How to Use STDEV and VAR in Excel

Introduction

Two production lines average the same output per day. One holds steady, the other swings from 25 to 60. AVERAGE can't tell them apart. STDEV and VAR measure the spread around that average, which is often the more important number.

The Dataset

Here's a week of daily output from two packaging lines at a small fulfillment center:

	A	B	C
3	Day	Line A	Line B
4	Monday	50	42
5	Tuesday	30	46
6	Wednesday	55	40
7	Thursday	25	48
8	Friday	60	44

Both lines average 44 units per day. But Line A bounces between 25 and 60, while Line B stays in a tight window around the mean. If you're planning tomorrow's staffing based on the average alone, Line A is going to surprise you.

STDEV

Standard deviation measures how far values typically fall from the average. The syntax is:

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

• number1: A range or set of values.

To measure the spread for Line A:

=STDEV.S(B4:B8)

The result is 15.6. On average, Line A's daily output lands about 15.6 units away from the mean of 44. That's a wide swing for a line that's supposed to produce predictable volume.

For Line B:

=STDEV.S(C4:C8)

The result is 3.2. Line B's output stays within about 3 units of the average on any given day.

Line	Average	STDEV.S
A	44	15.6
B	44	3.2

Same average, completely different story. Line A needs flexible staffing or buffer inventory. Line B is predictable enough to plan around.

Key takeaway: STDEV tells you how much your data moves around the average. A low number means consistency. A high number means the average is hiding real variation.

STDEV.S vs STDEV.P

Excel offers two versions:

• STDEV.S (sample) divides by n-1. Use it when your data is a sample of a larger population, which is almost always the case. Five days of production data is a sample of all the days the line has run and will run.
• STDEV.P (population) divides by n. Use it when you have every single data point that exists, like final exam scores for an entire class with no absent students.

Line	STDEV.S	STDEV.P
A	15.6	13.9
B	3.2	2.8

The gap between the two shrinks as the dataset grows. With hundreds of rows, they're nearly identical. With five data points the difference is noticeable, but the interpretation stays the same: Line A swings, Line B doesn't.

If you're not sure which to use, go with STDEV.S.

VAR

Variance is standard deviation before you take the square root. The numbers get bigger, but they tell the same story. The syntax is:

=VAR.S(number1, [number2, ...])

=VAR.S(B4:B8)

The result for Line A is 242.5. For Line B, it's 10.

Line	STDEV.S	VAR.S
A	15.6	242.5
B	3.2	10

Variance squares the units, which makes the raw numbers harder to interpret. If your output is in units per day, variance is in "units-per-day squared," and that doesn't mean much at a glance. Variance shows up when you need to combine spread from multiple sources or feed a statistical model, but for everyday analysis, STDEV is the one you'll reach for.

Like STDEV, variance comes in sample and population flavors: VAR.S (divides by n-1) and VAR.P (divides by n). Same rule: VAR.S unless you have the full population.

Conclusion

AVERAGE tells you the center. STDEV tells you how much to trust it. When two datasets share the same average but behave nothing alike, standard deviation is what separates them.

For other ways to look beyond the average, see our guide on MEDIAN, PERCENTILE, and QUARTILE. For basic aggregation, check out our guide on SUM, AVERAGE, and COUNT.