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Late Fee Calculator
Work out what to add to an overdue invoice. Enter the invoice amount, the number of days it is late and your late-payment interest rate — per year or per month — plus any flat fee. You get the interest, the total late fee and the new amount due.
- Total now due
- $2,044.38
- Interest portion
- $44.38
- Interest per day
- $0.99
Saved setups
Save a set of inputs you reuse — your usual rate, your loan, your room sizes — and load it back in one tap.
Your recent calculations
Results you calculate here are kept on this device so you can come back to them.
Formula
How to use it
- Enter the unpaid invoice amount.
- Enter the number of days past the due date.
- Enter the interest rate from your contract or invoice terms, and whether it is per year or per month.
- Add any flat late fee your terms allow.
Worked examples
A $2,000 invoice paid 45 days late at 18% a year
- Interest per day
- $0.99
- Interest portion
- $44.38
- Late fee
- $44.38
- Total now due
- $2,044.38
An $850 invoice 30 days late at 1.5% a month plus a $25 flat fee
- Interest portion
- $12.58
- Late fee
- $37.58
- Total now due
- $887.58
What you are allowed to charge
A late fee generally has to be agreed in advance — in the contract, the quote or the invoice terms the customer accepted — and it has to be within the legal limit where you do business. In the US, limits are set by state law and differ widely; 1.5% a month (18% a year) is a very common contract rate for business invoices, but some states cap rates lower.
In the UK, businesses can claim statutory interest on late commercial payments of 8% above the Bank of England base rate, plus a fixed sum for recovery costs, even if the contract is silent. Consumer debts follow different rules. Check the law that applies to you; this is not legal advice.
Simple interest, by the day
This calculator charges simple interest: the same amount for every day late, with no interest on interest. That is the usual method for invoice late fees. A monthly rate is converted to a yearly one and applied per day, so 1.5% a month on $1,000 comes to about 49 cents a day — $14.79 for 30 days rather than a round $15.
Questions people ask
How do you calculate a late fee on an invoice?
Multiply the invoice amount by the annual rate, divide by 365 and multiply by the days late. $2,000 at 18% a year for 45 days is $2,000 × 0.18 ÷ 365 × 45 = $44.38.
How much is 1.5% a month as an annual rate?
18% a year as simple interest (1.5% × 12).
Can I charge a late fee if it was not on the invoice?
In most places you need the customer’s prior agreement to the terms. Some jurisdictions, such as the UK for business-to-business debts, give a statutory right to interest regardless. Check your local rules.