This document specifies a standard method for determining the particle number concentration in ambient air in a range up to about 107 cm–3 for averaging times equal to or larger than 1 min. The standard method is based on a Condensation Particle Counter (CPC) operated in the counting mode and an appropriate dilution system for concentrations exceeding the counting mode range. It also defines the performance characteristics and the minimum requirements of the instruments to be used. The lower and upper sizes considered within this document are 10 nm and a few micrometres, respectively. This document gives guidance on sampling, operation, data processing and QA/QC procedures including calibration parameters.