NNT Calculator — Number Needed to Treat
Enter the event rates in the control and treatment groups to compute the Number Needed to Treat (or Harm), Absolute Risk Reduction, Relative Risk, RRR, and Odds Ratio — the core statistics of evidence-based medicine.
%
%
Treat this many patients to prevent one additional event vs control
- 1
Control event rate (CER)
20% ÷ 100 = 0.2 - 2
Treatment event rate (EER)
10% ÷ 100 = 0.1 - 3
Absolute risk reduction (ARR)
0.2 − 0.1 = 0.1 - 4
Number needed to treat (NNT)
⌈1 ÷ 0.1⌉ = 10Rounded up to the nearest whole patient.
此計算機如何運作？
NNT = 1 ÷ ARR; ARR = Control Event Rate − Treatment Event Rate. Small NNT means the treatment works well for fewer patients. This calculator also computes NNH (when treatment causes harm), Relative Risk, Relative Risk Reduction, and Odds Ratio from the same two inputs.
公式
How this is calculated
The Number Needed to Treat (NNT) answers the most clinically intuitive question about a treatment: how many patients must be treated to prevent one additional bad outcome compared with no treatment (or placebo)? It is the reciprocal of the Absolute Risk Reduction (ARR), rounded up to the nearest whole patient.
ARR is the arithmetic difference between the Control Event Rate (CER — proportion with the outcome in the untreated group) and the Experimental Event Rate (EER — proportion with the outcome in the treatment group). When CER exceeds EER the treatment is beneficial: ARR = CER − EER and NNT = 1/ARR. When EER exceeds CER the treatment increases harm: the Number Needed to Harm (NNH) = 1/(EER − CER).
The Relative Risk (RR = EER/CER) and Relative Risk Reduction (RRR = 1 − RR) are proportional measures that can appear more impressive than the NNT because they ignore baseline event rates. An RRR of 50% looks the same whether the ARR is 0.2% (NNT = 500) or 20% (NNT = 5). Always present ARR or NNT alongside relative measures. The Odds Ratio approximates RR when events are rare (< 10%) but diverges at higher event rates — it is reported here for studies using logistic regression or case-control designs.
常見問題
Lower NNT is better: NNT = 1 means every treated patient benefits. NNTs below 10 are considered clinically important for serious conditions; NNTs of 20–50 may still be worthwhile depending on the severity of the prevented outcome and the treatment burden. Context is essential — NNT = 100 to prevent a fatal stroke is very different from NNT = 100 to prevent a mild side effect.
ARR (Absolute Risk Reduction) is the percentage-point difference in event rates: CER − EER. RRR expresses this as a fraction of the baseline: (CER − EER) / CER × 100. With CER = 4% and EER = 2%, ARR = 2% and RRR = 50%. Media reports often highlight RRR because it looks larger; NNT and ARR provide the more grounded clinical picture.
Multiply by 100 before entering. For example if 0.30 of control patients had an event and 0.15 of treated patients had one, enter 30 and 15 respectively. The calculator treats inputs as percentages throughout.
也稱為
TG we-Calculate Editorial Team. (2026). NNT Calculator — Number Needed to Treat [Online calculator]. TG we-Calculate. https://we-calculate.com/zh-tw/calculator/nnt-calculator
TG we-Calculate Editorial Team. "NNT Calculator — Number Needed to Treat." TG we-Calculate. 2026. https://we-calculate.com/zh-tw/calculator/nnt-calculator.
TG we-Calculate Editorial Team, "NNT Calculator — Number Needed to Treat," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/zh-tw/calculator/nnt-calculator
@misc{wecalculate_nnt_calculator, title = {NNT Calculator — Number Needed to Treat}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/zh-tw/calculator/nnt-calculator}}, year = {2026}, note = {TG we-Calculate} }
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