3GPP release 15 focusing on 5G general outline has been published in December 2017. The major difference with respect to currently deployed LTE is the support of various physical layer numerologies. Making the physical layer scalable allows to properly address new services such as low latency or millimeter communications. However it poses the problem of numerology coexistence. Indeed the orthogonality of the OFDM waveform is broken by the use of different subcarrier spacings and therefore multiplexed communications may interfere with each others. A first and simple solution to limit the distortion is to consider guard bands. In this paper, the authors develop analytical metrics to quantify the level of distortion induced by 5G multi-service multiplexing. Besides, general comments and guard band dimensioning are carried out. It is shown that high interference rejection needs to be associated with side lobe reduction techniques to favor an efficient bandwidth use.