How much misinformation do people encounter? Published estimates range from a fraction of one percent to strong majorities. We systematically reviewed 443 studies reporting 1048 estimates of misinformation prevalence, exposure, sharing, and concentration. In behavioural studies, misinformation is a median 1.3% of what people consume online; it reaches 12.0% of people at least once and makes up 13.0% of what they share. In surveys, 57.7% say they have seen misinformation and 19.8% say they have shared it. In content analyses, 23.0% of items are classified as misinformation. How misinformation was identified (by fact-checkers, domain lists, researchers, or classifiers) and at which level (sources or items) explained more of the variance between estimates than what the study was about; platform and topic explained less. The top 1% of users account for a median 70% of misinformation sharing or exposure. These results help contextualize the diversity of misinformation estimates.