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Late payment interest calculator & overdue invoice letter

Work out the interest, see the breakdown and write your next reminder. Choose the applicable law yourself, then edit a letter you can copy or download.

Invoice details

Choose the law that applies to your invoice. Language and currency do not determine it.

For multiple invoices with the same amount and dates. Calculate different invoices separately.

How to set the due date

Defaults to today. If unpaid, use the date you want to calculate to.

Custom – no legal validation

Your estimate

B2B only (Spain also B2G). Indicative estimate, not legal advice. A contract rate overrides the statutory rate.

A clear breakdown, before you follow up.

Enter your invoice details to see overdue days, interest and fixed compensation.

Write your payment reminder

Choose a tone, add any details and edit the wording before sending. Nothing is sent for you.

Calculate a valid estimate first to generate your letter.

Calculations and letters stay in your browser. Your draft is saved locally for up to 7 days, then removed when next checked, or cleared when you use it after signup.

How the estimate works

Principal × annual rate ÷ 100 × days overdue ÷ 365. Days run from the day after the due date through the payment date. Spain splits interest at each half-year boundary. The UK keeps the rate from the half-year in which the first overdue day falls for the entire debt. Interest is rounded to cents at the end.

Enter the unpaid amount per invoice. The invoice count repeats that amount and those dates; calculate invoices with different balances or dates separately. The total is principal plus interest plus fixed compensation. No interest or fixed compensation is added when payment is on or before the due date.

Spain · Ley 3/2004

Business-to-business and business-to-government transactions only. The verified ECB main refinancing rate plus 8 percentage points applies by half-year. Article 8 provides EUR 40 compensation per overdue invoice.

United Kingdom · 1998 Act

B2B only. If the first overdue day falls in January–June, use Bank Rate on the preceding 31 December; for July–December, use Bank Rate on 30 June. Add 8 percentage points and keep that rate for the entire overdue period. The fixed recovery sum per invoice is GBP 40 below GBP 1,000, GBP 70 from GBP 1,000 to GBP 9,999.99, and GBP 100 from GBP 10,000.

Custom · worldwide

Enter your own annual rate and an optional fixed fee per invoice in EUR, GBP or USD. Custom — no legal validation. The letter cites no law. Use this for a contract rate or another rate you have independently checked.

This version does not convert currencies: Spain mode requires EUR and UK mode requires GBP so statutory fixed sums and thresholds stay in their legal currency. Select Custom for another currency. The law is never selected from your language or currency.

Verified rates & official sources

Last reviewed: .

Only the listed 2026 statutory periods are verified. Both legal modes require coverage for every overdue day. The UK selects the rate for the first overdue day and keeps it across later covered half-years. If a required period is missing or the registry expires without a newer verified period, legal mode stops and offers Custom rate, which has no legal validation.

Frequently asked questions

How is late payment interest calculated?

Principal × annual rate ÷ 100 × days overdue ÷ 365. Days run from the day after the due date through the payment date. Spain splits interest at each half-year boundary. The UK keeps the rate from the half-year in which the first overdue day falls for the entire debt. Interest is rounded to cents at the end.

Which law should I select?

Choose the law applicable to your transaction: Spain’s Ley 3/2004 for B2B and B2G, the UK Late Payment of Commercial Debts (Interest) Act 1998 for B2B, or Custom rate. Language and currency do not determine the law. A contract rate overrides the statutory rate.

What if my overdue period is outside 2026?

Only the listed 2026 statutory periods are verified. Both legal modes require coverage for every overdue day. The UK selects the rate for the first overdue day and keeps it across later covered half-years. If a required period is missing or the registry expires without a newer verified period, legal mode stops and offers Custom rate, which has no legal validation.

Is the calculator free and do I need an account?

The calculator and editable letter are free, with no signup. Calculation and letter generation happen in your browser. Your draft stays on this device for up to seven days so you can continue after signup; it is cleared when used or expired.

Late payment interest calculator & overdue invoice letter · FetchDue