A clearer view of your numbers
Invoice Late Fee & Interest Calculator
Model one flat fee, one percentage fee or simple interest on an overdue invoice balance.
Browser-local calculations · No login · No financial values sent
Your inputs
01 / EnterThese are editable examples, not recommended rates. Enter your own numbers.
Your result
02 / UnderstandLoading the example calculation…
Show the math
Estimates use unrounded numbers. Displayed amounts round to cents.
What this means
A flat or percentage late fee is applied once. Simple interest grows in proportion to days overdue without charging interest on interest.
Worked example
On $1,000 overdue for 30 days at 12% per fixed 365-day year, simple interest is $9.86 displayed. Alternatively, a $25 flat fee gives $25 once, or a 2% one-time fee gives $20.
How the math works
Flat fee = entered amount once. Percentage fee = balance × rate/100 once. Simple interest = balance × rate/100 × days/period days. A zero balance or zero days overdue gives zero charge.
About this estimate
No partial payments, compounding, tax or automatic grace period. The monthly model uses 30 days; the annual model uses 365 days even in leap years. Contracts and applicable law govern what may be charged; the result is not legal entitlement to collect.
Keep working through it
Related calculators for the next question.
Payment Terms & Due Date Calculator
Find a due date from an invoice date and common Net terms or a custom number of calendar days.
Open calculator Business OperationsBusiness Cash Runway Calculator
Explore remaining cash under constant monthly inflows, outflows and an immediate one-time expense.
Open calculatorNeed a different calculation? Suggest a calculator.
Formula v1.0.0 · Reviewed · Tool ID: invoice-late-fee-interest