Relative Risk Calculator with 95% Confidence Interval
Calculate Relative Risk (RR) and 95% confidence interval from a 2×2 contingency table. For cohort studies, RCTs, and intervention evaluations.
Relative Risk (RR) - also called Risk Ratio - compares the probability of an outcome between an exposed group and an unexposed group. It is the primary effect measure for cohort studies and randomised controlled trials. Enter the four cells of your 2×2 table below; the calculator returns RR with 95% CI, Risk Difference, NNT/NNH, Attributable Risk, and a chi-square significance test.
Enter counts from your 2×2 contingency table. All four cells must be whole numbers ≥ 1.
| Outcome + | Outcome − | Total | |
|---|---|---|---|
| Exposed | — | ||
| Unexposed | — | ||
| Total | — | — | — |
| RR value | Interpretation |
|---|---|
| RR = 1.0 | No association - outcome risk identical in both groups |
| RR > 1.0 | Increased risk in exposed group |
| RR < 1.0 | Protective effect - reduced risk in exposed group |
| 95% CI includes 1.0 | Not statistically significant at p < 0.05 |
| NNT (positive RD) | Number needed to treat - one additional outcome per NNT exposed |
| NNH (negative RD) | Number needed to harm - one extra adverse outcome per NNH exposed |
Source: Szklo M, Nieto FJ. Epidemiology: Beyond the Basics, 4th ed. Jones & Bartlett Learning, 2019. Chi-square p-value approximated via incomplete gamma function (Abramowitz & Stegun method).