Late Payment / Invoice Fee Calculator
Estimate simple daily-accrual interest on an overdue invoice.
How the late payment fee estimate works
This calculator estimates late payment interest using the simple daily-accrual method: an annual interest rate applied proportionally to the number of days an invoice is overdue.
Enter the invoice amount, the annual interest rate that applies, and the number of days the payment is late. The calculator returns the estimated fee and the total amount owed.
This models simple daily-accrual interest only. Some jurisdictions' statutory late-payment regimes (for example, UK Late Payment of Commercial Debts legislation) add a separate fixed compensation fee on top of interest — check your applicable law or contract terms for the complete calculation.
The formula
Fee = Invoice amount × (Annual rate ÷ 100) × (Days late ÷ 365)
Convert the annual interest rate to a daily rate by dividing by 365, multiply by the number of days late, then multiply by the invoice amount.
Worked example
$10,000 invoice, 8% annual rate, 30 days late
$10,000 × 0.08 × (30 ÷ 365) ≈ $65.75 in late fees, for a total owed of about $10,065.75.
Frequently asked questions
Is this the exact statutory late-payment interest for my country?
Not necessarily — this calculates simple daily-accrual interest, which is the common baseline method, but some jurisdictions add a separate fixed compensation fee or use a different accrual convention. Check your applicable law or contract terms.
What interest rate should I use?
Whatever rate applies under your contract, invoice terms, or the statutory late-payment rate in your jurisdiction — this calculator applies whatever rate you enter.
Does the fee compound?
No — this is simple (non-compounding) interest for the full period, the most common convention for invoice late fees.
Next steps in your analysis
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