Eighteenth-century ship tonnages were imprecise and unstandardized. Comparisons across British sources show considerable variation, both systematic and unsystematic. Comparisons with continental units of measurement confirm, on average, the usual conversions for tonneaux, toneladas and lasts, but implicit conversions for individual vessels vary so much that they were clearly not used by contemporaries. Tonnages usually displayed pronounced heaping. Variation and heaping suggest that rather than being calculated using official formulae, tonnages were often approximated by ship-owners, surveyors and local officials. The British Shipping Act of 1786 brought greater precision to calculation and reporting, with British tonnages becoming increasingly standardized.