Using a one-dimensional numerical model, this paper shows that level swell during top venting of a vessel containing a liquefied gas is affected by the axial distribution of void fraction in the vessel, which in turn depends on the distribution of vapour generation. Common methods for calculating level swell assume uniform vapour generation throughout the liquid, which results in considerable underprediction of level swell. This is attributed to their failure to include the effects of vapour generation at the vessel bottom. In this paper, an approximate theoretical analysis is proposed to estimate level swell, taking into account the effect of vapour generation at the bottom of the vessel. This approach leads to better (safer) results, irrespective of the way in which vapour generation and void fraction are related.