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Number Needed to Treat (NNT) / Harm (NNH)

From the control (CER) and experimental (EER) event rates: absolute risk reduction ARR = CER − EER, relative risk reduction RRR = ARR ÷ CER, and NNT = 1 ÷ ARR (rounded up). If the treatment increases events, the result is the number needed to harm (NNH).

Control event rate (%) (%)
Experimental event rate (%) (%)

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Formula

ARR = CER − EER · RR = EER ÷ CER · RRR = 1 − RR · NNT = 1 ÷ ARR (rounded up). If EER > CER: NNH = 1 ÷ (EER − CER).

Clinical pearls

Always state the time frame and outcome: an NNT of 50 over 5 years differs from 50 over 1 year.
NNT depends on baseline risk: the same RRR gives a smaller NNT in higher-risk patients.
Round NNT up to the next whole number by convention.

Examples

nnt control 20% treatment 15%number needed to treat placebo event rate 10 percent drug 7 percent nnh adverse events 2% control vs 5% treatment

FAQ

How do you calculate NNT?

NNT = 1 ÷ (control event rate − experimental event rate). If 20% of controls and 15% of treated patients have the event, ARR = 5% and NNT = 20.

What is the difference between relative and absolute risk reduction?

RRR is the proportional reduction (ARR ÷ CER); ARR is the simple difference in event rates and is what determines the NNT.

Evidence

Laupacis A, Sackett DL, Roberts RS. An assessment of clinically useful measures of the consequences of treatment. N Engl J Med 1988;318:1728–33.

General & Anthropometrics · Reviewed 2026-10-14 · Methodology