Household size distributions for 104 countries are examined. It is shown that a Poisson distribution truncated at zero can be used to derive models of household size distribution. An improved fit is obtained by adding a linear term to the truncated Poisson model. This distribution depends only on average household size which in turn is shown to be related to modified dependency ratios. This method can be used for comparisons of household size distributions across nations and for long-term forecasting.