This briefing paper addresses the question of when the UK should aim for zero (or net zero) carbon emissions. Starting from the global carbon budget which would allow the world an estimated 66% chance of limiting climate warming to 1.5oC, the paper derives a carbon budget for the UK of 2.5 GtCO2. The briefing then analyses a variety of emission pathways and target dates in terms of their adequacy for remaining within this budget. A key finding is that a target date for zero carbon is not sufficient to determine whether the UK remains within its carbon budget. Policy must specify both a target date and an emissions pathway. For a linear reduction pathway not to exceed the carbon budget the target year would have to be 2025. Nonlinear pathways, such as those with constant percentage reduction rates, have a higher chance of remaining within the available budget provided that the reduction starts early enough and the reduction rate is high enough. It is notable that reduction rates high enough both to lead to zero carbon (on a consumption basis) by 2050 and to remain within the carbon budget require absolute reductions of more than 95% of carbon emissions as early as 2030. On this basis, the paper argues in favour of setting a UK target for net zero carbon emissions by 2030 or earlier, with a maximum of 5% emissions addressed through negative emission technologies.