Abstract This paper examines the macro-economic benefits of the Single Market in goods and services by simulating a counterfactual scenario in which tariffs and non-tariff barriers are reintroduced. In this counterfactual scenario, intra-EU trade flows are significantly reduced. Lower trade openness also means reduced market size and less competition. Using empirical evidence on the effect of the Single Market on firms’ mark-ups over marginal costs, we add these competition effects and arrive at a total estimate of around 9% higher GDP on average for the EU, but with a strong degree of heterogeneity across EU countries.