How to Calculate Invoice Interest

When Can You Charge Interest on an Overdue Invoice?

Under Polish law, the right to charge statutory interest for late payment arises automatically the day after the invoice due date passes. You do not need a contractual clause, you do not need to warn the debtor in advance, and you do not need to prove any loss. The legal basis is the Ustawa o przeciwdziałaniu nadmiernym opóźnieniom w transakcjach handlowych (Act on Counteracting Excessive Delays in Commercial Transactions) of 8 March 2013.

The right applies to commercial transactions — meaning B2B. If you sell to consumers, different rules apply. For detailed information on rates, interest types, and the complete legal framework, see our statutory interest guide. This post is a practical how-to: five steps from overdue invoice to formal demand.

Step 1: Determine Which Rate Applies

The rate depends on who owes you money.

B2B transactions (most common): Statutory interest for late commercial transactions = NBP reference rate + 10 percentage points. As of April 2026, this is 5.75% + 10% = 15.75% per annum.

Transactions where the debtor is a public authority (e.g., a local government body, state-owned entity acting in a public capacity): A higher rate applies — NBP reference rate + 8 pp for most public entities, and a special rate for "large enterprises" when they are the debtor and the creditor is an SME. Check the current table in the statutory interest guide before calculating.

Contractually agreed rate: If your contract specifies an interest rate for late payment, that rate applies — provided it is not lower than the statutory minimum. If it is lower, the statutory rate takes precedence automatically.

Decision tree:

• Is the debtor a business entity? → Yes: use the B2B commercial rate (15.75% as of April 2026)
• Is the debtor a public authority? → Yes: check the specific public entity rate
• Does your contract specify a higher rate? → Yes: use that rate
• Does your contract specify a lower rate? → Ignore it; the statutory rate applies

Step 2: Calculate the Amount

The formula is straightforward:

Interest = Principal × Annual rate ÷ 365 × Number of days overdue

The number of days overdue runs from the day after the due date to the day of payment (inclusive). Use 365 days for standard years and 366 for leap years if your calculation spans 29 February.

Example 1: Small Invoice, Short Delay

Invoice: PLN 3,500 gross. Due date: 1 March 2026. Payment received: 31 March 2026. Days overdue: 30.

Interest: 3,500 × 15.75% ÷ 365 × 30 = PLN 45.21

Example 2: Mid-Size Invoice, Longer Delay

Invoice: PLN 22,000 gross. Due date: 15 January 2026. No payment received. Calculation as of 15 April 2026. Days overdue: 90.

Interest: 22,000 × 15.75% ÷ 365 × 90 = PLN 854.79

Plus: fixed compensation of 70 EUR (for invoices between 5,001 and 50,000 PLN), converted at the NBP mid-rate on the due date.

Example 3: Large Invoice, Rate Change Mid-Period

Invoice: PLN 80,000 gross. Due date: 1 October 2025. Assume NBP cuts by 0.5 pp on 1 July 2026, reducing the statutory rate from 15.75% to 15.25%. Calculation as of 1 August 2026 (305 days overdue, split across two rate periods).

Period 1 (1 Oct 2025 – 30 Jun 2026 = 273 days at 15.75%): 80,000 × 15.75% ÷ 365 × 273 = PLN 9,432.33

Period 2 (1 Jul 2026 – 1 Aug 2026 = 32 days at 15.25%): 80,000 × 15.25% ÷ 365 × 32 = PLN 1,070.14

Total interest: PLN 10,502.47. Plus fixed compensation: 100 EUR (invoice above 50,000 PLN).

For any invoice, use our statutory interest calculator to get the exact figure without manual arithmetic.

Step 3: Issue an Interest Note

An interest note (nota odsetkowa) is a separate document — distinct from your original invoice — that formally states the interest claim. It is not a VAT document; you do not charge VAT on interest for late payment. It is a civil-law settlement document.

What to include in an interest note:

• Your company details and the debtor's details (full legal names, tax IDs, addresses)
• Reference to the original invoice(s): number, date, amount, due date
• Legal basis: Article 7 of the Act of 8 March 2013
• Calculation: principal × rate ÷ 365 × days = interest amount
• Current statutory rate applied
• Total amount due (principal + interest)
• If claiming fixed compensation: state the compensation amount and cite Article 10
• Payment deadline and your bank account details
• Date of issue and your signature (or authorised representative)

There is no mandatory template format under Polish law, but the above elements ensure the note is legally sound and unchallengeable.

Step 4: Send a Payment Demand

An interest note can stand alone, but it is most effective when accompanied by — or incorporated into — a formal wezwanie do zapłaty (payment demand). The demand adds urgency and creates a documented paper trail that matters in court if you escalate.

Your payment demand should reference the original invoice, the interest note, and state a firm payment deadline (7–14 days is standard). For a complete template and sending instructions, see our payment demand guide. For invoices with a generator tool, use our payment reminder generator.

Send the demand by registered post (list polecony za potwierdzeniem odbioru) and by email simultaneously. Keep the postal receipt — you will need it if you go to court.

Step 5: What If the Debtor Still Does Not Pay?

If the payment deadline in your demand passes without a response, your options are:

1. Final pre-legal demand: A more urgent letter explicitly stating that legal proceedings will follow if payment is not received within 5–7 days. See our guide to final payment demands for a template.
2. Online payment order (EPU): File at e-sad.gov.pl. Court fee is 1.25% of the claim (minimum 30 PLN). The court issues a payment order without a hearing if the claim is undisputed. If the debtor does not object within 14 days, it becomes an enforcement title.
3. Automated reminder escalation: If you are not already using automated reminders, set them up now. Terminovo sends timed reminders by email and SMS, escalating in tone as the delay grows, so no invoice slips through without follow-up.

The most important thing: act quickly. Recovery rates drop sharply after 60 days. An interest note issued on day 10 of delay is far more effective than one issued on day 90.

Common Mistakes

• Using the wrong rate: Applying the general statutory rate (for civil matters) instead of the higher commercial rate for B2B transactions. The commercial rate is always higher — do not leave money on the table.
• Forgetting the fixed compensation: The 40/70/100 EUR compensation under Article 10 is separate from interest and does not require proof. Claim it on every overdue invoice. For the full details on entitlement, calculation, and how to claim it, see our complete guide to the 40 EUR compensation rule.
• Issuing the note too late: Interest accrues from day one of delay. You can issue the note at any time, but issuing it late means the debtor knows you are not paying attention.
• Using the wrong day count: The clock starts the day after the due date, not on the due date itself. Count carefully.
• Charging VAT on interest: Interest for late payment is not subject to VAT. Do not add VAT to your interest note.

Automatically Calculate and Claim What You're Owed

Terminovo computes statutory interest and the 40/70/100 EUR fixed compensation for every overdue invoice and populates your demand letters with the correct figures — no manual arithmetic, no missed entitlements. See pricing or learn how it works.