Calculate relative risk, risk difference, absolute and relative risk reduction and NNT with confidence intervals from cohort or trial 2×2 data.
About this tool
Relative risk, risk difference, ARR, RRR and NNT with confidence intervals.
How to use Relative Risk Calculator
- Enter the numbers with and without the outcome in the exposed and unexposed groups.
- Choose the confidence level.
- Read the relative risk, risk difference and number needed to treat or harm.
Worked example
30 of 100 exposed people and 15 of 100 unexposed people develop the outcome: relative risk 2.0 (95% CI 1.15 to 3.48) and a risk difference of 15%.
What does a relative risk of 2 mean?
The outcome is twice as likely in the exposed group. A value of 1 means no difference and below 1 means lower risk.
Why report the absolute difference too?
A large relative risk can hide a tiny absolute change when the outcome is rare. The risk difference and NNT show the real-world impact.
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