Late payment interest calculator for South Africa

Short answer: interest on an overdue invoice is the amount owed × the rate × the time overdue. For a business customer with no agreed rate, the default is the prescribed rate, 10.75% a year from 1 November 2026. For a private customer the most you can charge is 2% a month, and only if you disclosed it before the work. The calculator does the sum.

Which rate to enter

• You agreed a rate in your terms: enter that rate. This is the one that counts.
• You agreed nothing and the customer is a business: the prescribed rate applies, repo rate plus 3.5 percentage points. It is 10.75% from 1 November 2026, which is the figure pre-filled above. Under the Prescribed Rate of Interest Act the rate is generally the one in force when interest begins to run, so for an older debt check the rate for that date.
• The customer is a private individual: you can only charge interest you disclosed, and they accepted, on or before the date of supply, and not above 2% a month. Switch the calculator to a monthly rate to see it.

How the maths works

• Yearly rate: interest = amount × rate ÷ 100 × days overdue ÷ 365.
• Monthly rate: interest = amount × rate ÷ 100 × months overdue, where a month is a twelfth of a year (about 30.4 days).
• Simple interest only. The calculator does not compound, because interest generally cannot itself attract interest.
• The in duplum cap. Interest stops once the unpaid interest equals the original debt. The calculator stops at that line.

Our guide to interest on overdue invoices in South Africa explains each rule, with the sources and wording you can put in your terms. If the invoice stays unpaid, read how to recover it through the Small Claims Court, and use our follow-up messages first.

Questions people ask

How do I calculate interest on an overdue invoice?

Multiply the amount owed by the annual rate and by the number of days overdue, then divide by 365. For R10,000 at 10.75% a year for 30 days that is R10,000 × 10.75% × 30 ÷ 365, about R88.36. For a monthly rate, multiply the amount by the monthly rate and by the number of months overdue.

What interest rate should I use?

Use the rate in your agreed terms if you have one. If you agreed nothing and the customer is a business, the default is the prescribed rate of interest, which is the Reserve Bank repo rate plus 3.5 percentage points (10.75% from 1 November 2026). For a private customer you can only charge what you disclosed and they accepted before the work, and no more than 2% a month.

Does interest stop at some point?

Yes. Under the in duplum rule, interest stops running once the unpaid interest equals the original debt, so interest on a R10,000 invoice cannot grow past R10,000. The calculator applies that cap.

Is this legal advice?

No. It is a simple-interest calculator for working out a figure to put in a reminder or demand. Which rate applies, and when interest started to run, depends on your agreement and the customer. Read our guide, or ask an attorney before you sue.

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A simple-interest calculator, not legal advice. The prescribed rate and the rules that apply depend on your agreement and your customer; see the sources in our interest guide.