Late Payment Interest Calculator (South Africa)
Work out simple interest on an overdue invoice from due date to payment date, with the in duplum cap applied automatically. Free, no signup.
Can you charge interest on overdue invoices?
Yes. If your terms specify a rate, that rate applies. If not, the prescribed rate of interest applies by default — it changes with the repo rate, so confirm the current figure in the Government Gazette before relying on it.
How the calculation works
Simple interest: amount × annual rate ÷ 365 × days overdue. Under the in duplum rule, arrear interest stops accruing once it equals the outstanding principal.
The better fix: follow up yourself
Interest is leverage, but consistent follow-up is what actually gets SA invoices paid. PopPay does not send WhatsApp or SMS for you. You can still share invoice links yourself. A PopPay account is R0/month, with an optional card payment link. Hosted bank checkout is off. Built for South African SMEs.