Epidemiology
Incidence Rate
Incidence measures the frequency of new cases of a disease or event in a population at risk during a defined time interval. Public health agencies commonly report incidence per 100,000 population per week or per month.
Optional note—period is helpful when reporting “per 100,000 per 14 days”.
Inputs summary
- New cases: 125
- Population: 50000
- Period: 14 days
Incidence rate
250.0
Per 100,000 population
Sustain enhanced surveillance, targeted vaccinations, and communication with stakeholders.
Compare rates over time to monitor trends and intervention impact.
How to Use This Calculator
Count new cases accurately
Include only new cases occurring during the interval (exclude ongoing cases diagnosed earlier).
Define the population at risk
Use the population exposed to risk during the same period (e.g., county population, school enrollment).
Report transparently
Include the multiplier, time window, and data sources when communicating incidence rates.
Formula
Incidence rate = (Number of new cases ÷ Population at risk) × Multiplier
Common multipliers: 1,000; 100,000; 1,000,000 population.
Specify the time interval (e.g., weekly, monthly) alongside the rate.
Full Description
Incidence quantifies the appearance of new cases in a population over time and helps monitor outbreaks, assess intervention impact, and compare burden across regions. Accurate incidence reporting requires reliable case detection, clear definitions, and precise population denominators.
Incidence differs from prevalence (which measures existing cases) and attack rates (often used for short outbreaks). When communicating incidence, always specify the multiplier and time window (e.g., “87 cases per 100,000 population per 14-day period”).
Frequently Asked Questions
How is incidence different from prevalence?
Incidence counts new cases; prevalence counts all existing cases (new + pre-existing) at a point in time.
Can I use person-time instead of population?
Yes. Incidence density uses person-time denominators (cases ÷ person-years). This tool assumes a cumulative incidence.
What if case detection is incomplete?
Consider adjusting for under-reporting or complementing incidence with wastewater or syndromic surveillance data.
Why choose per 100,000 population?
Public health departments frequently use 100,000 to compare jurisdictions of different sizes, but you can select any multiplier.