Atmospheric clusters play a key role in atmospheric new particle formation and they are a sensitive indicator for atmospheric chemistry. Both the formation and loss of atmospheric clusters include a complex set of interlinked physical and chemical processes, and therefore their dynamics is highly non-linear. Here we derive a set of simple equations to estimate the atmospheric cluster concentrations in size ranges of 1.5–2 nm and 2–3 nm as well as 3–6 nm aerosol particles. We compared the estimated concentrations with measured ones both in a boreal forest site (the SMEAR II station in Hyytiälä, Finland) and in an urban site (the AHL/BUCT station in Beijing, China). We made this comparison first for 3–6 nm particles, since in this size range observations are more reliable than at smaller sizes, and then repeated it for the 2–3 nm size range. Finally, we estimated cluster concentrations in the 1.5–2 nm size range. Our main finding is that the present observations are able to detect a major fraction of existing atmospheric clusters.