A common question related to aerosol particle counters is how well particle counting results should match each other. An example of when this becomes important is new particle counters being added to an existing fleet. The new instruments could be supplied from the original manufacturer or from a new vendor. Even when the particle counters are supplied by the same manufacturer, it is not uncommon for different models to be purchased because of improved specifications or product obsolescence. All of these considerations are important because they can impact the resulting particle count data. The purpose of this short application note is to provide a practical guide for the expectations one should have when comparing particle count data from similar and dissimilar instruments.