Number Needed to Treat / Absolute Risk Reduction (NNT, ARR)
Absolute and relative risk reduction, relative risk, and the number needed to treat / harm from a control and an experimental event rate.
Open the calculator → Runs in your browser. Data leaves only if you deliberately send a problem report from the interactive tool.
Example
- Control event rate
- 20 %
- Experimental event rate
- 15 %
Result: ARR 5%; NNT 20; relative risk 0.75; RRR 25%.
The tool opens with these values already filled in. Replace them with your own.
What the result means
- Formulas
- ARR = CER − EER; RRR = ARR/CER; relative risk RR = EER/CER; NNT = 1/ARR (rounded up to whole patients).
- Harm
- When EER > CER the intervention increases events: the result is a number needed to harm NNH = 1/|ARR|. ARR = 0 is no measurable difference (NNT undefined).
What you enter
- Control event rate
- Experimental event rate
How this is calculated
Laupacis A, Sackett DL, Roberts RS. An assessment of clinically useful measures of the consequences of treatment. N Engl J Med. 1988;318(26):1728-1733. Read the source ↗
A reference and educational tool. Not medical, legal, or financial advice, and not a substitute for clinician judgment.