People are more than 80% of their time indoors. However, in contrast to ambient air, no (legal) limits for indoor particulate matter exist. This paper describes a measurement protocol to determine the yearly averaged indoor PM2,5 concentration and gives reference values for five quality classes: <2,5 μg/m3 (A+), <10 μg/m3 (A: WHO annual advisory value), <17,5 μg/m3 (B), <25 μg/m3 (C: EU limit value) and above (D). The indoor PM2,5 concentration is measured with an optical particle counter. The hourly outdoor concentration is derived from an outdoor station of an air quality monitoring network. This allows to determine an estimate for the infiltration coefficient and the indoor PM2,5 sources based on one week of data. The infiltration coefficient is a characteristic parameter for the efficiency of the air filters and the air tightness of the building. Based on these parameters and the yearly averaged ambient concentration a good approximation of the yearly averaged indoor PM2,5 concentration can be derived. Three case studies in offices are discussed: 4,4 (F7 filter), 5,8 (F7 filter) and 1 μg/m3 (F6 filter). The results clearly show that not only the filter quality but also the building air tightness is a very important parameter.