Two mathematical steady-state models for sizing single activated sludge systems aimed at nitrogen and organics removal from wastewater are proposed. The attention is focused on the combined nitrification system and the pre-denitrification system, considering three (soluble and particulate biodegradable organic matter and ammonia nitrogen) and four (the above cited and nitrate nitrogen) substrates, respectively. Growth and decay of two groups of bacteria (heterotrophic and autotrophic) are taken into account. If the influent flow composition and the kinetic parameters are known and the effluent nitrogen concentration is fixed, both models give explicit expressions of the volumes of the biological reactors. Moreover, the pre-denitrification model makes it possible to define a priori the applicability of the system for a particular wastewater. Model applications show that the nitrification reactor volume increases in the case of a higher influent organic load or to achieve a higher nitrogen ammonia removal efficiency. A smaller denitrification volume is required to face a higher influent organic load. A sensitivity analysis on the kinetic parameters shows an important influence of the maximum ammonia nitrogen removal rate and the ratio between the decay rates of heterotrophic and autotrophic biomass on the nitrification reactor volume.