# What is the NNT?

Computes the number needed to treat (NNT = 1 ÷ absolute risk reduction) from event counts or event rates in a control and a treated group, with the relative risk reduction.

- Page: https://www.acalculator.org/statistics/nnt-calculator
- JSON spec: https://www.acalculator.org/statistics/nnt-calculator.json
- Version: 217c47adcb1f

## Default answer

Example with the default inputs (I have Event counts, Control group: events 20, Control group: people 100, Treated group: events 10, Treated group: people 100): The number needed to treat is 10 (NNTB, a benefit).

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| mode | I have | Type event counts for each group, or the event rates as percents. |
| ce | Control group: events | People in the control group who had the bad outcome. |
| cn | Control group: people | Everyone in the control group. |
| te | Treated group: events | People in the treated group who had the bad outcome. |
| tn | Treated group: people | Everyone in the treated group. |
| cer | Control event rate (CER) | The share of the control group with the outcome, in percent. |
| eer | Treated event rate (EER) | The share of the treated group with the outcome, in percent. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| nnt | Number needed to treat | 1 ÷ \|ARR\|: how many people must be treated for one fewer (or, for a harm, one more) bad outcome. |
| whole | Rounded up to whole people | The NNT rounded up to the next whole number. |
| kind | Benefit or harm | NNTB when the treatment lowers the event rate, NNTH when it raises it. |
| cer | Control event rate | Control events ÷ control group, in percent. |
| eer | Treated event rate | Treated events ÷ treated group, in percent. |
| arr | Absolute risk reduction | CER − EER, in percentage points; negative for a harm. |
| rrr | Relative risk reduction | (CER − EER) ÷ CER, in percent. |
| rr | Relative risk | EER ÷ CER. |

## Method

CER = control events ÷ control group; EER = treated events ÷ treated group; ARR = CER − EER; NNT = 1 ÷ |ARR| (NNTB when ARR > 0, NNTH when ARR < 0); RRR = ARR ÷ CER; RR = EER ÷ CER.

## Assumptions

- The events are bad outcomes (deaths, strokes), so a lower rate in the treated group is a benefit.
- Both groups are followed for the same length of time; the NNT applies to that time.
- Rounding up to whole people is a page choice; the exact NNT is shown too.

## Worked examples

1. mode = counts, ce = 20, cn = 100, te = 10, tn = 100 gives nnt = 10, whole = 10, arr = 10%, rrr = 50%, rr = 0.5. Source: https://pmc.ncbi.nlm.nih.gov/articles/PMC1114210/.
2. mode = rates, cer = 10%, eer = 18% gives nnt = 12.5, whole = 13, arr = -8%, kind = NNTH, a harm. Source: Altman DG. Confidence intervals for the number needed to treat. BMJ 1998;317:1309 (Tramèr et al.’s ARR of −0.08 gives an NNT of −12.5, an NNTH of 12.5; Altman gives no event rates, so 10% and 18% are example rates with that ARR); https://pmc.ncbi.nlm.nih.gov/articles/PMC1114210/.
3. mode = counts, ce = 15, cn = 120, te = 9, tn = 130 gives nnt = 17.931034, whole = 18, cer = 12.5%. Source: https://pmc.ncbi.nlm.nih.gov/articles/PMC1114210/.

## FAQ

### How do I calculate NNT?

Subtract the treated event rate from the control event rate to get the absolute risk reduction (ARR), then take 1 ÷ ARR. If 20% of the control group and 10% of the treated group have the outcome, ARR = 0.10 and NNT = 1 ÷ 0.10 = 10.

### What does an NNT of 10 mean?

On average, 10 people must be treated, for the length of the study, for one of them to avoid the bad outcome. The lower the NNT, the more effective the treatment; an NNT of 1 means every treated person benefits.

### What is NNTH (number needed to harm)?

When the treated group has more bad outcomes than the control group, the ARR is negative. Altman (1998) suggests calling 1 ÷ |ARR| the number needed to treat for harm (NNTH): how many people treated lead to one extra bad outcome.

### Why is the NNT rounded up?

You cannot treat part of a person, so the page also shows the NNT rounded up to the next whole number. The exact value is shown too; 17.93 becomes 18.

### What is the difference between absolute and relative risk reduction?

The absolute risk reduction is the difference between the two event rates (20% − 10% = 10 points). The relative risk reduction divides that by the control rate (10 ÷ 20 = 50%). The same relative reduction gives a much larger NNT when the outcome is rare.

### What if the two rates are equal?

Then the ARR is 0 and the NNT is infinite: the treatment makes no difference to the outcome in this data. The page gives no answer in that case.

## Sources

- Altman DG. Confidence intervals for the number needed to treat. BMJ 1998;317:1309-1312, retrieved 2026-10-18. https://pmc.ncbi.nlm.nih.gov/articles/PMC1114210/
- Cook RJ, Sackett DL. The number needed to treat: a clinically useful measure of treatment effect. BMJ 1995;310:452-454, retrieved 2026-10-18. https://pmc.ncbi.nlm.nih.gov/articles/PMC2548824/