What this calculator does
Number needed to treat (NNT) works out how many patients must be treated to prevent one additional bad outcome, from two event rates. Enter your own figures above and the answer updates as you type: nothing is fixed in the code, so the result reflects exactly the numbers you supply.
The formula this calculator evaluates is printed under the tool and explained below, so you can check the working by hand or reuse it in a spreadsheet.
The formula
The inputs explained
| Field | What to enter |
|---|---|
| Control group event rate (%) | A number, measured in %. Starts at 20. |
| Treatment group event rate (%) | A number, measured in %. Starts at 15. |
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How the answer changes with control group event rate
Every other input is held at the calculator’s starting values while control group event rate varies. Select any row to load that scenario into the calculator.
| Control group event rate (%) | NNT | Absolute risk reduction | Note |
|---|---|---|---|
| 10 | 20.0 | 5.00% | Round up to the next whole person in practice. If the treatment increases the bad outcome instead, the same maths gives the number needed to harm (NNH). |
| 15 | Not defined: identical event rates | 0.000% | Round up to the next whole person in practice. If the treatment increases the bad outcome instead, the same maths gives the number needed to harm (NNH). |
| 20 | 20.0 | 5.00% | Round up to the next whole person in practice. If the treatment increases the bad outcome instead, the same maths gives the number needed to harm (NNH). |
| 30 | 6.7 | 15.0% | Round up to the next whole person in practice. If the treatment increases the bad outcome instead, the same maths gives the number needed to harm (NNH). |
| 40 | 4.0 | 25.0% | Round up to the next whole person in practice. If the treatment increases the bad outcome instead, the same maths gives the number needed to harm (NNH). |
| 60 | 2.2 | 45.0% | Round up to the next whole person in practice. If the treatment increases the bad outcome instead, the same maths gives the number needed to harm (NNH). |