F-Test Calculator
Compare two variances with F, df and the p-value.
Enter your values
Enter the sample 1 sd you are working with.
Enter the sample 1 size you are working with.
Enter the sample 2 sd you are working with.
Enter the sample 2 size you are working with.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- F statistic
- 2.25
- p-value (two-tailed)
- 0.09871
- Verdict
- Marginal (10% level)
- Degrees of freedom
- 19, 17
- Variance 1
- 144
- Variance 2
- 64
- Equal-variance assumption
- Reasonable — variances are similar
Quick answer
The F-test compares two variances as a ratio: F = larger variance ÷ smaller variance, tested against the F distribution.
F = s₁² ÷ s₂²
At a glance
| What it does | Compare two variances with F, df and the p-value. |
|---|---|
| Category | Statistics |
| Inputs needed | Sample 1 SD, Sample 1 size, Sample 2 SD, Sample 2 size |
| Main output | F statistic |
| Formula | F = s₁² ÷ s₂² |
| Cost | Free — no sign-up, no download |
How to use the F-Test Calculator
- 1Enter each sample's standard deviation and size.
- 2The calculator always puts the larger variance on top.
- 3Use the verdict to decide between the pooled and Welch versions of the t-test.
Inputs explained
- Sample 1 SD
- Enter the sample 1 sd you are working with.
- Sample 1 size
- Enter the sample 1 size you are working with.
- Sample 2 SD
- Enter the sample 2 sd you are working with.
- Sample 2 size
- Enter the sample 2 size you are working with.
Worked example
Using the values the calculator loads with:
Inputs
- Sample 1 SD12
- Sample 1 size20
- Sample 2 SD8
- Sample 2 size18
Results
- F statistic2.25
- p-value (two-tailed)0.09871
- VerdictMarginal (10% level)
- Degrees of freedom19, 17
- Variance 1144
- Variance 264
- Equal-variance assumptionReasonable — variances are similar
Frequently asked questions
Is the F-test sensitive to non-normality?
Very. If your data are skewed, prefer Levene's or Brown-Forsythe test, or just default to Welch's t-test.
Do I even need this test?
Often not — Welch's t-test handles unequal variances automatically, so many statisticians skip the pre-test entirely.
What do I need to enter into the F-Test Calculator?
Just 4 values: sample 1 sd, sample 1 size, sample 2 sd, sample 2 size. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the F-Test Calculator work out the answer?
The F-test compares two variances as a ratio: F = larger variance ÷ smaller variance, tested against the F distribution. It applies the formula F = s₁² ÷ s₂² and shows the working so you can check each step by hand.
Is the F-Test Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.