Relative Risk & Odds Ratio Calculator

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Updated October 27, 2026
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How to use this calculator

Fill in the 2×2 table: how many people in the exposed and unexposed groups had or did not have the outcome.

Outcome No outcome
Exposed a b
Unexposed c d

Formulas

RR = [a/(a+b)] ÷ [c/(c+d)] OR = ad ÷ bc

Worked example

30 of 100 exposed and 10 of 100 unexposed people developed the outcome:

  • Risk exposed = 30%, risk unexposed = 10%
  • RR = 3.00
  • OR = 30 × 90 ÷ (70 × 10) = 3.86
  • Absolute risk difference = 20 percentage points (NNH = 5)

Frequently asked questions

What is relative risk?

The risk of the outcome in the exposed group divided by the risk in the unexposed group. RR = 3 means three times the risk.

What is the difference between RR and OR?

RR compares probabilities; OR compares odds. They are similar when the outcome is rare, but OR exaggerates the effect when the outcome is common.

What is the number needed to treat (NNT)?

1 ÷ absolute risk difference. If a treatment reduces risk by 20 percentage points, NNT = 5.

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