How this calculator works
For an adverse outcome, subtract treatment risk from control risk over the same follow-up duration. A negative reduction indicates harm. The reciprocal is rounded up for a whole-person NNT or NNH. The person-years option returns person-years per event difference, not a patient count; interpreting that reciprocal as an NNT requires additional time-to-event assumptions. No confidence interval is inferred from risk percentages alone. The result is an estimate for information only and is not medical advice.
Formula
ARR = control risk - treatment risk. NNT = 1/ARR when ARR > 0; NNH = 1/|ARR| when ARR < 0. With event rates, reciprocal rate difference has units of person-years per event.
Worked example
Risks of 20% and 10% give ARR 0.10 and NNT 10. Risks of 10% and 15% give a 5 percentage-point increase and NNH 20. Equal risks have an infinite NNT.
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Frequently asked questions
Should 10% be entered as 10 or 0.10?
Enter 10 in a percentage input. The engine converts it to the fraction 0.10 before calculation.
Why must follow-up match?
Risks accumulate over time. A one-year control risk and five-year treatment risk do not form a meaningful NNT.
Can a non-integer reciprocal be rounded down?
No. A whole-person NNT is conventionally rounded up. The unrounded result is also shown.