Visual mental imagery is the ability to create a quasi-perceptual visual picture in the mind's eye. For people with the rare trait of aphantasia, this ability is entirely absent or markedly impaired. Here, we aim to clarify the prevalence of aphantasia in the general population, while overcoming limitations of previous research (e.g., recruitment biases). In Experiment 1, we screened a cohort of undergraduate students (n502) using the Vividness of Visual Imagery Questionnaire (Marks, 1973) and found that 4.2% had aphantasia. To establish the reliability of our estimate, we then screened a new sample of people (n502) at an online crowdsourcing marketplace, again finding that approximately four percent (3.6%) had aphantasia. Overall, our combined prevalence from over a thousand people of 3.9% - which shows no gender bias - provides a useful index for how commonly aphantasia occurs, based on measures and diagnostic thresholds in line with contemporary aphantasia literature.