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Effective Reproduction Number Calculator

Work out effective reproduction number instantly with clear inputs, formula shown and shareable results.

About Effective Reproduction Number Calculator

Scales the basic reproduction number by the susceptible fraction and the effect of control measures to give the effective reproduction number.

Formula

Effective reproduction number

Rt = R0 x susceptible fraction x (1 - intervention effect); further reduction needed = 1 - 1/Rt when Rt exceeds 1

Worked example

With the default values below, Effective Reproduction Number Calculator returns Effective reproduction number of 1.440. Change any input above and every figure here is recalculated in your browser.

Inputs used

Basic reproduction numberaccepts 0.1–20
3
Share of the population still susceptibleaccepts 1–100
60
Reduction in transmission from measuresaccepts 0–95
20

Results

Effective reproduction number
1.440
Rt without control measures
1.800
Epidemic direction
Above 1 — epidemic growing
Further transmission reduction needed
30.56%

Step by step

  1. 1Values used
  2. 2Effective reproduction number
  3. 3Effective reproduction number= 1.440
  4. 4Rt without control measures= 1.800
  5. 5Epidemic direction= Above 1 — epidemic growing
  6. 6Further transmission reduction needed= 30.56

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why does Rt fall as an epidemic proceeds?

The susceptible pool depletes, so each infectious person meets fewer people who can be infected.

How is Rt estimated in practice?

From the case time series and the serial interval distribution, using methods such as Cori or Wallinga-Teunis.

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