How this calculator works
Choose the denominator before entering the numbers. Incidence proportion uses the initially disease-free population over a specified period. A person-time rate uses the sum of observed time at risk, entered here in person-years. The same ratio arithmetic supports both measures, but the interpretation differs. Inverse results may be fractional expected counts. The result is an estimate for information only and is not medical advice.
Formula
Incidence per k = new cases / denominator × k. Cases = rate × denominator / k. Denominator = cases × k / rate.
Worked example
Twenty new cases among 1000 people give 20 per 1000 over that study period. Five cases over 250 person-years give 20 per 1000 person-years.
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Frequently asked questions
Is incidence the same as prevalence?
No. Incidence counts new cases. Prevalence counts people with a condition at a specified time or during a period.
What is a person-year?
One person observed for one year contributes one person-year. Add each participant's time at risk until the relevant study endpoint.
Does changing the scale change the underlying rate?
No. It changes only the reporting denominator, such as from per 1000 to per 100 000.