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Calculate an agreed flat fee, one-time percentage, or simple interest on an overdue balance. Enter the rate that applies to your invoice and see the dates, formula, daily accrual, cap, fee, and updated total without uploading financial data.
Calculation result
Enter the invoice facts and the fee or rate that applies.
This is an arithmetic estimate, not legal, accounting, collection, or tax advice. It does not decide whether a fee is enforceable and does not modify the invoice.
Transparent formulas
First subtract payments already credited from the original invoice total. Then count calendar days after the due date, remove the agreed grace period, and apply only the method and rate authorized for that transaction. This calculator never supplies a supposedly standard rate.
| Method | Formula after the grace period | Use only when |
|---|---|---|
| Flat fee | Agreed fixed amount, once | A fixed charge is permitted |
| One-time percentage | Outstanding balance × rate ÷ 100 | A single percentage charge is permitted |
| Annual simple interest | Balance × annual rate ÷ 100 ÷ 365 × days | An annual simple rate applies |
| Monthly simple rate | Balance × monthly rate ÷ 100 × 12 ÷ 365 × days | The agreement states a monthly simple rate |
Worked example
Assume an outstanding balance of $1,200, an entered annual rate of 10%, and 45 chargeable days after the grace period:
$1,200 × 10% ÷ 365 × 45 = $14.79
The estimated fee is $14.79 and the updated total is $1,214.79. The example demonstrates the arithmetic only; it does not say that 10% is permitted for a particular invoice.
Partial payments and grace periods
A $1,500 invoice with $300 already credited leaves a $1,200 outstanding balance. A five-day grace period on an invoice that is twelve days past due leaves seven chargeable days. Confirm whether the agreement or applicable rule uses that balance and timing before communicating a fee.
One fixed amount once the invoice passes the permitted trigger date. It does not grow with the balance or number of late days.
One percentage of the outstanding balance. It changes with the balance but not with additional late days.
Accrues in proportion to the entered rate and chargeable days. This tool does not add interest to principal or compound it.
For qualifying UK business-to-business transactions, GOV.UK explains when a commercial payment becomes late and provides separate statutory-interest and recovery-cost guidance. Its court-claim example demonstrates the same annual-rate ÷ 365 × overdue-days arithmetic used by this calculator.
That guidance is jurisdiction- and transaction-specific. Do not copy a UK rate into another country, consumer invoice, or contract without confirming the rules that actually apply.
Not automatically. The contract, notice given to the customer, transaction type, and applicable law may control whether a fee is permitted and how it is calculated. Confirm the rule that applies before using or communicating a result.
Use the balance allowed by the agreement or applicable rule. When payments have already been credited, this calculator subtracts them from the invoice total and applies the selected method to the remaining outstanding balance.
The grace period delays fee accrual after the due date. For example, an invoice that is 12 days past due with a 5-day grace period has 7 chargeable days in this calculator.
No. The annual and monthly methods calculate simple interest only. The monthly method annualizes the entered monthly rate by multiplying it by 12, then prorates it over 365 days.
No. It can read a saved browser-local draft after you select it, but it never writes the fee back to that invoice. A reminder handoff uses short-lived session storage rather than putting invoice values in the URL.
