A novel coronavirus (SARS-CoV-2) emerged as a global threat in December 2019. As the epidemic progresses, disease modellers continue to focus on estimating the basic reproductive numberR0—the average number of secondary cases caused by a primary case in an otherwise susceptible population. The modelling approaches and resulting estimates ofR0during the beginning of the outbreak vary widely, despite relying on similar data sources. Here, we present a statistical framework for comparing and combining different estimates ofR0across a wide range of models by decomposing the basic reproductive number into three key quantities: the exponential growth rate, the mean generation interval and the generation-interval dispersion. We apply our framework to early estimates ofR0for the SARS-CoV-2 outbreak, showing that manyR0estimates are overly confident. Our results emphasize the importance of propagating uncertainties in all components ofR0, including the shape of the generation-interval distribution, in efforts to estimateR0at the outset of an epidemic.