Statutory Interest Calculator: Poland and Russia

Statutory interest is the interest a late payer owes at a rate set by law rather than by a contract. This calculator works out that interest for a debt in Poland or Russia over a range of days, and because the legal rate changes whenever the central bank moves the index behind it, the tool splits the range at every change and charges each stretch of days at the rate in force on those days.

How the calculator works

The tool asks for four things: the country, the amount owed, the first day interest runs, and the last day it runs. Both dates are counted, so a claim from 1 June to 2 June is two days, not one.

It then builds the answer in three steps.

1. It reads the country's published rate table and cuts the claim period wherever a new rate took effect.
2. It works out the interest each stretch of days earns on its own.
3. It adds the stretches together and rounds the sum to two decimals once, at the end.

Each country brings its own rule about what the rate is and what a year divides by.

A claim that starts before the statute date in that table is refused rather than priced. Before those dates each country set the rate a different way, and applying today's mechanics to an older debt would give a confident wrong number.

Statutory interest formula

One stretch of days under a single rate earns:

$I_k = P \times \frac{r_k + m}{100} \times \frac{d_k}{B_k}$

where $I_k$ is the interest for stretch $k$, $P$ the amount owed, $r_k$ the published index rate for that stretch as a percentage, $m$ the percentage points the statute adds to the index, $d_k$ the number of days in the stretch counting both the first and the last, and $B_k$ the divisor for the year, either 365 or 366.

The whole claim is the sum of its stretches, rounded to money once:

$I = \operatorname{round}_2 \left( \sum_{k=1}^{n} I_k \right)$

where $n$ is the number of stretches and $\operatorname{round}_2$ means rounding to two decimals. Rounding every stretch first and then adding the rounded figures can drift by up to half a cent per rate change, so the tool never does that.

Interest does not compound here. Every stretch is charged on the original amount owed, not on the amount owed plus interest already run up.

Worked example

A Polish debt of 10,000 zloty runs from 1 June 2025 to 31 July 2025.

The NBP reference rate was 5.25% from 8 May 2025, and dropped to 5.00% on 3 July 2025. Adding the statutory 5.5 points gives 10.75% and then 10.50%. The claim period therefore has two stretches.

The first stretch runs from 1 June to 2 July, which is 32 days at 10.75%. It earns 10,000 × 0.1075 × 32 ÷ 365, about 94.25 zloty. The second runs from 3 July to 31 July, 29 days at 10.50%, earning 10,000 × 0.1050 × 29 ÷ 365, about 83.42 zloty.

The claim is 61 days long. The interest is 177.67 zloty and the total due is 10,177.67 zloty.

A period that crosses New Year

Russia divides by the real length of each calendar year, so a Russian claim that runs across 1 January is cut there as well as at each rate change.

A debt of 500,000 rubles running from 1 December 2023 to 31 January 2024 has three stretches. The key rate was 15% until 17 December 2023 and 16% from 18 December. December 2023 sits in a 365-day year; January 2024 sits in a leap year of 366 days.

• 1 to 17 December 2023: 17 days at 15% over 365, about 3,493.15 rubles.
• 18 to 31 December 2023: 14 days at 16% over 365, about 3,068.49 rubles.
• 1 to 31 January 2024: 31 days at 16% over 366, about 6,775.96 rubles.

Over 62 days that is 13,337.60 rubles of interest.

Where the rates come from

Rates go stale, and a stale rate lies quietly. Every figure in the tool carries its publisher, the page it was read from, and the date it was last checked. The page shows the whole table, and the reader can add or replace any period.

• Poland: Narodowy Bank Polski's own rate archive, published at static.nbp.pl as stopy_procentowe_archiwum.xml, where each entry carries the date it began to apply. Read row by row on 6 September 2026. The built-in table starts on 5 January 2022 and its last entry is 3.75% from 5 March 2026, which is a statutory rate of 9.25%.
• Russia: the Bank of Russia key-rate table at cbr.ru/hd_base/KeyRate/, using the date each value came into force rather than the date the board announced it. Read on 6 September 2026. The built-in table starts on 19 September 2022 and its last entry is 14.00% from 27 July 2026.
• The Russian divisor: no provision states 365 or 366. The 360-day year once used came from п. 2 of Постановление Пленумов ВС РФ и ВАС РФ от 08.10.1998 № 13/14, and п. 84 of Постановление Пленума ВС РФ от 24.03.2016 № 7 declared that part no longer applicable, which leaves the real calendar year.
• The Polish divisor: no provision settles it. Art. 114 Kodeksu cywilnego counts a year as 365 days, but it is written for periods whose days need not run without a break, and late-payment interest runs every day. The tool defaults to a fixed 365, the usual practice, and offers 366 for leap-year days. Over a full leap year the two answers differ by a factor of 366/365, about 0.27%.

Older decisions exist in both archives. They were left out to keep the shipped tables short, so a claim that reaches back further needs a rate period the reader adds. The tool refuses to guess a rate for a day before its earliest row.

What the result is, and what it is not

The result is an estimate of the arithmetic, not a legal opinion. The tool cannot know several things that change the real answer.

• Days after the last decision in the built-in table are charged at that last decision's rate. The page warns when a claim reaches into those days, because the central bank may have moved the rate since.
• Whether the day a debt is paid counts is settled in Russia and not in Poland. Russian law includes it, under ст. 395 п. 3 ГК РФ and п. 48 of Постановление Пленума ВС РФ от 24.03.2016 № 7. No Polish source of equal weight decides the point, so the tool leaves both endpoints to the user.
• Only the general civil rate is modelled. Poland's higher rate for commercial transactions between businesses, any rate agreed in a contract, and any statutory rate outside these two articles are not.
• Court-ordered pauses, insolvency stays, limitation periods and a judge's decision to cut a claimed period are not modelled.
• The currency follows the country. Nothing is converted between zloty and rubles.

The tool also checks its inputs in code rather than trusting the form. The amount owed must be above 0 and no higher than 100,000,000. The claim period cannot exceed 50 years. Up to 50 rate periods can be added by hand, each needing a real date and a rate between 0 and 100.

Frequently asked questions

Why does one claim use several interest rates?

Statutory interest follows a published index, and the central bank moves that index. A debt outstanding through a rate change is charged the old rate up to the day before the change and the new rate from the change onwards. Charging the whole claim at today's rate, or at the rate on the first day, gives the wrong money.

Does the last day count?

In this tool, yes. Both dates entered are included in the day count. For a Russian claim that matches the law, which counts the day the debt is actually paid. Polish practice on that day is not settled, so a user who wants to exclude it should enter the day before the payment as the last day.

Why do the rows in the breakdown not always add up to the total?

The rows are rounded to two decimals for display, but the total is the unrounded sum rounded once. When several rates are involved, the rounded rows can land a cent or two away from the total. The total is the figure that counts.

Does this cover late payments between businesses in Poland?

No. Commercial transactions between businesses fall under a separate Polish act with a higher rate, the reference rate plus 8 or 10 percentage points. This tool computes the general civil rate under art. 481 § 2 Kodeksu cywilnego. Polish contractual interest for late payment is capped at twice that statutory rate, so the figure here always sits below the cap.

Can it price a debt from 2015?

Not from the built-in tables, and not under the current statute wording. Poland's rate has followed the NBP reference rate only since 1 January 2016, and art. 395 ГК РФ has named the key rate only since 1 August 2016. An earlier claim was governed by a different rule, so the tool refuses it instead of pricing it wrongly.