An important uncertainty when estimating per capita consumption of e.g. illicit drugs by means of wastewater analysis (often referred to as sewage epidemiology) relates to the size and variability of the de facto population in the catchment of interest. In the absence of a day-specific direct population count any indirect surrogate model to estimate population size lacks a standard to assess associated uncertainties. Therefore, the objective of this study was to collect wastewater samples at a unique opportunity, i.e. on a census day, as a basis for a model to estimate the number of people contributing to a given wastewater sample. Mass loads for a wide range of pharmaceuticals and personal care products were quantified in influents of ten sewage treatment plants (STP) serving populations ranging from approximately 3,500 to 500,000 people. Separate linear models for population size were estimated with the mass loads of the different chemical as the explanatory variable: fourteen chemicals showed good, linear relationships, with highest correlations for acesulfame and gabapentin. Bayesian inference was then used to estimate de facto population using the population size provided by STP staff as prior knowledge and updated that with the measured chemical mass loads. Cross validation showed that large populations can be estimated fairly accurately with a few chemical mass loads quantified from 24-h composite samples. In contrast, the prior knowledge for small population sizes cannot be improved substantially despite the information of multiple chemical mass loads. In the future, observations other than chemical mass loads may improve this deficit, since Bayesian inference allows including any kind of information relating to population size.