What is the difference between point prevalence and period prevalence?

Point prevalence counts all individuals with the condition at a single moment in time (e.g., today). Period prevalence counts everyone who had the condition at any point during a defined time window (e.g., the past 12 months). Period prevalence is always greater than or equal to point prevalence.

What is the difference between prevalence and incidence?

Prevalence measures all existing cases in a population at a given time. Incidence measures only new cases arising over a period. Prevalence includes both new and long-standing cases, while incidence captures the rate of new disease onset.

How are the confidence intervals calculated?

This tool uses the Wilson score interval, which performs well for both small and large samples and avoids the impossible values (below 0 or above 1) that can arise from the normal approximation method.

When should I use per 1,000 vs. per 100,000?

Use per 1,000 for common conditions (e.g., hypertension, obesity). Use per 100,000 for rare diseases so the number is expressed as something greater than 1 and easier to communicate to non-specialist audiences.

Can cases exceed the population size?

No. The number of cases must be less than or equal to the population size. If cases equal the population, prevalence is 100% (1.0 as a proportion).

What population should I use?

Use the population at risk — the total number of people who could potentially have the condition. For many studies this is the total study population or census population of the area surveyed.

Prevalence Calculator

Name: Prevalence Calculator
Availability: InStock
Author: Nham Vu

Enter case counts and population size to calculate point or period prevalence with 95% confidence intervals.

Population Data

Prevalence Type

Point Period

Snapshot of cases at one moment in time.

Number of Cases

Individuals with the condition in the population.

Population Size

Total number of individuals in the study group.

Output Scale

Proportion (0–1) Per 1,000 Per 100,000

Quick Examples

Enter case count and population, then press
Calculate Prevalence to see results.

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Summary

Enter case counts and population size to calculate point or period prevalence with 95% confidence intervals.

How it works

Select the prevalence type: Point Prevalence (snapshot in time) or Period Prevalence (over a defined period).
Enter the number of existing cases (all affected individuals at the time of measurement).
Enter the total population size being studied.
Choose the output multiplier: proportion, per 1,000, or per 100,000.
The calculator divides cases by population, multiplies by the chosen scale, and computes exact Wilson score 95% confidence intervals.
Results include the prevalence estimate, confidence interval, and a plain-language interpretation.

Use cases

Cross-sectional survey analysis for chronic disease burden.
Public health surveillance reports and dashboards.
Estimating the prevalence of mental health conditions in a community.
Calculating period prevalence from registry data over a study year.
Grant proposals requiring epidemiological justification.
Teaching students the distinction between prevalence and incidence.
Health-needs assessments for resource allocation.

Frequently Asked Questions

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Last updated: 2026-05-23 · Reviewed by Nham Vu