NNT / NNH Calculator
Number needed to treat or harm, with absolute and relative risk reduction.
Enter your values to see the result.
The confidence interval of NNT is found by inverting the interval of the absolute risk difference (Altman 1998); it is only available from counts. Results use standard large-sample formulas and are meant to support, not replace, a statistician's review. Check the assumptions behind each method before you report the numbers.
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- Relative Risk Calculator — See relative risk alongside NNT.
- Attributable Risk Calculator — Measure the excess risk caused by an exposure.
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Clear instructions
Find steps, examples and limitations below.
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How to use the NNT / NNH Calculator
Number needed to treat or harm, with absolute and relative risk reduction.
- 1 Enter events and totals for the treatment and control groups, or the two event rates.
- 2 Say whether the event is harmful (like death) or beneficial (like cure).
- 3 Read the NNT or NNH, the absolute risk reduction and the confidence interval.
Example and practical tips
If 25% of controls and 15% of treated patients have a stroke, the absolute risk reduction is 10% and the NNT is 10.
Further reading
Frequently asked questions
Why is NNT rounded up?
You cannot treat part of a person, and rounding up gives the more conservative estimate of benefit.
Why does the confidence interval include infinity?
When the risk difference could be zero, its inverse runs from benefit through infinity to harm. Altman (1998) recommends showing both ends.
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