# Incidence Rate Calculator

URL: https://speedycalculator.com/calculators/health/incidence-rate/
Last updated: 2026-10-30

> The incidence rate calculator works out how quickly new cases of a disease occur in a population – per person-time or as cumulative incidence over a period – and expresses it per 1,000 or 100,000.

## Inputs

- New cases
- Population at risk
- Follow-up period (years)
- Express per

## Results

- Incidence rate
- Cumulative incidence (risk) over the period
- Person-years at risk

## 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
```

## Explanation

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

FieldWhat to enterNew casesA numberPopulation at riskA numberFollow-up periodIn yearsExpress perChoose 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 casesIncidence rate (per person-years (scaled))60609090120120150150180180240240
*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.*

## 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.

## FAQ

**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.


## Sources

- [CDC – Principles of epidemiology, lesson 3: measures of risk](https://archive.cdc.gov/www_cdc_gov/csels/dsepd/ss1978/lesson3/section2.html)