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Number Needed to Treat (NNT) Calculator

Find how many people you need to treat for one to benefit. Enter the event rate in the control group and in the treatment group.

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Number needed to treat

11

number needed to treat, absolute risk reduction 10%

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Calculate it yourself

Suppose an unwanted event occurs in 20% of a control group and 12% of a treatment group over the same follow-up period.

Follow this worked example, then repeat the steps with your own numbers.

  1. Convert rates to proportions

    Control rate = 0.20; treatment rate = 0.12.

  2. Find the absolute risk reduction

    ARR = 0.20 − 0.12 = 0.08, or 8 percentage points.

  3. Take the reciprocal

    NNT = 1 ÷ 0.08 = 12.5.

  4. Round up to a whole person

    Reported NNT = 13 people.

    This means about 13 people would need the treatment for one fewer event over the specified period, under the study assumptions.

Equal rates give an infinite NNT. If treatment increases the unwanted event rate, the negative risk reduction corresponds to a number needed to harm. NNT alone does not determine whether a treatment is appropriate.

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Use your PaperSurvey.io results

For an appropriate study, use control and treatment event rates over the same period to calculate NNT separately.

See howStart a free trial
PaperSurvey Odds Ratio widget showing observed outcome counts and relative risk for exercise and sick days.
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Example from PaperSurvey.ioThis observational example shows outcome counts and relative risk, not an NNT or evidence of treatment benefit.

What NNT tells you

The number needed to treat is how many people must receive a treatment for one additional person to benefit compared with the control condition. A smaller number means a more effective intervention. It is a plain-language way to express a difference in event rates that stakeholders find easier to act on than a percentage.

How it is calculated

First the absolute risk reduction is the control event rate minus the treatment event rate. The number needed to treat is 1 divided by that reduction, rounded up to a whole person. If the treatment increases the event rate instead, the same figure becomes a number needed to harm.

Frequently asked questions

What if the two rates are equal?

When the event rates are identical there is no risk reduction, so the number needed to treat is infinite: no amount of treating produces a difference.

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