau In linguistics, a blend—also known as a blend word, lexical blend, or portmanteau—is a word formed by combining the meanings, and parts of the sounds, of two or more words together.

of ''

guess Guessing is the act of drawing a swift conclusion, called a guess, from data directly at hand, which is then held as probable or tentative, while the person making the guess (the guesser) admittedly lacks material for a greater degree of certaint ...

'' and ''

estimate Estimation (or estimating) is the process of finding an estimate or approximation, which is a value that is usable for some purpose even if input data may be incomplete, uncertain, or unstable. The value is nonetheless usable because it is de ...

'', first used by American

statistician A statistician is a person who works with Theory, theoretical or applied statistics. The profession exists in both the private sector, private and public sectors. It is common to combine statistical knowledge with expertise in other subjects, a ...

s in 1934 or 1935.''guesstimate''
Dictionary.com Unabridged (v 1.1) It is defined as an estimate made without using adequate or complete information, or, more strongly, as an estimate arrived at by

work or

conjecture In mathematics, a conjecture is a conclusion or a proposition that is proffered on a tentative basis without proof. Some conjectures, such as the Riemann hypothesis or Fermat's conjecture (now a theorem, proven in 1995 by Andrew Wiles), ha ...

.''guesstimate''
American Heritage Dictionary Like the words estimate and guess, guesstimate may be used as a verb or a noun (with the same change in

pronunciation Pronunciation is the way in which a word or a language is spoken. To This may refer to generally agreed-upon sequences of sounds used in speaking a given word or all language in a specific dialect—"correct" or "standard" pronunciation—or si ...

as estimate). A guesstimate may be a first rough

approximation An approximation is anything that is intentionally similar but not exactly equal to something else. Etymology and usage The word ''approximation'' is derived from Latin ''approximatus'', from ''proximus'' meaning ''very near'' and the prefix ...

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 A pejorative word, phrase, slur, or derogatory term is a word or grammatical form expressing a negative or disrespectful connotation, a low opinion, or a lack of respect toward someone or something. It is also used to express criticism, hosti ...

sense if information for a better estimate is available but ignored. Guesstimation techniques are used: *in

physics Physics is the scientific study of matter, its Elementary particle, fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge whi ...

, where the use of guesstimation techniques to solve

Fermi problem A Fermi problem (or Fermi question, Fermi quiz), also known as an order-of-magnitude problem, is an estimation problem in physics or engineering education, designed to teach dimensional analysis or approximation of extreme scientific calculatio ...

s is taught as a useful skill to science students; *in

cosmology Cosmology () is a branch of physics and metaphysics dealing with the nature of the universe, the cosmos. The term ''cosmology'' was first used in English in 1656 in Thomas Blount's ''Glossographia'', with the meaning of "a speaking of the wo ...

, where the

Drake equation The Drake equation is a probability theory, probabilistic argument used to estimate the number of active, communicative extraterrestrial life, extraterrestrial civilizations in the Milky Way Galaxy.Physics Today 14 (4), 40–46 (1961). The e ...

is a well-known guesstimation method; *in

economics Economics () is a behavioral science that studies the Production (economics), production, distribution (economics), distribution, and Consumption (economics), consumption of goods and services. Economics focuses on the behaviour and interac ...

, where economic forecasts and statistics are often based on guesstimates; and *in

software engineering Software engineering is a branch of both computer science and engineering focused on designing, developing, testing, and maintaining Application software, software applications. It involves applying engineering design process, engineering principl ...

, where new development of features and release timelines are based on effort guesstimates of tasks. Lawrence Weinstein and John Adam's 2009 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 Old Dominion University (ODU) is a Public university, public research university in Norfolk, Virginia, United States. Established in 1930 as the two-year Norfolk Division of the College of William & Mary, it began by educating people with fewer ...

, promotes guesstimation techniques as a useful life skill. It includes many worked examples of guesstimation, including estimating the total number of miles that Americans drive in a year (about 2 trillion) and the amount of high-level nuclear waste that a 1 GW nuclear power plant produces in a year (about 60 tons).Weinstein & Adam (2008) Problem 10.5 __NOTOC__

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The Fermi Project: Collection of guesstimates for practice
{{Webarchive, url=https://web.archive.org/web/20200817075234/https://www.fermiproblems.in/ , date=2020-08-17 American slang Informal estimation Measurement

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