The measurement of non-market output, characterised by providing goods and services without economically significant prices, has always proved challenging for compilers of the National Accounts. Various approaches are available to meet these challenges, often resulting in slight differences in methodology between countries. Government policies, introduced in response to the coronavirus (COVID-19) pandemic exacerbated some of these existing differences, potentially influencing the GDP estimates across countries. This joint paper by the United Kingdom Office for National Statistics (ONS) and the Organisation for Economic Cooperation and Development (OECD) explains the methodological options available to statistical compilers and explores differences in methodologies used by countries to measure non-market output, analysing their implications for international comparisons of GDP growth during the COVID-19 pandemic.