What is an attack rate?

An attack rate is the proportion of people in a defined population who develop illness during a specified period. It is calculated as (number ill / total at risk) x 100, expressed as a percentage.

What is relative risk (RR)?

Relative risk is the ratio of the attack rate in the exposed group to the attack rate in the unexposed group. An RR > 1 suggests exposure increases disease risk; RR = 1 means no association; RR < 1 suggests a protective effect.

What is attributable risk?

Attributable risk (also called risk difference) is the attack rate in the exposed group minus the attack rate in the unexposed group. It estimates the excess risk of illness due to the exposure.

What counts as "exposed" vs "unexposed"?

Exposed individuals are those who had contact with the suspected source (e.g., ate a specific dish, attended an event). Unexposed individuals are those who did not have that contact during the same time and place.

Can I use this for a single group without a comparison?

Yes. You can enter zeros for the unexposed group and still get the exposed-group attack rate. However, relative risk and attributable risk require both groups.

Is this tool appropriate for case-control studies?

No. Attack rates and relative risk require cohort study data where you know the total at-risk population. For case-control studies, use odds ratio calculations instead.

Attack Rate Calculator

Calculate attack rates for exposed and unexposed groups, relative risk, and attributable risk during outbreak investigations.

Exposure Data

Enter counts for exposed and unexposed groups.

Exposed Group

Ill

Total Exposed

Unexposed Group

Ill

Total Unexposed

Exposure Label (optional)

Enter exposure data and click Calculate to see results.

Exposed AR

—

% became ill

Unexposed AR

—

% became ill

Relative Risk

—

RR (exposed / unexposed)

Additional Metrics

Attributable Risk —

Attributable Risk % (among exposed) —

Total Ill (both groups) —

Overall Attack Rate —

2x2 Contingency Table

	Ill	Not Ill	Total	Attack Rate
Exposed	—	—	—	—
Unexposed	—	—	—	—
Total	—	—	—	—

Interpretation

Summary

The Attack Rate Calculator helps epidemiologists and public health investigators quantify disease transmission during outbreak events. Enter the number of ill and total individuals for both exposed and unexposed groups to instantly compute attack rates, relative risk (RR), and attributable risk. This tool is commonly used in foodborne illness investigations, cohort studies, and field epidemiology.

How it works

Enter the number of people who became ill among those exposed to the suspected source.
Enter the total number of people exposed to the suspected source.
Enter the number of people who became ill among those not exposed.
Enter the total number of people not exposed.
Click Calculate to see attack rates for each group, relative risk, and attributable risk.
Review the 2x2 contingency table for a full summary of the data.

Use cases

Investigate foodborne illness outbreaks at restaurants or events.
Assess the risk associated with a specific exposure in a cohort study.
Determine whether a suspected food item or source is statistically linked to illness.
Calculate relative risk to compare disease rates between exposed and unexposed groups.
Compute attributable risk to estimate the excess risk due to exposure.
Support field epidemiology reports and outbreak summary documentation.
Train students and public health workers in basic epidemiologic calculations.
Cross-check manual calculations for accuracy during rapid outbreak response.

Frequently Asked Questions

Last updated: 2026-05-23 · Reviewed by Nham Vu