Late Fee Calculator
Enter the invoice amount, the due date and your late fee terms. The calculator shows the fee, the days late and the new total.
30 days late
$15.00 late fee
- New total
- $1,015.00
Charge only a late fee your client agreed to. Add it as a line on a new invoice. Make a free invoice
How do you calculate a late fee on an invoice?
Count the days late, then apply the late fee your client agreed to: a flat amount, a percentage of the invoice, or interest on the overdue amount for each day, month or year it is late.
Find the days late
Count the days from the day after the due date to the day the client pays. An invoice due on March 1 and paid on April 15 is 45 days late.
Apply your agreed rule
Use the late fee in your contract or invoice terms: a flat fee, a percentage of the invoice, or interest per month or per year.
For interest, work out the daily amount
Divide the yearly rate by 365 and multiply by the overdue amount. At 12% a year, 1,000 costs 0.33 a day.
Multiply and add it to the balance
Multiply the daily amount by the days late, then send the client a new invoice, or an invoice line, for the fee.
Late fee examples
For an invoice of 1,000 paid 45 days after the due date:
| Late fee terms | Fee | New total |
|---|---|---|
| Flat fee of 25 | 25.00 | 1,025.00 |
| 5% of the invoice, once | 50.00 | 1,050.00 |
| 1.5% a month, for each month or part month late | 30.00 | 1,030.00 |
| 1.5% a month, counted by the day | 22.19 | 1,022.19 |
| 12% a year, counted by the day | 14.79 | 1,014.79 |
| UK statutory interest (11.75% a year) plus fixed compensation | 84.49 | 1,084.49 |
The rates in this table are examples, not recommendations.
How is UK late payment interest calculated?
UK statutory interest is the overdue amount times 8% plus the Bank of England base rate, divided by 365, times the days late. It applies to business-to-business debts unless your contract sets a different rate. The base rate used is the one on 31 December (for debts that become late from January to June) or 30 June (July to December), and it stays fixed for that debt. The rate was 3.75% on both dates for 2026, so the statutory rate is 11.75% a year.
You can also claim fixed compensation once per invoice: £40.00 for a debt under £1,000, £70.00 for £1,000 to £9,999.99 and £100.00 for £10,000 or more. If you agreed no payment date, the debt is late 30 days after the customer gets the invoice or you deliver, whichever is later.
Sources: GOV.UK: charging interest, GOV.UK: debt recovery costs, GOV.UK: late commercial payments, Bank of England Bank Rate. Checked 1 November 2026.
How do you add a late fee to an invoice?
Put your late fee terms in your contract and on every invoice before the work is done, for example "Late payments are charged 1.5% per month". When an invoice is late, send a new invoice or a separate line for the fee, with the days late and how you worked it out. Many clients pay faster after a polite reminder than after a fee, so start with the overdue invoice email templates.
On the Pro plan, Due On Time can add the fee to an overdue invoice by itself, using the rule you set: a flat amount or a percent of the unpaid balance, once or repeating, with an optional cap. It stops when the invoice is paid. Compare plans.
Limits on late fees and interest differ by country and, in the US, by state. Check the rules where you and your client are. For more on payment terms, read the invoice payment terms guide, and work out due dates with the net 30 calculator.
Frequently asked questions
How do you calculate a late fee on an invoice?
Use the rule in your terms. A flat fee is a fixed amount. A percentage fee is the invoice amount times the percentage. Interest is the overdue amount times the yearly rate, divided by 365, times the days late.
How do you calculate 1.5% interest per month?
Multiply the overdue amount by 1.5%. On 1,000 that is 15 for each month. Some terms charge it for every month or part of a month late; others count it by the day, which is 18% a year divided by 365.
How much interest can I charge on a late invoice in the UK?
For business-to-business debts, the law allows statutory interest of 8% plus the Bank of England base rate, unless your contract sets a different rate. For debts that became late in 2026 that is 11.75% a year, plus fixed compensation of £40, £70 or £100.
How is UK statutory interest calculated?
Multiply the debt by 11.75% to get a year's interest, divide by 365 for the daily interest, and multiply by the days late. On £1,000 paid 45 days late that is £14.49, plus £70 fixed compensation (the band for £1,000 to £9,999.99).
Can I charge a late fee that is not in my contract?
Only charge a late fee your client agreed to, in your contract or the terms on your quote and invoice. In the UK, statutory interest on business debts applies by law unless your contract sets a different remedy.
Does this calculator give legal advice?
No. It does the arithmetic for the terms you enter. Limits on late fees and interest differ by country and US state, so check the rules where you and your client are.