Invoice Late Fee Calculator
Calculate late fees on unpaid invoices instantly. Choose monthly interest, annual interest, or a flat fee. Enter your numbers and the result updates in real time.
Advanced options
Based on 30 chargeable days.
This calculator provides general calculation assistance only and does not constitute legal or financial advice.
Late-fee rules vary by jurisdiction, contract terms, customer type, and other factors. Check your agreement and applicable local rules before charging late fees. Use the fee terms already stated in your contract or invoice.
How to Calculate an Invoice Late Fee
There are three common ways to calculate a late fee on an unpaid invoice. Use the method that matches the terms in your agreement or invoice.
Monthly interest
With a monthly interest rate, the late fee is prorated by the number of chargeable days.
Late Fee = Outstanding Balance × (Monthly Rate ÷ 100) × (Chargeable Days ÷ 30)
Example: An invoice balance of $1,000 at 1.5% monthly for 30 days produces $1,000 × 1.5% × 30÷30 = $15.00.
Annual simple interest
With an annual rate, the late fee uses 365-day simple interest.
Late Fee = Outstanding Balance × (Annual Rate ÷ 100) × (Chargeable Days ÷ 365)
Example: An invoice balance of $1,000 at 18% APR for 30 days produces $1,000 × 18% × 30÷365 ≈ $14.79.
Flat fee
A flat fee is a fixed amount stated in your contract or invoice terms. The total amount due is the outstanding balance plus the flat fee.
Example: An invoice balance of $1,000 with a $25 flat fee produces a total of $1,025.
If you are unsure which method applies, review your invoice terms or your client agreement before calculating.
Common Ways Businesses Structure Late Fees
Businesses use several approaches to structure late fees. The right one depends on your contract, industry norms, and local rules — not on a universal standard.
- Monthly percentage: A rate such as 1.5% per month (used here as an example only) charged on the outstanding balance, prorated by the number of overdue days.
- Annual simple interest: An annual percentage rate applied to the outstanding balance, prorated over 365 days.
- Flat fee: A fixed amount added to the invoice when payment is late, as specified in the agreement.
- Grace period: A set number of days after the due date before any fee begins. Chargeable days are counted only after the grace period ends.
- Flat fee plus interest: Some contracts specify both a flat fee and interest. This is only an option when your agreement explicitly allows it.
When sending a reminder for an overdue invoice, pair your late-fee calculation with a clear invoice reminder email. If you invoice as a freelancer, review your freelance invoicing workflow to make sure your payment terms and late-fee language are agreed before work begins.
Frequently Asked Questions
Is it legal to charge late fees on invoices?
Whether you can charge late fees depends on your local laws, your contract, your invoice terms, the type of customer, and other factors. Late-fee rules vary by jurisdiction. Check your agreement and applicable local rules before charging late fees. This calculator provides general calculation assistance only and does not constitute legal or financial advice.
What is a typical invoice late fee?
There is no single standard rate. Businesses use different approaches: a monthly percentage (for example, 1.5% per month as an illustration), an annual simple interest rate, or a flat fee. The right approach depends on what your contract or invoice states and what local rules allow.
How do you calculate late payment interest?
Two common methods are monthly prorated interest and annual simple interest. For monthly, you multiply the outstanding balance by the monthly rate and prorate by the number of chargeable days over 30. For annual, you multiply the outstanding balance by the annual rate and prorate over 365 days. This calculator handles both.
Can I charge a flat fee and interest?
Only when allowed by the applicable agreement and rules. If your contract specifies both a flat fee and interest, you can use the Advanced options in this calculator to combine them. Do not add charges that your agreement does not permit.
When is an invoice considered late?
An invoice is considered late after the payment due date specified in the invoice or agreement, subject to any applicable grace period. If a grace period applies, the chargeable days begin after the grace period ends.
Need an invoice template?
Send a clear invoice first. Late fees only apply after the due date passes and your agreement supports them.