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Fisher Exact Test Calculator

Enter the four counts in a 2×2 table to calculate Fisher’s exact test with fixed margins.

Observed 2×2 count table

Choose a one-sided direction only when specified before examining the data. The odds ratio is the uncorrected cross-product ratio; zero cells may make it undefined or infinite. This is not a causal conclusion.

Counts must be whole numbers. Maximum 10,000 per cell and 20,000 total. The two-sided test sums tables no more probable than the observed table under fixed margins.

When should I use Fisher’s exact test?

It is an exact test of association for a 2×2 table, often used when expected counts are small. The result depends on the study design and does not prove causation.

Frequently asked questions

When should I use Fisher’s exact test?

It is an exact test of association for a 2×2 table, often used when expected counts are small. The result depends on the study design and does not prove causation.

Are my counts uploaded?

No. The calculation runs in this browser. Counts are not uploaded or stored by this tool.

What is the null hypothesis?

The test evaluates whether the row and column classifications are independent, conditional on the observed margins. It does not explain why an association may exist.

How is a two-sided p-value calculated?

This calculator adds probabilities of all possible tables with the same margins whose conditional probability is no greater than that of the observed table. Other software conventions can differ.

Does a small p-value prove an effect?

No. A p-value is not an effect size or proof of causation. Review the odds ratio, study design, data quality, uncertainty and any multiple-testing plan.