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Invoice Late Fee Calculator

Work out the interest owed on an overdue invoice — by any rate, currency, and number of days late.

%
Rate is
Original amount
$1,000.00
Interest (30 days @ 8% p.a.)
$6.58
Total now due
$1,006.58

Uses simple (non-compounding) interest. Enter the rate from your contract or your local statutory rate — the calculator does not assume one.

How the calculation works

Late-payment interest is almost always calculated as simple interest — it accrues on the original amount and does not compound. The formula is:

interest = amount × rate% × (days overdue ÷ days in period)

Days in period is 365 for an annual rate, 30 for a monthly rate.

A one-off flat fee, if your terms include one, is simply added to the interest to give the total now due.

Setting a late fee that sticks

A late fee is only enforceable if your customer agreed to it, so the rate belongs in your payment terms before the invoice is late — on the invoice itself and, ideally, in the contract or quote. State the rate, the basis (per month or per year), and when it starts. Many jurisdictions also give you a statutory right to interest on overdue commercial invoices even without a contract term; where that applies, you can claim the statutory rate instead. This tool works with whichever rate governs your situation — it does not pick one for you.

Frequently Asked Questions

How is a late payment fee calculated?

The most common method is simple interest: fee = amount × interest rate × time overdue. For an annual rate, the time is days overdue ÷ 365; for a monthly rate, it is days ÷ 30. For example, a $1,000 invoice at 8% per year, 30 days late, accrues $1,000 × 8% × 30/365 = $6.58. Some contracts also add a one-off flat fee on top. The calculator above does both.

What late fee can I legally charge?

It depends on your contract and your country. Many businesses set a rate in their payment terms — often 1–2% per month or a fixed annual percentage — and that agreed rate governs. Several jurisdictions also set a statutory rate you can claim even without a contract term: in the UK, for commercial debts it is 8% plus the Bank of England base rate; the EU late payment directive works similarly; other countries have their own rules. Enter the rate that applies to you; the calculator does not assume one.

Should I charge a percentage or a flat fee?

Both are used. A percentage (interest) scales with the invoice size and the length of the delay, which is fairer on large or long-overdue amounts. A flat fee is simple and predictable and works well for smaller invoices. Many businesses combine them — a modest flat fee plus interest that accrues while the invoice stays unpaid. State whichever you use in your payment terms so it is enforceable.

When does a late fee start accruing?

From the day after the due date, once the invoice is genuinely overdue. Count the days from the due date to the date of payment (or today) and enter that as 'days overdue'. It helps to state your due date clearly on the invoice — for example 'Payment due within 14 days' — so the overdue period is unambiguous.

Is this late fee calculator free?

Yes, free and no sign-up, in any of the supported currencies. When you need to bill the overdue amount, you can create a fresh invoice — with the interest as a line item and your late-payment terms stated — using Bilto's free invoice generator.

Bill the overdue amount

Create a fresh invoice with the interest as a line item and your late-payment terms stated — free, no sign-up.

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