A set of handy formula and diagrams have been derived for calculating temperature in fire exposed concrete structures. Fire conditions according to the ISO 835 standard fire as well as natural fires may be assumed, and the influence of various material properties may be considered in a simple way. Two-dimensional structures may be analyzed. The method is particularly easy to use when the ISO 834 standard fire and normal concrete properties are assumed. The load capacity of a concrete element in a fire decreases as the strength of the reinforcement bars are reduces with the rising temperature. The method here outlined makes it possible to get good approximate temperatures of reinforcement bars as a function of distance to exposed surface and exposure time. The formula derived may easily be programmed into modern hand calculators. In the most simple case, when an ISO 834 standard fire and normal concrete properties are assumed, only time and depth need to be input to achieve a temperature of a reinforcement bar. The formula are obtained be comparison with more elaborate finite element computer calculations with the code TASEF-2. Although the computer analysis includes a material model with varying thermal conductivity and influences of water evaporating at about 100 oC, as well as non-linear boundary conditions, the simple method as outlined here predicts nearly the same results for a wide range of conditions as the more rigorous computer model.