The MRR waterfall formula
Ending MRR equals starting MRR plus new, expansion, and reactivation MRR, minus contraction and churned MRR. ARR is ending MRR multiplied by 12. This model keeps growth from new customers separate from retention within the existing customer base.
MRR is not trailing revenue
A payment processor can report revenue that is not recurring: lifetime deals, services, usage charges, annual prepayments, and one-time purchases. Use the calculator only with normalized recurring amounts. See the RevenueBug methodology for the distinction.