existing payment volume
Existing payment volume is everything a software platform's customers already take in payments, across every provider they use today. It covers every customer taking payments through or beside the platform's product, whoever processes them now, and is the starting point for working out what built-in payments could earn.
Most software platforms already have a payments business; it is just spread across other people's providers. Each customer takes cards on someone's terminal, collects Direct Debits through someone's service, sends invoices paid by bank transfer. Add it up and that is the platform's existing payment volume: the customers, the amounts, the methods and the providers, as they stand today.
Why it matters: this is the number everything else is measured against. What a platform can earn from payments is that volume, multiplied by the share of it that moves onto the platform's own payments, multiplied by the margin the platform keeps. Two of those three are decided by how well the move is planned. The first is a fact, and most platforms have never counted it, because the volume runs through providers they do not see.
What to watch: existing payment volume is not what runs through a built-in payments feature. It counts every customer who takes payments at all. A platform with 2,000 customers and 150 on its payments feature has 2,000 customers' payments to size, of which 150 have moved. Sizing the whole thing, by method and by provider, is the first stop in a payments review.