Microwave heating inside a cavity is not uniform and depends on various factors such as: cavity geometry, properties of treated material and frequency. In most applications of microwave (MW) heating there is only a small variation in the nature of the feed. At present, the assessment of uniformity of heating is possible only for the case of batch processing cavities. Therefore, it is desirable to have a parameter that will quantify uniformity of heating for continuously operated (non-batch) applicators. This paper has given an account of and the reasons for the widespread use of a new smoothness parameter (SP). This parameter is also compared to the already available parameter (designed for batch processes) through Finite-difference time-domain (FDTD) simulations. Assuming a uniform load, it was demonstrated that heating time and applied microwave power have only a small influence on the proposed uniformity parameter. Simulations of heating were carried out using a variable feed inside two microwave applicators with different uniformity parameters. The results are presented in this paper. (C) 2013 Elsevier Ltd. All rights reserved.