Start with the unpaid principal
Use only the balance that is actually overdue, excluding amounts already paid or formally credited.
- Unpaid amount
- No hidden compounding
- Separate fee result
Free business tool
Estimate a late fee with the formula left in plain sight
Enter the unpaid amount, annual percentage rate and days overdue. The calculator uses simple interest (amount × annual rate × days ÷ 365) and shows the fee and new balance separately.
- Transparent simple-interest formula
- Runs entirely in your browser
- No right to charge is assumed
Last updated:
Free browser tool
Calculate an estimated late fee
Adjust the amount, annual rate and days late. The result updates with the fee and total balance.
Result
0.00- Base amount
- 0.00
- Late fee
- 0.00
- Total
- 0.00
This estimate does not establish a right to charge a late fee. Confirm the signed contract, invoice terms and applicable law before adding any fee.
How to use it well
Arithmetic is the easy part and authorization is the important part
The calculator explains one simple-interest method. Your agreement and local rules determine whether a fee is allowed and how it must be disclosed.
Use the formula
A predictable estimate from three explicit inputs.
Convert the annual rate by days
The calculation is amount × annual rate ÷ 100 × days ÷ 365. It does not compound prior fees.
- Annual rate
- 365-day basis
- Simple interest
Check permission first
A calculation is not authorization.
Read the signed agreement
Confirm that the customer accepted the rate or fee terms before the payment became overdue.
- Signed contract
- Invoice terms
- Customer notice
Check applicable limits
Consumer, commercial, usury and prompt-payment rules vary by jurisdiction and transaction type.
- Jurisdiction matters
- Rate caps may apply
- Professional advice may be needed
Communicate clearly
Show the customer how the amount was derived.
State the period and formula
Include the original due date, overdue days, rate basis and resulting fee on any notice.
- Original due date
- Overdue period
- Transparent breakdown
Keep the underlying invoice unchanged
Preserve the original invoice and record any authorized fee as a separate adjustment or document according to your process.
- Preserve source facts
- Separate adjustment
- Maintain an audit trail
Questions
Late Fee Calculator, answered
How is the estimated late fee calculated?
The calculator uses simple interest: unpaid amount × annual rate ÷ 100 × days overdue ÷ 365. It does not compound prior fees.
Does this calculator mean I can charge the result?
No. A numerical result does not create a right to charge. Check the signed agreement, invoice terms and laws that apply to the transaction.
Should I include amounts the customer already paid?
No. Use the portion of principal that remains unpaid for the overdue period you are calculating.
Why does the formula use 365 days?
This tool converts an annual rate using a 365-day simple-interest basis. Your contract or jurisdiction may prescribe a different method.
Does the calculator compound the fee?
No. It estimates a fee on the entered unpaid principal only. It does not add past late fees to the interest-bearing amount.
Is any invoice information uploaded?
No. The three numeric inputs and result remain in your current browser tab.
Ready to keep the work, not just the calculation?
Use Invoice Crowd to save customers, products, taxes and business documents in one connected workspace.
- No card required to start
- Your records stay yours
- Cancel anytime