Sigma
The sigma symbol has three forms: capital Σ, lowercase σ and the word-final ς. Tap one to copy it, then see its codes, how to type it, what sigma means in each field and the lookalikes to avoid.
By Maya Chen, lettering designer and type educator · Checked by Jordan Hale · · How we review content
Copy sigma
One tap copies one character. Each form below has its own code point.
Key facts
- Sigma is the 18th letter of the Greek alphabet, with a numeral value of 200.
- Lowercase sigma has two shapes, ς at the end of a word (U+03C2) and σ everywhere else (U+03C3), while capital Σ (U+03A3) has one.
- In statistics, lowercase σ is the population standard deviation and σ² the variance, while the sample standard deviation is usually written s.
- Capital Σ (U+03A3) is a letter, and the summation sign ∑ (U+2211) is a separate operator character even where fonts draw them alike.
- Stigma ϛ (U+03DB), the numeral letter for 6, can look like final sigma ς in many fonts but is a different character.
Codes for every form
Unicode names and code points for each form, with the escapes you need in HTML, CSS, JavaScript and LaTeX.
| Form | Unicode name | Code point | Decimal | HTML | CSS / JavaScript | LaTeX |
|---|---|---|---|---|---|---|
| sigma, capital | GREEK CAPITAL LETTER SIGMA | U+03A3 | 931 | Σ Σ | \03A3 / \u03A3 | \Sigma |
| sigma, lowercase | GREEK SMALL LETTER SIGMA | U+03C3 | 963 | σ σ | \03C3 / \u03C3 | \sigma |
| final sigma, lowercase, used at the end of a word | GREEK SMALL LETTER FINAL SIGMA | U+03C2 | 962 | ς ς | \03C2 / \u03C2 | \varsigma |
| lunate sigma, lowercase | GREEK LUNATE SIGMA SYMBOL | U+03F2 | 1010 | ϲ | \03F2 / \u03F2 | none |
| lunate sigma, capital | GREEK CAPITAL LUNATE SIGMA SYMBOL | U+03F9 | 1017 | Ϲ | \03F9 / \u03F9 | none |
How to type sigma
The quick methods for this letter. Step-by-step instructions for each system, and what we have and have not tested, are on how to type Greek letters.
| Method | What to do |
|---|---|
| Copy and paste | Use the buttons at the top of this page. Pasted text is plain Unicode. |
| Word on Windows | Type the code, then press Alt+X: 03A3 for Σ; 03C3 for σ; 03C2 for ς; 03F2 for ϲ; 03F9 for Ϲ. Windows steps. |
| Excel and Google Sheets | =UNICHAR(931) for Σ; =UNICHAR(963) for σ; =UNICHAR(962) for ς; =UNICHAR(1010) for ϲ; =UNICHAR(1017) for Ϲ. Office steps. |
| Windows Alt code | Hold Alt and type on the numeric keypad: 229 gives σ. From the old code page 437 table; not tested by us on a live Windows machine. |
| Phone | Add a Greek keyboard: iPhone and iPad or Android. |
| Web page or LaTeX | Σ, \Sigma for Σ; σ, \sigma for σ; ς, \varsigma for ς. LaTeX commands work in math mode. |
Characters that look like sigma
These are separate characters, so each has its own code point. Copy the one that matches the job.
How to say sigma
| Reading | Value |
|---|---|
| English (maths and science) | SIG-muh, /ˈsɪɡmə/ |
| Modern Greek | The letter is /s/. Before μ, ν, β, δ and γ it is voiced to /z/, so σμ sounds like "zm". |
| Classical Greek (reconstructed) | /s/ |
English speakers say the name "SIG-muh" in maths and science. The Greek name is σίγμα. The sound rule for modern Greek comes from the Wikipedia article on sigma; classical Greek is reconstructed, so treat that value as the scholarly consensus rather than a recording. See the pronunciation page for the whole alphabet.
What sigma means, field by field
"Standard" marks a definition or notation that is fixed, such as an SI unit or a named function. "Convention" marks common practice that varies by author, so check the definitions in your own text. The same letter often means different things in different fields; the full table for every letter is on Greek letters in math and science.
Mathematicssymbol ΣStandard
Capital sigma stands for a sum. Σ from i = 1 to n of xᵢ means x₁ + x₂ + ... + xₙ. In typeset maths the operator form ∑ is drawn larger so it can carry limits above and below.
Number theorysymbol σ(n)Convention
The sum-of-divisors function. σ(6) = 1 + 2 + 3 + 6 = 12. Here σ is a function name, not a spread.
Statisticssymbol σ, σ²Convention
The population standard deviation and the variance. For a normal distribution, about 68 percent of values lie within one σ of the mean, 95 percent within two and 99.7 percent within three. The sample standard deviation is usually written s.
Physicssymbol σStandard
The Stefan-Boltzmann constant, about 5.670 × 10⁻⁸ W m⁻² K⁻⁴, relates radiated power to temperature. In particle physics σ is also a cross-section, measured in barns.
Electrical engineeringsymbol σConvention
Electrical conductivity in siemens per metre, the reciprocal of resistivity ρ. Some texts use γ for the same quantity.
Mechanicssymbol σConvention
Normal stress, force per area in pascals, with τ for shear stress. Tension is usually positive in engineering texts and sign conventions differ in other fields.
Chemistrysymbol σ bondStandard
A sigma bond forms by head-on overlap of orbitals along the line between two nuclei. It is the first bond between two atoms; π bonds are added on top for double and triple bonds.
