| Study | Type | Cases at point/start | New cases | Total population | Excluded | Effective population | Prevalence % |
|---|---|---|---|---|---|---|---|
| Community Survey A | Point | 48 | 0 | 1,200 | 0 | 1,200 | 4.00% |
| School Screening B | Period | 30 | 12 | 900 | 20 | 880 | 4.77% |
| Clinic Audit C | Point | 76 | 0 | 2,500 | 100 | 2,400 | 3.17% |
| Regional Follow-up D | Period | 110 | 24 | 4,000 | 200 | 3,800 | 3.53% |
These rows show how both point and period prevalence use case counts and the effective population after exclusions.
Point prevalence
Point prevalence = Current cases ÷ Effective population
Use this when you want the proportion of people affected at one specific time point.
Period prevalence
Period prevalence = (Cases at start + New cases during period) ÷ Effective population
Use this when you want the proportion affected at any time during a defined period.
Effective population
Effective population = Total population − Excluded population
This improves the denominator when missing, duplicate, or ineligible records should not be counted.
Confidence interval
Standard error = √[p × (1 − p) ÷ n]
Confidence interval = p ± Z × Standard error
Here, p is the prevalence proportion and n is the effective population.
Rate conversion
Rate per selected multiplier = Prevalence proportion × Multiplier
- Enter a study or dataset name for your report.
- Choose Point prevalence or Period prevalence.
- Fill in the total population and any excluded people.
- For point mode, enter the number of current cases.
- For period mode, enter cases at the start and new cases during the period.
- Select a confidence level and a rate multiplier.
- Choose how many decimal places you want displayed.
- Press Calculate prevalence to show the result above the form.
- Use the export buttons to download a CSV or PDF version.
1. What does prevalence measure?
Prevalence measures how common a condition is within a population. It reports the proportion of people who currently have the condition, or who had it during a defined period.
2. What is the difference between point and period prevalence?
Point prevalence uses cases observed at one specific moment. Period prevalence includes cases already present at the start plus new cases that appeared during the selected period.
3. Why is excluded population included?
Excluded population lets you remove people who should not be part of the denominator. This is useful for missing records, ineligible participants, or duplicate data entries.
4. Can prevalence be shown per 1,000 or 100,000?
Yes. The calculator converts the prevalence proportion into a rate using your chosen multiplier. This makes results easier to report in health, survey, and demographic studies.
5. Why is a confidence interval helpful?
A confidence interval shows the likely range around the estimated prevalence. Narrow intervals suggest more precision, while wider intervals suggest more uncertainty in the estimate.
6. Does this calculator measure incidence?
No. Incidence focuses on new cases only. This tool measures prevalence, which reflects the overall burden of existing cases at a point or during a period.
7. When should I use period prevalence?
Use period prevalence when your report covers a time window, such as a semester, quarter, or year. It is helpful when cases can appear at any time during that window.
8. Can I use this for non-medical datasets?
Yes. Prevalence methods can also summarize non-medical conditions, such as risk states, behaviors, defects, compliance gaps, or survey attributes within a population.