A theoretical method is proposed to extrapolate the steel temperature under standard fire tests for various cross sectional areas of steel and insulation thickness. The method utilizes the parameter estimation technique, where the parameter in the lumped heat transfer model is estimated to fit to the steel temperature rise in a standard fire resistance test. By using the estimated parameters, the steel temperature rise of similar assemblies can be calculated changing the steel cross sectional area and insulation thickness. An example is shown for a steel column (H-section, 300 × 300 × 13 × 15) insulated with calcium silicate board. The extrapolated results are compared with fire test data. It is shown that the extrapolated results give conservative results for most of the cases. Thus the method is useful for engineering calculations. However, the extrapolation overestimates the time to critical temperature if the method is applied to thinner columns under more severe mechanical conditions thus identifying the limitations of this method.