Invoice Late Fee Calculator
Calculate the late fee on an overdue invoice from an annual interest rate, the days past due, and an optional flat penalty — with the total now due.
How to use this tool
- Enter the unpaid invoice amount.
- Enter the annual interest rate from your payment terms (e.g. 18%).
- Enter how many days the invoice is past due.
- Add a flat fee if your terms include one, then read the total due.
An invoice has gone past due. Enter the amount, your annual late-payment rate and how many days late it is — plus any flat fee — to see the interest charge and the total now owed.
Formula
Daily interest = Invoice × (Annual rate ÷ 365)
Interest charge = Daily interest × Days past due
Total late fee = Interest charge + Flat fee
Total now due = Invoice + Total late fee
How it works
A late fee compensates you for being financed by a client who has not paid. The standard method is simple daily interest: convert the annual late-payment rate in your terms to a daily rate by dividing by 365, multiply by the unpaid invoice to get interest per day, and multiply by the number of days overdue. Many freelancers also add a one-time flat penalty; this calculator supports both and reports the total now due.
The interest is simple, not compounding — it accrues on the original invoice rather than on a growing balance — which matches how most freelance and small-business terms are written (for example, '1.5% per month', i.e. 18% per year). Because permissible late-fee rates are set by contract and capped by local law in some places, the annual rate is an input, not a fixed value; check that your stated rate is enforceable in your jurisdiction before applying it. A clear late-fee clause agreed up front is what makes the charge collectible.
Prepared by the AbraCalc Freelance Desk for general use. This is not legal advice; usury limits and consumer-protection rules vary, so confirm enforceable rates locally.
Worked example
$2,000 invoice, 18% annual rate, 30 days late, no flat fee
- Daily interest = 2,000 × (18% ÷ 365) = 2,000 × 0.00049315 = 0.9863 per day.
- Interest charge = 0.9863 × 30 = 29.589041.
- Total late fee = 29.589041 + 0 = 29.589041.
- Total now due = 2,000 + 29.589041 = 2,029.589041.
Total now due = $2,029.59
Late-fee interest on a $2,000 invoice at 18% annual (no flat fee)
| Days late | Interest charge | Total due |
|---|---|---|
| 7 | $6.90 | $2,006.90 |
| 15 | $14.79 | $2,014.79 |
| 30 | $29.59 | $2,029.59 |
| 45 | $44.38 | $2,044.38 |
| 60 | $59.18 | $2,059.18 |
| 90 | $88.77 | $2,088.77 |
Key terms
- Late fee
- A charge added to an overdue invoice to compensate for delayed payment.
- Simple interest
- Interest charged on the original amount only, not on accrued interest.
- Net terms
- The payment window on an invoice, e.g. Net 30 means payment is due 30 days after issue.
- Flat penalty
- A fixed one-time fee added when an invoice goes past due, independent of how late it is.
Frequently asked questions
- What late fee can I legally charge?
- It depends on your contract and local law. Common freelance terms are 1.5% per month (18% per year), but some jurisdictions cap late-payment or interest rates, so confirm the figure you enter is enforceable where you operate.
- Is the interest simple or compounding?
- This calculator uses simple daily interest on the original invoice, which matches how most freelance terms are written. It does not compound the accrued interest into the balance.
- Why divide the annual rate by 365?
- To turn an annual percentage into a daily one. Multiplying the daily rate by the invoice gives interest per day, and by the days overdue gives the total interest charge.