Knowledge of the TTT or CCT diagrams of steels is an important factor in the thermomechanical processing of steels. Much experimental work has been undertaken to determine such diagrams. However, the combination of wide alloy specification ranges, coupled with sharp sensitivity to composition changes plus a dependency on grain size, means that it is impossible to produce enough diagrams for generalised use. To this end significant work has been undertaken over recent decades to develop models that can calculate TTT and CCT diagrams for steels. Almost without exception, these models have been shown to be limited in applicability to carbon and low alloy steels. The aim of the present work is to develop a model that can provide accurate TTT and CCT diagrams for general steels, including medium to high alloy types, tool steels, 13%Cr steels etc., for inclusion in the software programme JMatPro. This aim has been achieved and the present paper provides a background to the calculation method and present results of an extensive validation of the model against experiment.