Late Fee Calculator: Overdue Invoice Interest

How to use Late Invoice Interest Calculator with a clear next step.

Enter the unpaid invoice amount, the annual interest rate that applies and the number of days the payment is overdue. The calculator shows the simple interest to add, the total now due and the daily accrual, so you can put an exact figure in a payment reminder.

Use it to work out late payment interest on an overdue invoice. The answer depends on the rate you enter. If your contract or payment terms set a late-payment rate, that rate normally applies. In the UK, business-to-business debts with no agreed rate can carry statutory interest under the Late Payment of Commercial Debts (Interest) Act 1998 at 8% plus the Bank of England base rate. The base rate on 30 June applies to debts that become overdue from July to December, and the rate on 31 December applies from January to June. Other countries have their own rules, and consumer debts are treated differently. The calculation runs in the current browser session. Use a neutral arithmetic check before entering a real claim: 1,000 outstanding at an illustrative annual 10% for 30 days gives about 8.22 under the calculator’s 365-day simple-interest convention. That example does not establish an allowed rate or entitlement. Confirm the original due date, the amount still unpaid and whether a later partial payment changed the balance. A figure calculated on the original invoice total can overstate interest when part has already been paid. Check whether the agreement or applicable process uses a different day-count basis or excludes certain periods. If the result seems too large, make sure a monthly percentage has not been entered as an annual rate and that days, rather than months, were entered. Keep a dated calculation alongside the source records. Any fixed fee, collection cost or statement about legal consequences needs separate verification before inclusion in a reminder.

Interest = outstanding amount × annual rate ÷ 100 × days overdue ÷ 365. This is simple interest on a 365-day year and it does not compound. Daily accrual = interest ÷ days overdue, and total due = outstanding amount + interest. Fixed debt-recovery fees, such as the UK's £40, £70 or £100 compensation, are not added automatically.