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- A late fee is a charge added to an invoice that is paid after its due date. It is usually a percentage per month, a percentage per year, a flat amount, or a mix.
- This calculator turns the invoice amount, days overdue and your fee terms into the late charge and the new total due.
- It handles a grace period, simple or monthly compounded interest and an optional cap on the total fee.
- The result is an estimate. Your contract and local law decide what you can actually charge.
How to use the invoice late fee calculator
Enter the invoice and days late
Type the invoice amount and the number of days past the due date. Count from the due date, not from the day the invoice was issued.
Choose the fee type
Pick percent per month, percent per year, a flat fee, or a flat fee plus a monthly percentage, then enter the rate or amount written in your terms.
Adjust the advanced options
Add a grace period, switch to monthly compounding, or set a maximum fee as a percentage of the invoice if your contract has one.
Read the result
The calculator shows the total late fee, the new amount due, the equivalent annual rate and how much each extra day adds.
How an invoice late fee is calculated
There are three common ways to charge for a late payment. Each one is simple on its own, and many contracts combine a flat fee with interest. The calculator applies the same steps shown below.
Percent per month
Late interest = invoice × monthly rate × (days late ÷ 30)
Example: $2,500 × 1.5% × (30 ÷ 30) = $37.50
Days late means days after the due date, minus any grace period.
Percent per year
Late interest = invoice × annual rate × (days late ÷ 365)
Example: $2,500 × 18% × (30 ÷ 365) = $36.99
An annual rate of 18% is the same as 1.5% per month, but the daily amounts differ slightly because of the 30 and 365 day bases.
Flat fee, compounding and total due
Total late fee = flat fee + late interest
Compounded monthly: interest = invoice × ((1 + monthly rate) ^ months late − 1)
Total due = invoice + total late fee
A flat fee is charged once, no matter how many days late the invoice is.
Invoice late fee examples
Example 1: 30 days late at 1.5% per month
A freelancer sent a $2,500 invoice and it is 30 days overdue. The terms say 1.5% per month.
- Invoice amount $2,500.00
- Late interest ($2,500 × 1.5% × 1) $37.50
- Total now due $2,537.50
This equals 18% a year, or about $1.25 for every extra day the invoice stays unpaid.
Example 2: flat fee plus monthly interest
The same $2,500 invoice is 45 days late. The terms add a $25 flat fee on top of 1.5% per month.
- Flat fee $25.00
- Late interest ($2,500 × 1.5% × 1.5) $56.25
- Total late fee $81.25
- Total now due $2,581.25
The flat fee is a one-time charge, so it matters most on smaller invoices.
Example 3: simple versus compounded interest
A $10,000 invoice is 90 days late at 1.5% per month.
- Simple interest $450.00
- Compounded monthly $456.78
- 18% per year, simple (90 ÷ 365) $443.84
Compounding adds only a few dollars over three months, but the gap grows on invoices that stay unpaid for a long time.
Common late fee structures
Late fee per $1,000 overdue for 30 days
These figures use simple interest and a 30 day month.
- 1% per month (12% a year) $10.00
- 1.5% per month (18% a year) $15.00
- 2% per month (24% a year) $20.00
- Flat fee The amount written in your terms
Higher rates are not always enforceable. Many places limit what can be charged on overdue invoices, so check the rules that apply to you before you set your terms.
When an invoice late fee calculator helps
Chasing a late client
See the exact amount to put in your reminder email instead of guessing.
Writing payment terms
Test a rate on sample invoices before you put it into a contract.
Comparing with factoring
Check what waiting costs against the price of selling the invoice.
Tracking cash flow
See how much a slow payer adds to the money you are owed.
A late fee is easiest to defend when the client agreed to it in advance. Put the rate, any grace period and any cap in your contract or on the invoice. This calculator gives estimates only and is not legal or accounting advice. If you would rather get paid sooner, try the Invoice Factoring Cost Calculator or work out your real rate with the Effective Hourly Rate Calculator.
Invoice late fee calculator FAQ
How do you calculate a late fee on an invoice?
For a monthly rate, multiply the invoice by the rate and by the number of months late (days late ÷ 30). For an annual rate, multiply by the rate and by days late ÷ 365. Then add any flat fee. The calculator does this for you and shows the new total due.
What is a normal late fee on an invoice?
Many businesses use between 1% and 1.5% per month, which is 12% to 18% a year, sometimes with a small flat fee. The right amount depends on your industry, your contract and the rules where you work.
Can I charge a late fee that was not in the contract?
A late fee is much easier to enforce when the client agreed to it before the work began. If it was not agreed, charging one can cause a dispute and may not be allowed. Laws differ, so this is not legal advice and you should check the rules that apply to you.
What is a grace period?
A grace period is a set number of days after the due date when no late fee is charged yet. If you enter 7 days, an invoice that is 20 days late is charged for 13 days.
Should late interest be simple or compounded?
Simple interest is easier to explain and is the more common choice on invoices. Compounding is only appropriate if your terms say so, and it makes only a small difference unless the invoice is very late.
Does the late fee start from the invoice date or the due date?
From the due date. If an invoice is issued on the 1st with 30 day terms, it becomes late on the 31st, and the days late are counted from then.