Financesymbol σConvention
Volatility, the standard deviation of returns over a period. It uses the statistics meaning, applied to price changes.
Lookalikes and common mistakes
ς and σ are one letter
Use ς only at the end of a word and σ everywhere else. Greek text such as Ὀδυσσεύς has σσ in the middle and ς at the end. If you replace letters one by one in code you will get this wrong. Python str.lower() and JavaScript toLowerCase() apply the rule themselves when they lowercase Σ.
ς is not ϛ
Stigma ϛ (U+03DB) is the numeral letter for 6. In many fonts it looks like final sigma, and a pasted one will not match a search for ς. Copy from the buttons above rather than from a numeral chart.
Σ is not ∑
The summation operator ∑ (U+2211) and the letter Σ (U+03A3) look the same in some fonts and different in others. Use ∑ for a sum in a formula and Σ when sigma is a letter, such as in a name or a variable.
The lunate forms pose as C
Ϲ and ϲ are shaped like Latin C and Cyrillic С but have their own code points. Text that mixes them will not sort or search as you expect. If you only need the standard letter, use Σ σ ς.
Case-insensitive search needs folding
ς, σ and Σ are three different code points. Folding with casefold() (Python) or a locale-aware comparison treats ς and σ as equal, but a plain character comparison does not, so a code-point comparison of "οδυσσευσ" (with σ at the end) against "οδυσσευς" fails even though they are the same word apart from the ending rule.
Where does the sigma symbol come from?
Sigma is the 18th letter of the Greek alphabet and has a numeral value of 200. It descends from the Phoenician letter shin, and the Latin S descends from sigma. The Cyrillic С traces back to the lunate form, which explains why the C-shaped Ϲ looks the way it does (Wikipedia, Sigma).
That history is the reason sigma behaves differently from most letters in text. It has three shapes in everyday use (Σ, σ and ς), a C-shaped variant and several maths symbols that borrow its outline.
How do you read a sigma in a formula?
Check where the sigma sits. A large Σ with limits (i = 1 below, n above) is a sum. A lowercase σ with a subscript, such as σₓ, is a spread: the standard deviation of x. A σ standing alone next to a number and a unit, such as 5.67 × 10⁻⁸ W m⁻² K⁻⁴, is a physical constant. And σ(n) next to an integer is the divisor function. The same letter works as an operator, a measure and a constant, so the context and the units tell you which.
If you are writing, define your symbols on first use. A footnote saying “σ denotes the population standard deviation” costs one line and prevents the question from your reader.
How does sigma behave in software?
Sigma is the main case-mapping gotcha in Greek. Lowercasing the capital string ΟΔΥΣΣΕΥΣ gives οδυσσευς in both Python and JavaScript, with the final ς correct, because both follow the Unicode rule. Going the other way is simpler: ς, σ and Σ all uppercase to Σ.
For comparison, use casefolding. In Python, "ς".casefold() returns “σ”, so a casefolded search treats the two lowercase forms as equal.
Sources
These sources back the facts on this sigma symbol page. Sigma’s meaning shifts from field to field, so go by the definition your text sets out.
- Wikipedia, Sigma (history, final form, sound values, uses)
- Unicode Greek and Coptic code chart (PDF)
- NIST, CODATA fundamental physical constants (Stefan-Boltzmann constant)
- Python documentation, str.casefold
- Code points, names and the final-sigma behaviour of
str.lower()andtoLowerCase()were checked with Python 3 and Node.js.
Frequently asked questions
Basics
Why does sigma have two lowercase forms?
What does the sigma symbol mean in statistics?
Where does the sigma symbol come from?
Where it works
What does the sigma symbol mean in physics and chemistry?
How it works
How do I type σ, Σ and ς?
How do I lowercase Σ correctly in Python or JavaScript?
Tips & limits
What is the difference between Σ and the ∑ sum sign?
What is a lunate sigma?
Is final sigma ς the same as stigma ϛ?
Related Greek pages and symbols
- Greek Letters in Math and ScienceGreek letters in math and science are conventional symbols: π is the circle constant, Δ a change, σ a standard deviation, λ a wavelength and Ω the SI ohm. Only a few meanings are fixed by standards, so they vary by field and author. Look up a letter below; each one lists its meanings grouped by field.
- How to Type Greek LettersHow to type Greek letters on any device: add a Greek keyboard on Windows, Mac, iPhone or Android, or use Alt+X in Word and UNICHAR in Excel for a single character. Copying from the alphabet page works everywhere. Each method notes what we verified and which traps to avoid.
- Greek Alphabet UnicodeThe Greek alphabet Unicode block, Greek and Coptic (U+0370 to U+03FF), holds all 24 letters: capitals at U+0391 to U+03A9 and lowercase at U+03B1 to U+03C9. This page lists each code point, HTML entity and LaTeX command, plus the micro sign, ohm sign and homoglyph traps.
- Math SymbolsTap a symbol to copy it. Every symbol shows its Unicode name and code point, and the page explains which lookalikes to avoid.
- Greek Letter GeneratorA Greek font generator that outputs real Greek letters, not a font: type a name or word and the tool spells its sounds in Unicode Greek, with final sigma handled. Copy it anywhere. Look-alike Greek-style text sits below, clearly marked as not Greek.
- Greek AlphabetThe Greek alphabet has 24 letters, from alpha to omega, each with an uppercase and a lowercase form. Tap any letter to copy it, check its name, sound and numeric value in the table, or print a clean one-page chart. This page treats the letters as characters and symbols, not as a language course.