This document specifies the measurement methods and strategies for determining the total number of airborne particles per unit volume of air indoor, using a condensation particle counter (CPC) for particles approximately between 10 nm to 3 µm. NOTE As the particle number concentration is usually dominated by the ultrafine particle (UFP) fraction, the obtained result can be used as an approximation of the UFP concentration. Quality assurance, determination of the measurement uncertainty and minimal reporting information are also discussed in this document. This document is applicable to indoor environments as specified in ISO 16000-1. This document does not address the determination of bioaerosols or the chemical characterization of particles. Nevertheless, some bioaerosols can be detected by the CPC and then contribute to the measured count of particles.