Use this invoice late fee calculator to estimate a fee from an unpaid balance, a due date, a check date, and the rate stated in your agreement. It separates simple daily interest from a one-time fixed or percentage charge and shows the total amount due.
Invoice late fee results
How the invoice late fee calculator works
The invoice late fee calculator first counts calendar days between the original due date and the date you choose. It subtracts any grace days, never letting chargeable days fall below zero. The due date itself does not earn a charge. If the check date is on or before the due date, the estimate is zero even when a fixed fee or a percentage has been entered.
Chargeable days = max(0, overdue days − grace days).
Simple interest = unpaid balance × annual rate ÷ 100 × chargeable days ÷ 365.
One-time fee = fixed fee + unpaid balance × one-time percentage ÷ 100, applied once when chargeable days are positive.
Total due = unpaid balance + simple interest + one-time fee.
The interest uses a fixed 365-day denominator, including leap years, and does not compound. It accrues on the entered unpaid balance only. A jurisdiction or contract may instead use a 360-day basis, monthly compounding, a changing rate, or a different convention. This tool does not decide which method applies.
Worked example: a 30-day overdue invoice
Suppose an invoice has an unpaid balance of $1,200, was due on September 12, 2026, and is checked on October 12, 2026. It is 30 calendar days past due. With a five-day grace period, the calculator counts 25 chargeable days, beginning September 18. At a 12% annual simple rate, interest is $1,200 × 12% × 25 ÷ 365 = $9.86 after rounding.
Add a one-time $10 fee and a one-time 2% fee ($24). The estimated late fee is $43.86, and the total is $1,243.86. Each part is shown separately, so you can set any component to zero if your agreement does not provide for it. The invoice late fee calculator rounds the displayed amounts to the currency’s usual number of decimal places.
When a late charge starts
In the invoice late fee calculator, grace days are counted after the due date. With five grace days, September 13 through September 17 are free of charges in this example, and September 18 is the first chargeable day. This is an arithmetic convention for the tool. Your contract may describe grace differently, and some laws restrict or prohibit particular fees. Verify the agreed due date and whether a late fee was disclosed before using the result on an invoice.
For date terms such as Net 30, calculate the due date first with the Invoice Due Date Calculator. The late fee tool expects the actual due date as an input. A reminder, payment plan, dispute, or a later revised invoice can affect what is owed, so review the account record rather than relying on a date alone.
Interest versus one-time charges
The invoice late fee calculator treats simple interest as growing with the number of chargeable days. A one-time fixed fee is a set amount; a one-time percentage is a single percentage of the balance. Neither is automatically assessed again each month. For example, if you enter 2% in the one-time field, this tool adds 2% once when the grace period has ended; it is not a 2% monthly interest rate. Do not enter a fee that your agreement or local rules do not allow.
The invoice late fee calculator uses the current unpaid balance, assuming it remained constant for the whole late period. If a customer made a partial payment during that time, split the timeline into periods with their actual balances and calculate each separately. The result does not handle payment allocation, taxes, discounts, collection costs, or interest on interest.
Use the estimate responsibly
The invoice late fee calculator works best when payment terms are clear on invoices and agreements. The Australian Government’s payment terms guidance recommends setting expectations for payment and overdue collection. The UK Small Business Commissioner’s interest calculator is an example of a jurisdiction-specific statutory approach. Its legal rules and changing reference rate are separate from the user-entered contractual estimate here.
Before sending a charge, check the actual agreement, customer type, jurisdiction, notice requirements, applicable caps, disputes, and payments already received. A legal right to a charge is not established by this calculator. For collection planning, pair the figure with Days Sales Outstanding and Accounts Receivable Turnover to understand broader receivables performance.
More examples and common mistakes
In the invoice late fee calculator, on the due date an unpaid $1,200 invoice shows zero overdue days and zero fees. Six calendar days after the due date with a five-day grace period, one day is chargeable. With the 12% rate, daily interest is about $0.3945 before currency rounding. The tool keeps the unrounded daily calculation internally and rounds the final total for display, which avoids accumulating a cent-level rounding error on each day.
A common invoice late fee calculator mistake is to enter 12 in an annual-rate field when the contract says 12% per month. Those are different terms and should not be substituted. Another is to use the whole original invoice when part was paid. Keep the balance and late-period dates consistent. The invoice late fee calculator is designed for a simple single-balance estimate and does not generate a legally compliant demand notice.
Invoice late fee calculator FAQs
Does the due date count as a late day?
No. The first overdue day is the following calendar day. Grace days are then subtracted.
Does the tool compound interest?
No. It calculates simple interest on the entered unpaid balance with a 365-day denominator.
Are the fixed and percentage fees charged every month?
No. Both are one-time amounts added only after the grace period ends.
Can I enter zero for a fee component?
Yes. Use zero for any annual interest, fixed fee, percentage, or grace days that do not apply.
Can I use a statutory late-payment rate?
Enter a rate only after checking the current rule that applies to the transaction. This tool does not retrieve or verify statutory rates or legal eligibility.
What if the invoice was partly paid?
Calculate separate time periods using each actual outstanding balance. A single balance for the whole period would overstate interest after a partial payment.