What is a person-year?

A person-year is one person followed for one year. If 100 people are each followed for 6 months, that equals 50 person-years. It accounts for varying follow-up durations across participants.

What is the difference between incidence rate and cumulative incidence?

Cumulative incidence (attack rate) is the proportion of a fixed population that develops the disease over a set period. Incidence rate (incidence density) accounts for varying follow-up time and is expressed per person-time unit.

How is the confidence interval calculated?

This tool uses the exact Poisson 95% confidence interval method, which is appropriate when case counts are small. For large counts it converges with the normal approximation.

When should I use per 1,000 vs. per 100,000?

Use per 1,000 for common conditions (e.g., influenza). Use per 100,000 for rare diseases (e.g., certain cancers) so the rate is expressed as a number greater than 1 for readability.

Can I enter person-months or person-days instead of person-years?

Yes. Simply select the matching unit. The result will be expressed as cases per 1,000 (or 100,000) of that same time unit, which you should then label accordingly.

Incidence Rate Calculator

Name: Incidence Rate Calculator
Availability: InStock
Author: Nham Vu

Enter new case counts and person-time data to calculate incidence rates per 1,000 or 100,000 person-years.

Study Data

New Cases

Number of new disease cases observed.

Total Person-Time at Risk

Sum of all individual follow-up durations.

Rate Multiplier

Per 1,000 Per 100,000

Quick Examples

Enter your study data and press
Calculate to see results.

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Summary

Enter new case counts and person-time data to calculate incidence rates per 1,000 or 100,000 person-years.

How it works

Enter the number of new cases observed during the study period.
Enter the total person-time at risk (person-years, or equivalent months/days).
Select the multiplier: per 1,000 or per 100,000 person-years.
The calculator divides cases by person-time and multiplies by your chosen multiplier.
Results include the incidence rate, 95% confidence interval, and a plain-language interpretation.

Use cases

Summarizing disease burden in cohort study reports.
Comparing incidence rates between exposed and unexposed groups.
Calculating incidence density for vaccine efficacy studies.
Public health surveillance of chronic or infectious diseases.
Supporting grant proposals and epidemiological manuscripts.
Teaching students the difference between incidence rate and cumulative incidence.

Frequently Asked Questions

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