Calculate period prevalence from cases and population at risk, or determine the missing value from any pair of inputs.
Customize This Calculator
Build your own version. Describe what you want changed, added, or compared.
Period Prevalence Formula
Period prevalence measures the proportion of a population that had a condition at any time during a defined period.
- Period Prevalence (%) = percentage of the at-risk population with the condition during the period
- Number of Cases = people who had the condition at any point during the period
- Population at Risk = people who could have developed or had the condition during the period
The period must be clearly defined, such as one month, one year, or a study interval. The population at risk should match that same period and setting.
Useful Tables
| Measure | What it tells you | Example wording |
|---|---|---|
| Point prevalence | Cases present at one specific time | Prevalence on January 1 |
| Period prevalence | Cases present at any time during a period | Prevalence during 2024 |
| Incidence | New cases occurring during a period | New diagnoses in 2024 |
| Prevalence | Equivalent proportion | Approximate interpretation |
|---|---|---|
| 1% | 0.01 | 1 in 100 people |
| 5% | 0.05 | 5 in 100 people |
| 10% | 0.10 | 10 in 100 people |
| 25% | 0.25 | 1 in 4 people |
Example
If 80 people in a population of 2,000 had a condition at any time during one year, the period prevalence is:
(80 / 2,000) × 100 = 4%
That means 4% of the at-risk population had the condition during that year.
Do not count the same person more than once for the same condition in the same period.
