To derive design diagrams of insulated steel columns, a methodology was proposed to extrapolate the temperature rise of specific steel column assembly under fire resistance test to those differing in cross sectional area of steel. Having a set of steel column temperature measurement, the thermal conductance between fire resistance furnace and steel was estimated by the technique of parameter estimation. Then, using the estimated thermal conductance, temperature rise of steel columns with arbitrate cross sectional area was calculated while keeping the construction of insulation unchanged. The results were compared with fire resistance test results that were conducted separately, The agreement was good or conservative if the cross sectional area were larger than the reference cross sectional shape. Thus it was shown that the methodology is feasible to develop design diagrams for insulated steel columns. A practical chart and simple design formula was produced for columns insulated by calcium silicate boards.