Excel STDEV Function — Measure Data Spread (with Examples)

What does it do?

STDEV calculates the standard deviation of a sample — a measure of how spread out values are from their mean. A low standard deviation means values cluster closely around the average. A high one means they are scattered widely. Used in quality control, finance, performance analysis, and academic research.

Syntax

=STDEV(number1, [number2], ...)

Argument	Description
=STDEV(number1, [number2], ...)
number1 required	The first value or range. Usually a range like B2:B100.
[number2] ... optional	Additional ranges. Up to 255 arguments.

Real examples

Example 1

Basic standard deviation

=STDEV(B2:B100)

Measures spread in a sample dataset.

Example 2

Flag values more than 2 standard deviations from mean

=IF(ABS(B2-AVERAGE(B$2:B$100))>2*STDEV(B$2:B$100),"Outlier","")

Identifies statistical outliers (Z-score method).

Example 3

Coefficient of variation (relative spread)

=STDEV(B2:B100)/AVERAGE(B2:B100)

Normalises spread — useful for comparing variability across different scales.

Example 4

Process control limit (UCL)

=AVERAGE(B2:B100)+3*STDEV(B2:B100)

Upper control limit — values above this trigger a quality review.

Example 5

Volatility of investment returns

=STDEV(C2:C13)*SQRT(12)

Monthly STDEV annualised by multiplying by square root of 12.

FAQ

What is the difference between STDEV and STDEVP?

STDEV calculates standard deviation for a sample (most common — you have a subset of all possible data). STDEVP calculates for the entire population. Use STDEV unless you genuinely have every possible data point.

What does a standard deviation of 0 mean?

All values are identical. There is no spread.

How do I interpret standard deviation practically?

About 68% of values fall within 1 standard deviation of the mean. About 95% fall within 2. About 99.7% fall within 3. Values beyond 3 standard deviations are strong outliers.

Standard deviation in quality control and finance

In quality control, standard deviation tells you how consistent a process is. A manufacturing line producing components with low standard deviation is a consistent, reliable process. High standard deviation means variability — some components are too large, some too small. The goal in quality management is to reduce standard deviation, not just hit a target average.

In finance, standard deviation of returns is the most common measure of investment risk. A portfolio with a standard deviation of 20% is riskier (more volatile) than one with 10%. The Sharpe ratio — (return - risk free rate) / standard deviation — uses STDEV as its risk measure. Higher Sharpe ratio means better return per unit of risk.

The 68-95-99.7 rule is useful for interpreting STDEV results. In a normal distribution, 68% of values fall within one standard deviation of the mean, 95% within two, and 99.7% within three. Values beyond three standard deviations are statistically unusual — they warrant investigation as outliers or errors.

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Related formulas

AVERAGE PERCENTILE CORREL LARGE VAR MEDIAN

The Excel STDEV Functionexplained simply