Monthly Retained Revenue (MRR Retention) measures the percentage of recurring revenue retained from existing customers from one month to the next. It helps businesses understand the stability of their revenue stream and the effectiveness of their retention strategies.

MRR Retention is calculated by dividing the MRR at the end of the month from existing customers by the MRR at the beginning of the month, excluding new customers.
MRR from existing customers includes renewals, expansion (eg plan upgrades, additional licenses) and contraction (eg plan downgrades, churn)

\[\text{MRR Retention} = \left(\frac{\text{MRR at End of Month from Existing Customers}}{\text{MRR at Start of Month}}\right) \times 100\]

1. Basic Example: A company has $100,000 in MRR at the start of the month and $95,000 from the same customers at the end of the month. The MRR retention is:

\begin{align}\text{MRR Retention} &= \left(\frac{95,000}{100,000}\right) \times 100 \cr&= 95\%\end{align}

2. Growth Example: A company starts the month with $200,000 MRR and retains $210,000 from existing customers by the end. The increase is due from customers upgrade to a more expensive plan. 
The MRR retention is:

\begin{align}\text{MRR Retention} &= \left(\frac{210,000}{200,000}\right) \times 100 \cr&= 105\%\end{align}