Number Needed to Treat Calculator

What NNT actually tells you

Number needed to treat (NNT) is how many patients need to receive a treatment, compared to the control, for one additional patient to benefit. A lower NNT means a more effective treatment relative to control — an NNT of 5 means treating 5 patients prevents one additional bad outcome compared to not treating them.

The formula

Absolute risk reduction (ARR) is the difference between the control event rate (CER) and the experimental event rate (EER): ARR = |CER − EER|. NNT is the reciprocal, rounded up to the next whole patient: NNT = 1 / ARR. When the experimental rate is higher than control (a harmful effect), the same math gives number needed to harm (NNH) instead.

Why NNT beats relative risk reduction alone

Relative risk reduction can make a small effect sound dramatic — a treatment that cuts risk from 2% to 1% is a 50% relative risk reduction, but the absolute benefit is 1 in 100 patients treated. NNT keeps that absolute scale visible, which is usually the number that should actually inform a treatment decision.

This calculator does the arithmetic only — it doesn't know your patient's baseline risk or whether the underlying trial applies to them. Confirm event rates against the actual source before using them clinically.