What the SIR model does
The SIR model is a foundational tool in epidemiology that divides a population into three groups: Susceptible (not yet infected), Infected (currently infectious), and Recovered (immune or removed). It tracks how people move between these groups over time to project the shape of an outbreak.
How this calculator works
Using your inputs — population size, initial infections, the basic reproduction number (R0), and the average infectious period — the calculator derives the model’s transmission rate (beta) and recovery rate (gamma), then steps the classic SIR differential equations forward in small time increments to estimate the peak number of simultaneous infections, when that peak occurs, and the total share of the population ultimately infected (the attack rate).
Note: the SIR model is a simplified, idealized representation. It assumes uniform mixing and constant parameters, and does not account for interventions, immunity waning, age structure, or new variants. This tool is for general and educational exploration only, not for operational public health planning.