Polish Bank Deposit Calculator

A deposit earns simple interest for each term: amount × yearly rate × number of months ÷ 12, or for a deposit in days, amount × rate × number of days ÷ 365. With monthly capitalisation we work out each month this way (rate ÷ 12), with daily capitalisation each day (rate ÷ 365), and add the interest to the deposit straight away, so it earns interest itself in the next month or day. A deposit in months with daily capitalisation is turned into days so that the total number of days comes out right: term number k ends on day k × number of months × 365 ÷ 12, rounded to a whole day. For example, 4 × 3 months is 91 + 92 + 91 + 91 = 365 days.

At every capitalisation the interest is rounded to the grosz, and the bank takes 19% Belka tax from it and adds the rest to the deposit. Since 2012 the tax on interest has been rounded to whole grosze (Article 63 § 1a of the Polish Tax Ordinance): fractions below half a grosz are dropped, and those of half a grosz or more are rounded up to a whole grosz. We round every amount the same way after each step, and work out the tax from the already rounded interest. For example, PLN 10,000 at 3.75% for 1 day earns PLN 1.03 of interest and PLN 0.20 of tax, so PLN 0.83 goes into the deposit. With daily capitalisation this rounding happens every day, so on small amounts it shows in the result.

After each term the deposit renews with the whole amount — what you paid in plus the interest after tax — at the same rate and with the same capitalisation. The yearly return after tax is the average yearly growth of the amount: (final amount ÷ amount paid in)^(1 ÷ years) − 1, where years are the total number of months ÷ 12, or days ÷ 365. The real return is (1 + return after tax) ÷ (1 + inflation) − 1, and the amount in today's money is the final amount divided by (1 + inflation)^years. The rate at which the deposit keeps up with inflation is the lowest gross rate at which a deposit with the same amount, term, capitalisation and number of terms gives a real return of at least zero. We find it by halving the range from 0 to 50% again and again (bisection), to within 0.01 percentage points. When inflation is zero or negative, any rate will do. When even 50% isn't enough, we show “out of reach”. That happens at very high inflation, and at any positive inflation when the amount is so small that the interest rounds to zero even at 50% — say, 1 zł for a day or two, or a few złoty with daily capitalisation.

Assumptions: the same rate at every renewal (in reality the bank can change it), each term held to the end with no early break, a 365-day year (no leap years or 360-day convention), no top-ups along the way and none of the conditions of promotional offers, such as “new money” or opening an account with the bank. We count each month of a deposit as 1/12 of a year, but many banks pay interest for the actual number of days (3 months is 89–92 days), so the result can differ from your statement: PLN 10,000 at 3.75% for 3 months earns PLN 93.75 of interest here, but PLN 94.52 for 92 days, PLN 0.77 more (PLN 91.44 for 89 days). For an exact result, enter the term in days from your contract. Inflation is the same throughout — a scenario, not a forecast.