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Population SD: σ = √(Σ(xᵢ−μ)²/N). Sample SD: s = √(Σ(xᵢ−x̄)²/(N−1)). Divide by N−1 (Bessel's correction) for samples to get an unbiased variance estimate. SD measures spread around the mean—useful alongside mean for comparing datasets. For normal distributions: 68% of data within ±1SD, 95% within ±2SD, 99.7% within ±3SD (empirical rule). Outliers inflate SD significantly.
σ / s = 2.138090