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The calculation of incidence rates of work-related ill-health (WRIH) is essential when assessing employment sectors at risk. The Health and Occupational Reporting network in General Practice (THOR-GP) collects information on WRIH from approximately 250 GPs. To calculate rates, GP numerator data has to be divided by a compatible denominator.To formulate the best methods of determining incidence rates from THOR-GP dataThe THOR-GP denominator is the registered population of all participating GPs’ practices. Census data linked to practice postcodes were used to establish how the population was employed; practice list data were used to estimate the size of the population and adjusted for GP participation. Numerator data (incidence cases of WRIH) from 2006–2015 were adjusted for response rates and time sampling frequency. Numerator data were then divided by the denominator and rates calculated per 1 00 000 persons employed per annumThe incidence rate of WRIH (from 6491 numerator cases) was 1350 per 1 00 000 persons employed per annum. Musculoskeletal disorders had the highest rates (640) followed by mental ill-health (516). Rates for musculoskeletal disorders were highest in agriculture (2232) and construction (1048) and for mental ill-health, in financial services (1113) and public administration/defense (1124).All employed persons have access to a GP therefore calculating incidence rates from GP data allow identification of industries/occupations at higher risk of WRIH Using postcode-based census data to characterise the denominator (therefore specifically related to the numerator) enables rates to be calculated with greater accuracy compared with using national employment data.