Statutory Interest Calculator

How much interest is owed on an unpaid invoice under Polish law? Enter the amount, the due date and the date it was paid — the calculator does the rest and shows where every grosz came from.

AN UNOFFICIAL APP
This calculator is independent. It is not affiliated with Narodowy Bank Polski, the Polish Ministry of Finance, the National Revenue Administration or any other government body, and does not represent them.

OFFICIAL SOURCES
Every rate and every rule comes from an official publication. The addresses are in the app as well — on the "NBP rates" screen and in settings:
• NBP interest rates — https://nbp.pl/polityka-pieniezna/decyzje-rpp/podstawowe-stopy-procentowe-nbp/
• Civil Code, art. 359 and 481 — https://isap.sejm.gov.pl/isap.nsf/DocDetails.xsp?id=WDU19640160093
• Tax Ordinance, art. 54 and 56 — https://isap.sejm.gov.pl/isap.nsf/DocDetails.xsp?id=WDU19970137926
• act on excessive delays in commercial transactions — https://isap.sejm.gov.pl/isap.nsf/DocDetails.xsp?id=WDU20130000403
• rate announcements, Monitor Polski — https://monitorpolski.gov.pl/

SPLIT BY RATE PERIOD
Polish rates move several times a year, so almost any delay spans more than one of them. The calculator cuts the period on the days the rate actually changed and shows a table: from, to, days, rate, amount. The rows add up to the total exactly, so the breakdown can be pasted into a demand for payment without correcting it.

EIGHT KINDS OF INTEREST
• statutory late-payment interest (art. 481 § 2 KC)
• statutory capital interest (art. 359 § 2 KC)
• commercial transactions
• commercial transactions, public health entity debtor
• tax arrears (art. 56 Ordynacja podatkowa)
• reduced (art. 56a) and increased (art. 56b) tax arrears
• an agreed rate you type yourself

Commercial transactions do not work like the rest: the rate is fixed for a whole half-year and does not move when the central bank cuts in March. The calculator accounts for that.

TAX RULES
The base and the result are rounded to whole złote, and no interest is charged when it would fall below the threshold in art. 54 § 1 pkt 5 of the Ordynacja podatkowa. The app says plainly when that happens.

MAXIMUM INTEREST
Type an agreed rate above the statutory maximum in art. 481 § 2¹ KC and you get a warning and both figures. The calculation is not blocked: a contract term above the cap is void only as to the excess, and you are entitled to know by how much.

WORKS WITH NO NETWORK
Every rate and every grosz is computed on the phone. Nothing is sent to any server.

HONEST ABOUT ITS DATA
The rates screen shows the NBP reference and lombard rates, today's rate for each kind, and the history of changes. It states the day the data was read from NBP. When your period runs past that day, the app says so instead of feigning certainty — and lets you enter a newer rate by hand, so it never refuses an answer just because nobody has updated it in six months.

COPY AND SHARE
A ready-made text with the amount, the dates, the rates and the breakdown, to paste into an email to the debtor.

NO ADS
A one-off purchase removes the banner. It also covers what later versions add — saved calculations, a multi-invoice mode, the EUR 40/70/100 recovery fee and a PDF interest note. Buy today and you get those at no extra cost.

The result is an arithmetic calculation, not legal advice.