Incidence Rate Calculator — Epidemiology Rate per Population

Formula

How this is calculated

Incidence measures how often new disease events occur in a population over a defined time period. The incidence rate (or incidence density) is expressed per a standard population size — most commonly per 100,000 person-years in chronic-disease epidemiology, or per 1,000 or 100,000 for outbreak reporting. A higher multiplier does not mean a higher rate; it simply changes the scale of expression (e.g., 0.005 per person = 500 per 100,000).

The incidence proportion (also called the "attack rate" in outbreak investigations) is the fraction of the population that develops the disease during the observation period: cases ÷ population × 100. It is a dimensionless probability between 0 and 100%. In contrast to the incidence rate, the proportion does not explicitly capture the length of the observation window — it simply states what fraction got the disease in that period.

Important limitations: this calculator assumes a closed cohort (no new entrants or losses during the period) and that all individuals in the population are equally at risk from the start. For open cohorts (people entering and leaving), the correct denominator is person-time at risk (sum of each individual's time under observation), which requires individual-level data. Published disease rates are often age-standardised to account for differing age structures between populations — raw rates are not directly comparable across populations with different age profiles.