Choose one group or compare exposed and unexposed groups, enter whole-person counts, and select Calculate. Use new cases and the population initially at risk over the same defined period. Comparison results include relative risk and the risk difference in percentage points.

Use new cases during a defined period and the people initially at risk in that group. Include cases in the group total.

For educational/informational use only. This calculator does not provide medical advice, diagnosis, or outbreak confirmation. Results depend on using correct case definitions, a defined time window, and appropriate study design; for suspected outbreaks or urgent health concerns, contact a clinician and your local public health authority. For official guidance, consult outbreak investigation resources from your national public health agency (e.g., CDC). This tool may save inputs in your browser (local storage) for convenience—do not enter personal or identifiable health information.

Attack Rate Formula

The following formula is used to calculate an attack rate (expressed as a percent).

A(%) = (NC) / (PE) × 100
  • Where A(%) is the attack rate (percent)
  • NC is the number of new cases among people exposed to a specific event (during the specified time period)
  • PE is the number of exposed people initially at risk, including those who later become cases

To calculate an attack rate, divide new cases among the exposed group by the exposed population initially at risk, then multiply by 100. For two groups, relative risk is the exposed attack rate divided by the unexposed attack rate; risk difference is exposed minus unexposed in percentage points. A zero unexposed attack rate gives no defined finite risk ratio.

Attack Rate Definition

Attack rate (incidence proportion) is the proportion of an at-risk group that becomes ill during a specified time period after a defined exposure or event (often used in outbreak investigations). A clear case definition and time window are required for meaningful results.

Attack Rate Example

How to calculate an attack rate?

  1. First, determine the number of cases of a disease in people exposed.

    Calculate the number of people that contracted the disease.

  2. Next, determine the number of people exposed.

    Measure the total number of people exposed.

  3. Finally, calculate the attack rate.

    Calculate the attack rate using the equation above.

FAQ

What is attack rate?

The attack rate is a medical term used to describe the proportion of people who became sick after being exposed to a specific event.