Prevalence Calculator
Prevalence as a percentage, per 100,000 people, and as a one-in count.
What it does
Prevalence describes how much of a population has a condition at a chosen time or during a chosen period. It is a snapshot of burden, not a measure of how fast new cases appear.
The numerator is every current case, including people diagnosed earlier who are still counted. The denominator is the population at risk, including those cases. Mixing a count of new cases with a prevalence denominator gives a number that is neither prevalence nor incidence.
The defaults describe 468 cases in a population of 1,150. Prevalence is 40.7%, or 40,696 per 100,000, which is about 1 in 2.5. The result can be read as a percentage or as a rate per 100,000.
Prevalence rises when a condition lasts longer, even if new cases arrive at the same rate. That is why a chronic disease can have a high prevalence while its incidence stays low.
How to use
- Enter the number of current cases at the chosen time or during the period.
- Enter the population at risk, including the cases.
- Press Calculate to see the percentage.
- Read the per 100,000 figure for public health reporting.
- Use the one-in value when it is easier to explain than a percentage.
- State whether the estimate is point prevalence or period prevalence.
Formula
- cases
- People with the condition at the chosen time or during the period.
- population
- Everyone in the group being described, including the cases.
- prevalence
- The proportion of the population with the condition.
The population must be at least as large as the case count. A zero case count gives a prevalence of 0 and no one-in value, because the reciprocal is undefined. Point prevalence uses one date; period prevalence counts anyone with the condition at any time during the window.
Worked example
Example: 468 cases in a population of 1,150
Defaults: 468 cases and a population of 1,150.
- Prevalence: 468 / 1,150 = 0.4069565.
- As a percentage: 0.4069565 x 100 = 40.7 %.
- Per 100,000: 0.4069565 x 100,000 = 40,696.
- One-in count: 1,150 / 468 = 2.457.
- So the result is 40.7 %, 40,696 per 100,000, or about 1 in 2.5.
- The denominator includes the 468 cases, as a prevalence denominator should.
Reference table
| Cases | Population | Prevalence | Per 100,000 |
|---|---|---|---|
| 1 | 100 | 1 % | 1,000 |
| 10 | 1,000 | 1 % | 1,000 |
| 50 | 1,000 | 5 % | 5,000 |
| 468 | 1,150 | 40.7 % | 40,696 |
Limits
- Point prevalence and period prevalence answer different questions.
- The denominator must be the population at risk, not a convenient total.
- Prevalence says nothing about how long people stay cases or how fast new cases appear.
- Counts from a sample need a margin of error before they describe a whole population.
Common mistakes
- Using new cases only and calling the result prevalence.
- Leaving cases out of the denominator.
- Mixing a one-year case count with a mid-year population.
- Comparing prevalence across places with different age structures without adjustment.
- Reading one in 2.5 as a lifetime risk.
Try it yourself
Quick checks. Answers sit tucked under each question.
What is the prevalence for 50 cases in 1,000 people?
5 %, or 5,000 per 100,000, which is 1 in 20.0.
What is the prevalence for 2 cases in 500 people?
0.4 %, or 400 per 100,000, which is 1 in 250.0.
What happens when there are zero cases?
Prevalence is 0 % and 0 per 100,000. The one-in value is not applicable.
Questions
What is the difference between prevalence and incidence?
Prevalence counts all current cases. Incidence counts new cases over a period, so it measures how often the condition develops.
Is prevalence a percentage or a rate?
It is a proportion. It can be written as a percentage or scaled to a rate per 100,000 for reporting.
What is point prevalence?
The share of the population with the condition on one specific date.
What is period prevalence?
The share with the condition at any time during a defined window, such as a year.
Why does prevalence include old cases?
Prevalence measures current burden, and people diagnosed earlier still have the condition if they have not recovered or died.
Can prevalence be above 100%?
No. Cases cannot exceed the population, so the tool rejects a case count above the denominator.
Sources
- CDC, Principles of Epidemiology, morbidity frequency measures
- NIMH, what is prevalence?
- StatPearls, Prevalence