What this calculator does
Incidence rate is a standard epidemiological measure: the number of new cases of something occurring within a defined population over a defined stretch of time, expressed relative to the total person-time that population was actually at risk. Unlike a simple proportion, it accounts for the fact that not everyone in a study or population is observed for the same length of time.
The formula is deliberately simple: new cases divided by total person-time at risk, then scaled by a multiplier so the result reads as a sensible rate rather than a tiny decimal. How to calculate incidence rate this way is the standard approach used in public health surveillance, clinical trials and occupational safety reporting alike, and it applies equally to disease onset, workplace injuries or any other countable event.
The formula
Person-time is the sum of the time each individual in the population was actually observed and at risk, added together across the whole group, usually measured in person-years. Dividing new cases by that total gives the raw incidence rate; multiplying by 100, 1,000, 10,000 or 100,000 rescales it into a figure that reads naturally, rather than a fraction like 0.005.
| Term | Meaning |
|---|---|
| New cases | The count of newly occurring cases during the observation period, not existing (prevalent) cases. |
| Person-time at risk | The total time all at-risk individuals were observed, summed across the population, usually in person-years. |
| Incidence rate | New cases divided by total person-time at risk, scaled to a chosen denominator such as per 1,000 person-years. |
The inputs explained
| Field | What to enter |
|---|---|
| New cases during the period | The number of new cases counted during the observation period. |
| Total person-time at risk | The total person-time at risk across the whole population, in person-years. A group of 500 people followed for 2 years contributes 1,000 person-years, for example. |
| Express per | The denominator the rate is expressed against. Rarer events are usually reported per 10,000 or per 100,000; more common ones per 100 or per 1,000. |
When to use it
Public health surveillance
Health departments report disease incidence rates per 100,000 person-years to compare how fast a condition is newly occurring across regions or time periods, independent of population size.
Workplace safety reporting
Occupational injury rates are commonly expressed per 100 or per 1,000 full-time-equivalent person-years, since two workplaces with different headcounts and hours need a common footing to compare safely.
Clinical trial follow-up
When participants join a trial at different times and are followed for different lengths of time, incidence rate is more accurate than a simple percentage of participants affected, because it accounts for that unequal follow-up.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What is the incidence rate meaning at different new case counts?
A fixed 5,000 person-years, against a range of new case counts, expressed per 1,000 person-years.
| New cases | Incidence rate | Person-time per new case |
|---|---|---|
| 5 | 1.00 per 1,000 person-years | 1,000 person-years |
| 10 | 2.00 per 1,000 person-years | 500 person-years |
| 25 | 5.00 per 1,000 person-years | 200 person-years |
| 50 | 10.00 per 1,000 person-years | 100 person-years |
| 100 | 20.00 per 1,000 person-years | 50 person-years |
| 200 | 40.00 per 1,000 person-years | 25 person-years |
How does incidence rate change with the chosen multiplier?
The same 25 cases and 5,000 person-years, expressed against different multipliers.
| Expressed per | Incidence rate |
|---|---|
| 100 person-years | 0.50 per 100 person-years |
| 1,000 person-years | 5.00 per 1,000 person-years |
| 10,000 person-years | 50.00 per 10,000 person-years |
| 100,000 person-years | 500.00 per 100,000 person-years |
Questions
What is the incidence rate formula?
Incidence rate = new cases ÷ total person-time at risk, multiplied by a chosen denominator such as 1,000 or 100,000 so the result reads as a sensible rate rather than a small decimal.
How is incidence rate different from prevalence?
Incidence counts only new cases arising during a defined period, relative to person-time at risk. Prevalence counts all existing cases, new and old, at a single point in time, relative to the whole population. They answer different questions: how fast something is newly occurring versus how much of it exists right now.
Why use person-time instead of just the number of people?
People are often observed for different lengths of time within the same study or population. Person-time credits each individual only for the time they were actually at risk and being watched, which avoids overstating or understating the rate when follow-up periods are uneven.
What multiplier should I use?
It depends on how common the event is. Common events are often reported per 100 or per 1,000 person-years; rarer diseases or events typically use per 10,000 or per 100,000 so the resulting number is easy to read and compare.
For a simple share of a whole population at one point in time rather than a rate over person-time, use the difference between two numbers calculator to compare two related figures directly.