A common question related to aerosol particle counters is how well particle counting results should match each other. New airborne particle counters added to an existing fleet won’t always match the counts of the old. This could be as a result of different manufacturer’s, but even when the particle counters are supplied by the same manufacturer, it is not uncommon for variation to exist. In this short paper, we provide a practical guide for the expectations one should have when comparing particle count data from similar and dissimilar instruments.