Number Needed to Treat Calculator

Calculate the number needed to treat and compare it against harms.

About Number Needed to Treat Calculator

Comparing the number needed to treat against the number needed to harm gives the likelihood of being helped rather than harmed. A ratio well above one is required for a treatment to be worthwhile. Treatments with a favourable NNT can still be net harmful if the NNH is lower, which is why both must be reported together.

Formula

Number Needed to Treat

NNT = 1 ÷ ARR; NNH = 1 ÷ absolute risk increase; LHH = NNH ÷ NNT

LHH: Likelihood of help versus harm, the ratio of the two numbers needed

Worked example

With the default values below, Number Needed to Treat Calculator returns Number Needed to Treat of 20.00. Change any input above and every figure here is recalculated in your browser.

Inputs used

Control event rateaccepts 0–100 %: 20 %
Treatment event rateaccepts 0–100 %: 15 %
Adverse event rate without treatmentaccepts 0–100 %: 2 %
Adverse event rate with treatmentaccepts 0–100 %: 6 %

Results

Number Needed to Treat: 20.00
Number Needed to Harm: 25.00
Likelihood of Being Helped Rather Than Harmed: 1.2500
Absolute Risk Reduction: 5.000%
Verdict: Marginally favourable

Step by step

1Values used
2Number Needed to Treat
3Number Needed to Treat= 20.00
4Number Needed to Harm= 25.00
5Likelihood of Being Helped Rather Than Harmed= 1.2500
6Absolute Risk Reduction= 5.000
7Verdict= Marginally favourable

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is Number Needed to Treat calculated?

NNT = 1 ÷ ARR; NNH = 1 ÷ absolute risk increase; LHH = NNH ÷ NNT Comparing the number needed to treat against the number needed to harm gives the likelihood of being helped rather than harmed. A ratio well above one is required for a treatment to be worthwhile.

Why does Number Needed to Treat matter?

Treatments with a favourable NNT can still be net harmful if the NNH is lower, which is why both must be reported together.

What values do I need to enter?

This calculator takes 4 inputs: Control event rate, Treatment event rate, Adverse event rate without treatment, Adverse event rate with treatment. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.