When considerations of molecular pakcing are combined with a simple thermodynamic treatment, it is possible to calculate many of the characteristics of a microemulsion from the molecular dimensions of the detergent. The only parameters required are the length and volume of the non-polar tail of the detergent and the apparent molar volume in water of the head-group, all of which can readily be estimated. It is shown that a cosurfactant is required when the structure of the detergent is such that the polar head-groups surrounding the microemulsion droplet would otherwise pack too closely together. Equations are derived which give (1) the size of the droplets as a function of the molar ratio of water to detergent, (2) the maximum droplet size that can be attained (the solubilisation capacity) and (3) indicate the size distribution. These equations give excellent agreement with experimental data.