Relative Risk Calculator

Work out relative risk instantly with clear inputs, formula shown and shareable results.

About Relative Risk Calculator

Relative risk divides the incidence in the exposed group by that in the unexposed. The absolute risk difference matters more for decisions, and its reciprocal is the number needed to treat or harm.

Formulas

Relative risk

RR = (a/n₁) / (c/n₂)

Number needed

NNT = 1 / |risk difference|

Worked example

With the default values below, Relative Risk Calculator returns Relative risk of 3.00000000. Change any input above and every figure here is recalculated in your browser.

Inputs used

Cases among the exposedminimum 0: 30
Total exposedminimum 1: 100
Cases among the unexposedminimum 0: 10
Total unexposedminimum 1: 100

Results

Relative risk: 3.00000000
Absolute risk difference: 0.20000000
Number needed to treat or harm: 5.0000
95% confidence interval: 1.5508 to 5.8033

Step by step

1Values used
2Relative risk
3Number needed
4Relative risk= 3.00000000
5Absolute risk difference= 0.20000000
6Number needed to treat or harm= 5.0000
795% confidence interval= 1.5508 to 5.8033

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 8 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

For 30/100 versus 10/100?

RR = 3 with a risk difference of 0.2, so one extra case arises for every 5 people exposed.

Why report the absolute difference too?

A relative risk of 3 sounds alarming, but if the baseline risk is 1 in a million the absolute effect is negligible.