How to calculate debt interest: examples and limits

To calculate interest, check the balance, rate, period and method in the agreement. An annual rate, a monthly rate, APR and a flat rate are not interchangeable. Actual statements may also reflect repayments, new borrowing, fees and daily interest.

Simple interest

With a constant principal, simple interest is principal × annual rate × time in years. Express 5% as 0.05. For £100 at 5% simple interest for two years, interest is £100 × 0.05 × 2 = £10 and the total is £110, assuming no repayments or other charges.

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Compound interest

Where interest is added to the balance and itself attracts interest, the debt compounds. At 5% compounded annually, £100 becomes £100 × 1.05 × 1.05 = £110.25 after two years, assuming no payments or fees. The compounding interval and agreement determine the actual calculation.

This is a guidance tool only and not an assessment. For accurate interest calculations, contact the company issuing the credit. Do not rely solely on this calculator’s results.

Can I divide the annual rate by 12?

Only where that matches the stated nominal-rate method. Dividing an effective annual rate or APR by 12 is not generally the correct equivalent monthly rate. For an effective annual rate, the equivalent monthly rate is (1 + annual rate) raised to the power 1/12, minus 1. Use the lender’s statement or an appropriate calculator for a repayment schedule.

APR versus a flat rate

APR expresses the annualised cost under prescribed assumptions, including relevant compulsory charges. It is useful for comparison but does not mean every possible default or optional charge is included. A flat rate calculates interest using the original principal even as payments reduce the balance, so it should not be compared directly with APR. Check total repayment and fees as well.

Is statutory interest added to every late debt?

No. Interest may arise under a contract, a particular statute or a court order. The commercial late-payment regime can allow 8% above the Bank of England base rate for qualifying business-to-business debts, but that is not a universal charge on consumer borrowing. See the commercial-payment rules.

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Is 25% the maximum legal UK interest rate?

No general UK “criminal usury” rule caps every loan at 25%. Specific products have their own restrictions. For example, high-cost short-term credit covered by the FCA price cap has daily-cost, default-fee and total-cost limits. Those rules must not be applied indiscriminately to every personal loan. See FCA CONC 5A.

What if I pay late?

A missed due date may result in consequences under the agreement, but an additional fee is not inevitable in every case. Reporting arrangements vary; there is no universal safe 30-day grace period or UK seven-year credit-report rule. Contact the lender promptly if payment is difficult and ask how the account will be reported.

Can interest be reduced after the agreement starts?

Yes. You can request support after borrowing. For regulated credit, firms must consider appropriate forbearance for customers in financial difficulty. In an agreed forbearance repayment arrangement, further interest and charges must be reduced, waived or cancelled as needed to stop the balance increasing while the arrangement is met. See FCA CONC 7.3.

Check any calculation you dispute against statements and the agreement, ask the firm for a breakdown and use its complaint process if needed. A free debt adviser can help assess affordability and options. Do not agree to payments that leave essential living costs uncovered.