Rytvae Consulting · Think Right · Invest Right
Standard Deviation Calculator
Result
Chart
Show formula & working
Mean = Σx ÷ n · Sample variance = Σ(x − mean)² ÷ (n − 1) · Population variance = Σ(x − mean)² ÷ n · SD = √variance
Annualised SD = SD × √(periods a year) · Annualised mean = mean × periods a year
Details
How this works
Standard deviation measures how widely returns swing around their average — the usual measure of volatility.
Monthly figures are annualised by multiplying by √12 (the square-root-of-time rule), which assumes each period is independent of the last.
Frequently asked questions
Sample or population?
Sample (n − 1) for a slice of history — the usual case for investment returns. Population (n) only when you have every value there is.
Does this match Excel?
Yes: sample = STDEV.S, population = STDEV.P; variance = VAR.S / VAR.P.
Is a higher SD bad?
It means a bumpier ride, not necessarily a worse result. Compare it with the return earned — see the Sharpe ratio.