How the interest is worked out
Simple interest on the unpaid amount at the yearly rate, divided over 365 days, from the day after the due date up to and including the day of payment.
The in duplum rule stops interest once the unpaid interest equals the amount owed; the calculator applies that cap and says when it does.
Which rate
Use the rate in your contract data first. Where the contract says nothing, the prescribed rate of interest under the Prescribed Rate of Interest Act applies; it follows the repo rate with a lag and changes during the year, so the rate shown is the one in force today.
Worked example
A certificate of R1 000 000 was due on 1 October 2026 and paid on 31 October. That is 30 days late. At 10.5% a year the interest is R287.67 a day, R8 630.14 in total.