Mu
Mu (Μ, μ) is the twelfth letter of the Greek alphabet. μ is the metric prefix micro, one millionth (μg is a microgram); the mean in statistics; the coefficient of friction; and magnetic moment and the muon in physics.
How to read it
Mu is said MYOO in Britain and MOO in America (Cambridge Dictionary: /mjuː/ and /muː/). The University of Arizona’s guide gives its Modern Greek name as Μύ, “mi”, with an m sound.
Also searched as: micro symbol, mu letter, mu symbol, μ, Μ, μ meaning.
Capital, small and look-alike forms
| Form | Character | Code point | Unicode name |
|---|---|---|---|
| Capital | Μ | U+039C | Greek capital letter mu |
| Small | μ | U+03BC | Greek small letter mu |
| Micro sign | µ | U+00B5 | Micro sign |
The capital Μ looks like the Latin M (Cambridge Dictionary gives the symbols M and μ). The micro sign µ is a separate character from the Latin-1 set that looks the same as the small mu; Unicode cross-refers each to the other.
Mu as micro
μ is the SI prefix micro, 10⁻⁶ or one millionth, as in μg for a microgram (NIST, Metric (SI) Prefixes).
Mu in statistics
μ is the mean of a normal distribution. Jeff Miller’s history of mathematical symbols says that use was surprisingly late to settle: R. A. Fisher wrote m from 1912 and switched to μ in the 1936 edition of Statistical Methods for Research Workers.
In AI and machine learning
Google’s Machine Learning Crash Course normalises a feature to its Z-score with x′ = (x − μ)/σ, where μ is the mean and σ the standard deviation. PyTorch’s stochastic gradient descent optimiser uses μ for momentum, which carries part of each step into the next (bₜ = μbₜ₋₁ + …).
In quantum physics
μ is magnetic moment. OpenStax’s University Physics gives the orbital magnetic dipole moment of an electron’s current loop as μ = IA, the current times the loop’s area. The Bohr magneton is written μ_B; NIST’s 2022 CODATA value is 9.274 010 0657 × 10⁻²⁴ joules per tesla. μ is also the muon, a particle over 200 times heavier than the electron but otherwise like it (OpenStax).
Other uses of mu
In mechanics μ is the coefficient of friction: μₛ for static friction and μₖ for kinetic (OpenStax, University Physics). In electromagnetism μ₀ is the vacuum magnetic permeability, 1.256 637 061 27 × 10⁻⁶ N A⁻² in NIST’s 2022 CODATA values.
More greek letters
Browse symbols drawn with letters and numbers.
Sources
- Greek and Coptic: Range 0370–03FF (code chart), The Unicode Standard, Version 18.0, Unicode Consortium, Greek and Coptic, pp. 39–41
- C1 Controls and Latin-1 Supplement (code chart, Range 0080–00FF), The Unicode Standard, Version 18.0, Unicode Consortium, C1 Controls and Latin-1 Supplement, 00B5
- mu (Cambridge Dictionary entry), Cambridge Advanced Learner’s Dictionary & Thesaurus, Cambridge University Press, mu, noun
- mu (American Heritage Dictionary entry), The American Heritage Dictionary of the English Language, Fifth Edition, HarperCollins Publishers, mu, n.
- Modern Greek Alphabet And Phonetics, University of Arizona, Center for Middle Eastern Studies, Letter table
- Metric (SI) Prefixes, U.S. National Institute of Standards and Technology, SI prefixes table
- Earliest Uses of Symbols in Probability and Statistics, Jeff Miller, Earliest Uses of Various Mathematical Symbols, MacTutor History of Mathematics, University of St Andrews, Moments; the mean of the normal distribution
- Numerical data: Normalization, Machine Learning Crash Course, Google for Developers, Z-score scaling
- torch.optim.SGD, PyTorch 2.14 documentation, PyTorch Foundation, Algorithm
- University Physics Volume 1, 6.2 Friction, OpenStax (Rice University), Equations 6.1 and 6.2
- University Physics Volume 3, 8.2 Orbital Magnetic Dipole Moment of the Electron, OpenStax (Rice University), Equation 8.13
- CODATA Value: Bohr magneton, 2022 CODATA recommended values, U.S. National Institute of Standards and Technology, Value
- CODATA Value: vacuum magnetic permeability, 2022 CODATA recommended values, U.S. National Institute of Standards and Technology, Value
- University Physics Volume 3, 11.1 Introduction to Particle Physics, OpenStax (Rice University), Section 11.1, leptons
Last reviewed September 30, 2026 by Rory Hansen. How we research