Formula
Sum of MRR increases from existing customers in a period
How to measure
Compare each existing customer's MRR at the start and end of the period, summing only the positive increases.
Why track it
Expansion MRR shows how much growth is coming from deepening existing relationships versus acquiring new ones, usually the cheaper growth source.
Example
A company adds $12,000 in expansion MRR this month purely from existing customers adding seats.