NNT calculator (number needed to treat, ARR, RRR)
Enter your values. The answer and the full worked solution update as you type.
Result
The events are unwanted outcomes (such as a stroke or death): a lower rate in the treatment group is a benefit. For exam practice only. This is a study aid, never a clinical decision: always follow the prescription, local protocols and the product information.
Calculate the number needed to treat (NNT) from a trial: enter the events and group sizes in the control and treatment groups, or the two event rates. The calculator works out the absolute risk reduction (ARR), relative risk reduction (RRR), relative risk and odds ratio, and the NNT rounded up to a whole person. When the treatment group does worse, it gives the number needed to harm (NNH).
Step-by-step solution
Event rates
Absolute and relative risk reduction
Number needed to treat
Key formulas
- Risk measures
- RR = EER/CER, ARR = CER − EER, RRR = ARR/CER
- Number needed to treat
- NNT = 1/ARR, rounded up; NNH = 1/(EER − CER)
How to calculate the number needed to treat
- Event rates: CER = events ÷ people in the control group, EER = events ÷ people in the treatment group.
- Absolute risk reduction: ARR = CER − EER, for example 15% − 10% = 5%.
- NNT = 1 ÷ ARR, with ARR as a fraction: 1 ÷ 0.05 = 20. Round up to the next whole number.
- Relative risk reduction: RRR = ARR ÷ CER = 5% ÷ 15% = 33%. Relative risk: RR = EER ÷ CER.
- If the EER is higher than the CER, the treatment causes harm and 1 ÷ |ARR| is the number needed to harm.
Questions and answers
Why is the NNT rounded up?
You cannot treat part of a person, and rounding up keeps the estimate of benefit conservative. An NNT of 19.2 is reported as 20.
What is the difference between ARR and RRR?
ARR is the difference in event rates in percentage points; RRR is that difference relative to the control rate. A drug that lowers the risk from 2% to 1% has an RRR of 50% but an ARR of only 1% and an NNT of 100.
What is a good NNT?
It depends on the outcome, the time frame and the harms. An NNT of 2 to 5 is a large effect for treating an acute condition; for preventing rare events over years, NNTs of 50 or more can still be worthwhile.
Where does the NNT come from?
It was introduced by Laupacis, Sackett and Roberts (New England Journal of Medicine, 1988) as a clinically useful measure of treatment effect.
Practise it yourself
Practise Evidence-based practice statistics
A calculator gives the answer. In the exam you need the method: practise it with new problems, hints and worked solutions.