Free legal calculator
Interest on a debt at the prescribed rate
Work out the interest on an unpaid debt at the prescribed rate of interest, a rate that changed with every repo-rate decision, or the rate the parties agreed. Add part-payments and see where the in duplum rule stops the clock.
Nothing you enter is saved. Rates, tariffs and tax tables come from the published sources we maintain, dated so each result uses the figures in force on the dates you give. Every result shows where its numbers came from.
A calculation aid, not legal advice. Check the result against the Act, the rules of court and any practice directive of the division before relying on it. If a deadline matters, speak to an attorney.
Good to know
Questions, answered
- What is the prescribed rate of interest?
- The rate the Prescribed Rate of Interest Act 55 of 1975 applies to a debt when no law, agreement or trade custom sets a different one. Section 1(2) makes it the Reserve Bank repo rate plus 3.5 percentage points, effective from the first day of the second month after the Reserve Bank sets a new repo rate.
- Which rate applies to my debt?
- Section 1(1) applies the prescribed rate in force when interest begins to run, and that rate stays for the life of the debt even if the prescribed rate later changes. That is the default here. If an agreement or court order says interest runs "at the prescribed rate from time to time", choose the option that follows each change. If the parties agreed a rate, enter it instead.
- From when does interest run?
- Interest for late payment (mora interest) runs from the date the debtor is in default: the agreed due date, or where there is none, the date of a demand. For an unliquidated claim such as damages, section 2A of the Act runs it from the date of a written demand or service of summons, whichever is earlier. Enter that date as the start date.
- What is the in duplum rule?
- Unpaid interest stops running once it equals the unpaid capital, so interest can never more than double the debt. If later payments bring the unpaid interest below the capital, interest runs again. The rule is suspended while litigation is pending and applies afresh after judgment, so switch it off for a period that runs while an action is pending if that applies to you.
- How are payments allocated?
- Each payment pays off the interest owing on its date first, and only the balance reduces the capital. That is the common-law default and the order the National Credit Act prescribes (interest, then fees, then capital). An agreement can provide otherwise.
- Is the interest compounded?
- No. Interest at the prescribed rate is simple interest, calculated per day on a 365-day year. Interest is not charged on interest.
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