South Africa Late Payment Interest Calculator

How this calculator works

When a South African debtor pays a liquidated debt late, the creditor is entitled to mora interest (interest for delay). Unless the contract says otherwise, this is simple interest at the prescribed rate of interest set under the Prescribed Rate of Interest Act 55 of 1975. Since the Judicial Matters Amendment Act 2015, the prescribed rate is linked to the South African Reserve Bank repo rate plus 3.5 percentage points. The calculation is:

• Rate = repo rate + 3.5% (or your contract rate, if you enter one)
• Daily interest = invoice amount x (rate / 100) / 365
• Interest accrued = daily interest x number of days overdue
• Days overdue = payment date minus due date

The prescribed rate applicable to a debt is the rate in force when the debt fell due (when interest begins to run), so this calculator selects the rate from your due date. You can override it in the rate field.

The current prescribed rate and how it changes

The prescribed rate of interest is 10.5% per annum, effective 1 July 2026, following the SARB's increase of the repo rate to 7.0% in late May 2026 (7.0% + 3.5% = 10.5%). Before that the prescribed rate was 10.25% (repo 6.75%). A change in the repo rate takes effect for the prescribed rate on the first day of the second month after the change, which is why the late-May 2026 repo increase applied only from 1 July 2026. No Government Gazette notice is required for the change to take effect. Rates checked July 2026 against the repo rate published by the South African Reserve Bank.

Simple interest and the in duplum rule

Mora interest is calculated as simple interest unless a contract specifically provides for compounding. Two limits are worth knowing. First, the common-law in duplum rule stops arrear interest from exceeding the outstanding capital: once unpaid interest equals the capital sum, it stops running until a payment is made. Second, if your agreement is a credit agreement under the National Credit Act, statutory caps on interest and fees may apply instead of the prescribed rate. This calculator computes straight simple interest and does not apply the in duplum cap for you, so check it on long-outstanding debts.

Contract rate vs prescribed rate

If your invoice terms or contract state an interest rate for late payment, that agreed rate applies instead of the prescribed rate. Enter the agreed annual percentage in the rate field to override the default. Where a monthly rate is quoted (for example 2% per month), multiply by 12 to get the annual figure to enter here. If no rate was agreed, the prescribed rate is your fallback entitlement.

Charge late payment interest automatically with Tidybill

Tidybill lets you set a default late fee and interest rate and override it per client, so your prescribed-rate or contract interest is applied without manual calculation. It sends automated reminders before and after the due date and tracks every overdue invoice. Read the guide to small business invoicing in South Africa, try the generic late payment interest calculator, or browse all free calculators.

This calculator is a general guide, not legal or financial advice. The prescribed rate and repo rate change over time; figures were checked in July 2026. It does not apply the in duplum cap or National Credit Act limits. Confirm the applicable rate and your right to claim before relying on any figure, and seek professional advice for a specific dispute.