Basic Reproduction Number (R0) Calculator

Formula used

• Contact-based: R0 = c × p × D, where c is contacts/day, p is transmission probability per contact, and D is infectious duration.
• SIR rates: R0 = β / γ, where β is the transmission rate and γ is the recovery/removal rate.
• Growth approximation: R0 ≈ exp(r × Tg), where r is exponential growth rate and Tg is generation time.
• Adjusted effective value: Re = R0 × s × (1 − u), where s is susceptible proportion and u is intervention reduction fraction.
• Herd immunity threshold: HIT ≈ 1 − 1/R0 (when R0 > 1).

How to use this calculator

1. Select the method that matches your available measurements.
2. Enter the required inputs; use realistic units and time scales.
3. Optionally set susceptible proportion and intervention reduction.
4. Press Calculate to view results above.
5. Use CSV/PDF buttons to save a compact report for sharing.

FAQs

1) What does R0 represent?

R0 is the average number of secondary infections caused by one infectious person in a fully susceptible population. It summarizes transmission potential under specific biological and behavioral conditions.

2) How is Re different from R0?

Re adjusts transmission for current conditions, such as immunity and interventions. This calculator estimates Re by applying your susceptible proportion and reduction settings to the base R0 value.

3) Which method should I choose?

Use contact-based if you can estimate contacts, probability, and duration. Use SIR rates when β and γ are known. Use growth approximation when you have early growth rate and generation time.

4) Why can different methods give different values?

Each method relies on different assumptions and data sources. Measurement error, changing behavior, and heterogeneous mixing can shift estimates. Consistency improves when inputs describe the same setting and time window.

5) What does “herd immunity threshold” mean?

It approximates the fraction of the population that must be immune to bring sustained spread below one, assuming uniform mixing. It is computed as 1 − 1/R0 when R0 is above one.

6) How should I set the susceptible proportion?

Use 1.0 for a mostly susceptible population. If prior immunity exists, enter the estimated fraction still susceptible, such as 0.7. This affects Re but does not change the base R0.

7) What is the uncertainty band used for?

It provides a quick sensitivity view by applying a +/- percentage around computed outputs. It is not a statistical confidence interval, but it helps you understand how results might vary with uncertainty.

8) Can I use this for clinical or policy decisions?

This is an educational calculator, not medical advice. For decisions, use validated epidemiological analyses with surveillance data, uncertainty quantification, and expert review aligned to local context.