Number Needed to Treat (NNT) Calculator
Calculate NNT, NNH, ARR, RRR, relative risk, and odds ratio from clinical trial event rates.
Essential tool for evidence-based medicine.
What Is NNT? The Number Needed to Treat (NNT) tells you how many patients must receive a treatment for one additional patient to benefit. It was introduced by Laupacis et al. in 1988 and is now a cornerstone of evidence-based medicine.
Key Formulas
ARR (Absolute Risk Reduction) = CER − EER NNT = 1 / ARR
Where:
- CER = Control Event Rate (proportion of events in the control/placebo group)
- EER = Experimental Event Rate (proportion of events in the treatment group)
- ARR = absolute reduction in risk due to treatment
Relative Measures
RR (Relative Risk) = EER / CER RRR (Relative Risk Reduction) = 1 − RR = ARR / CER Odds Ratio = (EER / (1 − EER)) / (CER / (1 − CER))
NNH (Number Needed to Harm) When EER > CER, the treatment increases risk. In this case: ARI (Absolute Risk Increase) = EER − CER NNH = 1 / ARI
Interpreting NNT
- NNT = 1: Every patient treated benefits (ideal, very rare)
- NNT = 2–5: Excellent treatment effectiveness
- NNT = 10–20: Moderate effectiveness, may be worthwhile
- NNT = 50–100: Modest benefit — weigh against cost and side effects
- NNT > 100: Treatment has minimal population-level impact
Why ARR Beats RRR A drug that reduces risk from 2% to 1% has a 50% RRR — which sounds dramatic. But the ARR is only 1%, and the NNT is 100. Headlines often misuse RRR to make treatments sound more effective than they are. ARR and NNT tell the real story.
Odds Ratio vs Relative Risk Odds ratios are used in case-control studies and logistic regression. For rare outcomes (< 10%), OR ≈ RR. For common outcomes, OR overstates the relative risk.