Parameters have been specified for electron beams with energies 5--45 MeV from a Brown Boveri betatron for use in computerised dosimetry calculations. A semi-empirical equation is given for the dose at any point at various depths in water. This equation is a modification to Kawachi's predictive model which was based on solutions of a general age--diffusion equation. The depth doses and isodose curves are predicted as a function of the practical range, source skin distance (SSD) and field size. Depth dose accuracy requirements of +/-2% above 50% depth dose and +/-5% at lower doses, relative to maximum dose, have been set and achieved. Also, the shape of the isodose curves with the constrictions at higher doses and bulging at lower values are accurately predicted. Computer calculated beams have been used to generate summed isodose distributions for certain clinical situations.