Monthly Recurring Revenue (MRR) is the total amount of predictable revenue that a subscription-based business expects to receive every month. It is the lifeblood of the SaaS business model, providing a level of financial stability that traditional transaction-based businesses lack. MRR is calculated by summing the monthly fees from all active paying subscribers. It is important to distinguish between 'New MRR' (from new customers), 'Expansion MRR' (from upgrades), and 'Churn MRR' (lost revenue). MRR allows founders to make data-driven decisions about hiring, product development, and marketing spend because it provides a reliable floor for future income. It excludes one-time implementation fees or consulting revenue, focusing strictly on the recurring software component. Tracking MRR growth trends is the most effective way to measure the momentum of a subscription business and is the primary metric used by investors to determine early-stage company valuations.