Activation of machine components and air by primary and secondary beams is an inevitable byproduct of a particle accelerator operation. Evaluation of radiation field type and intensity during and shortly after accelerator run is crucial in its planning phase and for obtaining necessary licenses. Final results are usually obtained by employing numerical codes. In the present paper a transparent ab initio method is given for upper limit dose estimation in a linear electron accelerator. The gamma radiation level is calculated for the linear accelerator FLUTE. In addition possible nuclear reactions are identified and corresponding values for activations are quantified. This simple method was used extensively with success in our official application for an operational license of FLUTE.