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Based on an energy balance for the compartment gases the thermal exposure in fully developed fires is determined as a function of opening factor, fuel load and thermal inertia of the bounding walls. A conversion factor is calculated which makes it possible to carry out all calculations in a standard compartment by applying an equivalent fuel load and an equivalent opening factor. The conversion factor is determined as the thermal inertia in the standard compartment divided by the thermal inertia in the real compartment. The errors from this approximate method are estimated from the maximum steel temperatures of an unprotected and a protected steel structure placed inside the standard compartment and the real compartment. Nine different compartments are used and apart from the compartment made of steel the greatest error is smaller than 13%. The normalized heat load concept introduced by Harmathy ( 7 ) is used as a principle of equivalent fire duration in the fire test furnace, in the standard compartment and in a compartment with low thermal inertia. The accuracy of the concept is estimated from the maximum steel temperature of a protected structure.