Are There 10³¹ Virus Particles on Earth, or More, or Fewer?

The number of virus particles on Earth is frequently reported in the scientific literature and in general-interest publications as being on the order of 10³¹, with some confusion about whether this is a high or low estimate. This number is often given without a source, although it should be attributed to a paper by Hendrix et al. published in 1999 (R. W. Hendrix, M. C. Smith, R. N. Burns, M. E. Ford, and G. F. Hatfull, Proc Natl Acad Sci U S A 96:2192-2197, 1999, https://doi.org/10.1073/pnas.96.5.2192). As with any oft-repeated statistic, it is informative to know how it has been derived and whether it should be revised in the light of new evidence. I review the history of the 10³¹ estimate and use more recent assessments of the number of bacterial and viral particles in various habitats to conclude that the best estimate of the number of virus particles on Earth ("the Hendrix product") remains close to 10³¹ and is unlikely to be either much less or much more than that.