Free browser tool
Late fee calculator
Check the arithmetic of a late payment fee using the terms in your own agreement. Choose a one-time flat fee, a monthly percentage or both. The result shows the added charge and unpaid balance; it does not determine whether your terms are lawful or enforceable.
Fee and balance
Enter the invoice and the fee from your own agreement to calculate a balance.
The formulas
Percentage fee = unpaid invoice × monthly percentage ÷ 100 × (days late ÷ days used for one month).
Total late fee = applicable flat fee + percentage fee. The flat fee is added once, and neither part compounds.
New balance = unpaid invoice + total late fee. If days late is zero, both fee portions are zero.
If your terms use a different convention, such as complete months, compounding, grace periods or a cap, this calculation may not match them.
How it works.
- Find the unpaid invoice principal and the date your agreed fee starts. Enter the days late after any grace period you have agreed, keeping earlier fees out of the invoice amount.
- Select the fee structure in your terms. For a flat fee, enter the amount to apply once. For a monthly percentage, enter both the percentage and the number of days your agreement uses as one month.
- Read the separate flat and percentage portions. The monthly calculation is simple and prorated by days; it does not compound or charge another fee on an earlier fee.
- Before charging, check your contract and your state's own government website, reached through the official state directory if needed, and ask an attorney about applicable limits. No legal maximum is built into this tool, and nothing is sent to your client.
Sources.
Useful answers.
Does this tell me what fee I am allowed to charge?
No. Limits vary by state, contract and circumstances. Use your state's own site and an attorney to check your terms before applying a charge. The calculator evaluates the numbers you supply without checking legal permission.
How is a monthly percentage converted to days?
The tool divides days late by the day basis you enter for one month, then multiplies by the unpaid invoice and monthly percentage. You must supply that basis from your terms. Complete-month fees or other conventions need a different calculation.
What happens if the invoice is not late?
Enter zero days late and the fee is zero, including the flat portion. The result leaves the unpaid invoice balance unchanged. The tool does not read a due date or decide when a grace period ends.
Can this compound fees or apply a contractual cap?
No. It applies one flat fee and a simple percentage to the principal you enter. It does not compound, process partial-payment dates, apply a cap or send a reminder. Check those details separately if your terms require them.
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