Incidence measures new cases, while prevalence counts all existing cases. Incidence rates are the basis for comparing disease risk between groups and over time.
How to use the Incidence Rate Calculator
- New cases
- Population at risk
- Follow-up period
- Express per
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the number of new cases, the population at risk and the time period.
Tips
- Use counts and person-time from the same population and period.
- Report confidence intervals with rates and ratios.
- Check whether figures are crude or age-adjusted before comparing.
- Larger samples give more stable estimates.
Common mistakes to avoid
- Confusing incidence (new cases) with prevalence (all cases).
- Comparing rates per 1,000 with rates per 100,000.
- Reading a relative risk without the baseline risk.
Inputs explained
| Field | What to enter |
|---|---|
| New cases | A number |
| Population at risk | A number |
| Follow-up period | In years |
| Express per | Choose one: 1,000, 10,000, 100,000 |
What each result means
- Incidence rate = [New cases]÷([Population at risk]×[Follow-up period])×[Express per]
- Cumulative incidence (risk) over the period = [New cases]÷[Population at risk]×100
- Person-years at risk = [Population at risk]×[Follow-up period]
Example values
How the main result changes when New cases changes and everything else stays at the example values above:
| New cases | Incidence rate (per person-years (scaled)) |
|---|---|
| 60 | 60 |
| 90 | 90 |
| 120 | 120 |
| 150 | 150 |
| 180 | 180 |
| 240 | 240 |
Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.
Formula
Incidence rate = new cases ÷ person-time at risk
Cumulative incidence (risk) = new cases ÷ population at risk at the start
Rate per 100,000 = rate × 100,000
Worked example
Inputs — New cases: 120; Population at risk: 50,000; Follow-up period: 2 years; Express per: 100,000.
Results — Incidence rate = 120 per person-years (scaled); Cumulative incidence (risk) over the period = 0.24 %; Person-years at risk = 100,000.
Key terms
- Person-time
- Sum of the time each person was at risk.
Frequently asked questions
What is the difference between incidence and prevalence?
Incidence counts new cases over time; prevalence counts all current cases.
Why use person-time?
It accounts for people followed for different lengths of time.
Why per 100,000?
Rare diseases are easier to compare when scaled to a large population.