Mathematics · Glossary
What is Standard Deviation?
Quick definition
A measure of how spread out numbers are from their mean. Low = consistent, high = volatile.
Full explanation
Standard deviation (σ or s) measures the average distance of values from the mean. A low standard deviation means values cluster tightly; high means they are spread out. For a normal distribution: ~68% of values fall within 1 standard deviation, ~95% within 2, ~99.7% within 3 (the empirical rule). In finance, standard deviation is the most common volatility measure. Stocks with 15% annual standard deviation are considered moderate risk; 30%+ is high. The square of standard deviation is variance. Standard deviation is sensitive to outliers. Always pair it with the mean to get the full picture.
Related calculators
Calculators that use or explain Standard Deviation.
Related terms
More from Mathematics
P-Value
The probability of observing results at least as extreme as those observed, assuming the null hypothesis is true.
Mean (Average)
The sum of values divided by the count. The most common type of "average." Sensitive to outliers.
Median
The middle value in a sorted list. Half the values are above, half below. Robust to outliers.
Pythagorean Theorem
In a right triangle, the square of the hypotenuse equals the sum of the squares of the two legs: a² + b² = c².
Logarithm
The exponent to which a base must be raised to produce a given number. log(100) = 2 means 10² = 100.
Exponent
A number that indicates how many times a base is multiplied by itself. 2³ = 2×2×2 = 8.
Last reviewed: June 15, 2026 • Category: Mathematics