Secondary Attack Rate Calculator

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

About Secondary Attack Rate Calculator

Divides secondary cases by the susceptible contacts, excluding the primary cases themselves from the denominator.

Formula

Secondary attack rate

SAR (%) = secondary cases / (total contacts - primary cases) x 100

Worked example

With the default values below, Secondary Attack Rate Calculator returns Secondary attack rate of 28.571%. Change any input above and every figure here is recalculated in your browser.

Inputs used

Secondary casesaccepts 0–100,000: 8
Total household or close contactsaccepts 1–100,000: 40
Primary cases among those contactsaccepts 0–100,000: 12

Results

Secondary attack rate: 28.571%
Susceptible contacts in the denominator: 28
Secondary cases per primary case: 0.667
Interpretation: Moderate household transmission

Step by step

1Values used
2Secondary attack rate
3Secondary attack rate= 28.571
4Susceptible contacts in the denominator= 28
5Secondary cases per primary case= 0.667
6Interpretation= Moderate household transmission

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

Why exclude primary cases?

They were the source of exposure, so they were never susceptible contacts within the household.

How does SAR relate to R0?

Household SAR measures transmission in a closed high-contact setting and usually exceeds the community average.