Absolute Risk Reduction Calculator

Calculate absolute risk reduction and the number needed to treat.

About Absolute Risk Reduction Calculator

Absolute risk reduction is the difference in percentage points, and its reciprocal is the number needed to treat. NNT is the most decision-relevant measure because it accounts for baseline risk. An NNT of 100 means 99 patients receive treatment, and any side effects, without benefit — which is the trade-off relative measures conceal.

Formula

Absolute Risk Reduction

ARR = control rate − treatment rate; NNT = 1 ÷ ARR

NNT: Patients who must be treated to prevent one event

Worked example

With the default values below, Absolute Risk Reduction Calculator returns Absolute Risk Reduction of 7.000%. Change any input above and every figure here is recalculated in your browser.

Inputs used

Control event rateaccepts 0–100 %: 25 %
Treatment event rateaccepts 0–100 %: 18 %
Population treatedminimum 1: 10,000

Results

Absolute Risk Reduction: 7.000%
Number Needed to Treat: 14.3
Relative Risk Reduction: 28.000%
Events Prevented in the Population: 700
Interpretation: Moderately effective

Step by step

1Values used
2Absolute Risk Reduction
3Absolute Risk Reduction= 7.000
4Number Needed to Treat= 14.3
5Relative Risk Reduction= 28.000
6Events Prevented in the Population= 700
7Interpretation= Moderately effective

Assumptions and limits

The result is derived only from the 3 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 3 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 Absolute Risk Reduction calculated?

ARR = control rate − treatment rate; NNT = 1 ÷ ARR Absolute risk reduction is the difference in percentage points, and its reciprocal is the number needed to treat. NNT is the most decision-relevant measure because it accounts for baseline risk.

Why does Absolute Risk Reduction matter?

An NNT of 100 means 99 patients receive treatment, and any side effects, without benefit — which is the trade-off relative measures conceal.

What values do I need to enter?

This calculator takes 3 inputs: Control event rate, Treatment event rate, Population treated. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.