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Number Needed to Treat (NNT) Calculator

Compute the number needed to treat from control and treatment event rates — how many patients to treat for one to benefit.

Control event rate (CER)

Experimental event rate (EER)

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How it works

NNT = 1 ÷ (CER − EER) = 1 ÷ ARR · rounded up to a whole patient

Number needed to treat translates a trial result into a tangible clinical unit: the count of patients who must receive a treatment for one additional good outcome, over the study period. It is the reciprocal of the absolute risk reduction, so a large treatment effect gives a small NNT (closer to the ideal of 1). NNT is always rounded up, because you cannot treat a fraction of a patient and rounding down would overstate the benefit.

Worked example

If a therapy lowers the event rate from 20% to 12%, ARR = 8% = 0.08, so NNT = 1 ÷ 0.08 = 12.5, rounded up to 13. Treating 13 patients is expected to prevent one event.

Sources: Number needed to treat — Wikipedia

Frequently asked questions

What is a good NNT?

Lower is better — an NNT of 2 means treating two people yields one benefit, while an NNT of 100 means most treated patients gain nothing. What counts as "good" depends on the cost, side effects, and severity of the outcome being prevented.

Does NNT depend on the follow-up period?

Yes. Event rates accumulate over time, so an NNT quoted for one year differs from a five-year figure. Always report the time horizon alongside the NNT so it can be compared fairly.

What is number needed to harm?

NNH uses the same reciprocal formula but for an adverse effect: how many patients are treated before one extra harm occurs. Comparing NNT against NNH helps weigh benefit versus risk for a therapy.

Related calculators

Absolute Risk Reduction Calculator

Calculate absolute risk reduction (ARR), relative risk reduction (RRR), relative risk and the number needed to treat or harm from event rates or a 2 × 2 table, with 90%, 95% or 99% confidence intervals (Wald or Newcombe).

Relative Risk from a 2×2 Table Calculator

Compute relative risk with a 95% confidence interval from the raw event counts in exposed and unexposed groups.

Relative Risk Calculator

Calculate relative risk (the risk ratio) from a 2×2 table with its confidence interval, plus the odds ratio, absolute risk difference, NNT or NNH, chi-square and Fisher exact p-values, and the worked steps.

Likelihood Ratio Calculator

Compute positive and negative likelihood ratios for a diagnostic test from its sensitivity and specificity.

Predictive Value Calculator

Enter the four cells of a diagnostic 2×2 table to get positive and negative predictive value, sensitivity and specificity.

Educational statistics tool, not medical advice.