Late Fee Calculator
$
$
1.5%
3
Total Now Owed$1,070.00
Interest Charge$45.00
Flat Late Fee$25.00
Total Fees Added$70.00
When an invoice or bill goes unpaid, many contracts add both a one-time flat penalty and ongoing interest that accrues each period it stays overdue. This calculator adds a flat late fee to simple interest charged on the outstanding balance, showing the interest portion, the total fees, and the full amount the debtor now owes.
Formula
lateFee = flatFee + (balance × rate × periods); totalOwed = balance + lateFee
- flatFee
- One-time flat penalty charged once
- balance
- Outstanding amount past due
- rate
- Interest rate per period as a decimal
- periods
- Number of periods overdue
How it works
- Enter the outstanding balance, the flat late fee charged once, the interest rate per period (for example 1.5% per month), and how many periods the payment is overdue.
- Interest is computed as simple interest: balance × periodic rate × periods late. It is not compounded between periods.
- The total fees add the flat fee to that interest, and the total owed adds those fees back onto the original balance.
Worked example
A $1,000 invoice is three months late with a $25 flat fee and 1.5% monthly interest.
- Interest = 1,000 × 0.015 × 3 = $45.00.
- Total fees = 25 + 45 = $70.00.
- Total owed = 1,000 + 70 = $1,070.00.
The debtor owes $1,070.00, including $45 interest and a $25 flat fee.
Frequently asked questions
- Is the interest simple or compound?
- This calculator uses simple interest, charging the rate on the original balance for each period. It does not roll prior interest into the balance, which keeps results conservative and easy to verify.
- How much can I legally charge for late fees?
- Caps on late fees and interest are set by state, country, and contract law and vary widely. Check your jurisdiction and the terms in your agreement before charging, as excessive penalties may be unenforceable.
- Should the rate be monthly or annual?
- Use the rate that matches your period count. If you enter periods in months, use a monthly rate; an 18% annual rate is 1.5% per month.