What is the formula to calculate $R_0$ (basic reproduction number)?
$$R_0 ∝ \left(\frac{\text{infection}}{\text{contact}}\right) · \left(\frac{\text{contact}}{\text{time}}\right) · \left(\frac{\text{time}}{\text{infection}}\right)$$
  
  More specifically:
  
  $$R_0 = τ · \bar{c} · d \quad \quad (1)$$
  
  where $τ$ is the transmissibility (i.e., probability of infection given contact between a susceptible and infected individual), $\bar{c}$ is the average rate of contact between susceptible and infected individuals, and $d$ is the duration of infectiousness.



Notes On $R_0$ (2007)




Further reading:


https://en.wikipedia.org/wiki/Basic_reproduction_number
Complexity of the Basic Reproduction Number (R0), emerging Infectious Diseases (2019)
Theory versus Data: How to Calculate R0?, PLoS One (2007)
The R0 package: a toolbox to estimate reproduction numbers for epidemic outbreaks, BMC Medical Informatics and Decision Making (2012)