Attack Rate Calculator

Work out attack rate instantly with clear inputs, formula shown and shareable results.

About Attack Rate Calculator

Divides those who became ill by those exposed, the cumulative incidence measure used in outbreak investigation.

Formula

Attack rate

Attack rate (%) = ill / population at risk x 100

Worked example

With the default values below, Attack Rate Calculator returns Attack rate of 30.000%. Change any input above and every figure here is recalculated in your browser.

Inputs used

People who became illaccepts 0–1,000,000: 45
People exposed or at riskaccepts 1–1,000,000: 150

Results

Attack rate: 30.000%
Cases per 1,000 exposed: 300.0
Exposed who stayed well: 105
Assessment: High attack rate

Step by step

1Values used
2Attack rate
3Attack rate= 30.000
4Cases per 1,000 exposed= 300.0
5Exposed who stayed well= 105
6Assessment= High attack rate

Assumptions and limits

The result is derived only from the 2 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

Why is it called a rate?

Conventionally, though it is really a proportion because it has no time dimension beyond the outbreak period.

How are food-specific attack rates used?

Comparing attack rates among those who ate and did not eat each food identifies the likely vehicle.