Sigma symbol Σ

• Summation: Σ with limits means “add these terms.” The sum of i from i = 1 to n equals n(n + 1)/2; for n = 10, that is 55.
• Standard deviation: lowercase σ is the population standard deviation, and σ² is the variance. The sample standard deviation is usually written s.
• Six Sigma: the quality-management method takes its name from σ.
• Greek letters on campus: Σ appears in the names of many US fraternities and sororities.
• Greek text: Σ is simply the capital S of the Greek alphabet.

• ∑ (n-ary summation) is a separate math character designed for formulas. It is larger, and it is what math software and LaTeX (\sum) produce. The Greek letter Σ works in plain text, but in typeset formulas ∑ is the better match.
• σ is the regular lowercase sigma.
• ς is the final sigma, used only at the end of a word in Greek, as in λόγος. Don’t use it in math.

On a Mac, Option+W types ∑, the summation sign, not the Greek capital letter. That is usually what you want for math. For Greek words and fraternity or sorority names, copy Σ from this page instead, so the text spells a real Greek letter.

• Word: in an equation (Alt+=), type \sum and press space for the summation operator, \Sigma for the capital letter and \sigma for σ.
• Excel: the AutoSum button on the Home tab shows ∑ and inserts =SUM(). Standard deviation is =STDEV.S() for a sample and =STDEV.P() for a population.
• LaTeX: \sum_{i=1}^{n} i, \Sigma, \sigma, \varsigma.
• Phones: add the Greek keyboard to type Σ, σ and ς, or copy them here.