Albert Einstein allegedly called compound interest the "eighth wonder of the world." Whether or not he said it, the principle is powerful: earning interest on your interest turns modest savings into meaningful wealth over time — but only if you understand how it works and what can erode your gains.
What Is Compound Interest?
Simple interest is calculated solely on your original principal. Compound interest, by contrast, is calculated on the principal plus all previously accumulated interest. Each period, your balance grows a little larger, and so does the interest you earn — creating an exponential curve rather than a straight line.
The Compound Interest Formula
The standard formula is:
- FV = PV × (1 + r)ⁿ
Where: FV = future value, PV = present value (initial deposit), r = interest rate per period, n = number of periods.
For more frequent compounding (quarterly, monthly), the formula becomes:
- FV = PV × (1 + r/m)^(n×m)
Where m = number of compounding periods per year.
Compounding Frequency Matters
The more often interest compounds, the more you earn. On a 10,000 PLN deposit at 6% annual interest over 10 years:
- Annual compounding: FV = 10,000 × (1.06)¹⁰ ≈ 17,908 PLN
- Quarterly compounding: FV = 10,000 × (1.015)⁴⁰ ≈ 18,061 PLN
- Monthly compounding: FV = 10,000 × (1.005)¹²⁰ ≈ 18,194 PLN
Polish bank deposits typically compound interest at the end of the term (annually or at maturity). Investment accounts and some savings products compound monthly.
The Rule of 72 — Quick Mental Maths
To estimate how many years it takes to double your money, simply divide 72 by the annual interest rate:
- At 6%: 72 ÷ 6 = 12 years
- At 4%: 72 ÷ 4 = 18 years
- At 9%: 72 ÷ 9 = 8 years
The Belka Tax: What Poland Takes from Your Returns
In Poland, interest income and capital gains are subject to a 19% flat tax, informally known as the podatek Belki. Banks withhold this automatically, so your effective net return is lower than the advertised rate.
Returning to our example — 10,000 PLN at 6% annual compounding over 10 years:
- Gross future value: ≈ 17,908 PLN (gain of 7,908 PLN)
- Belka tax (19% of gain): 7,908 × 19% ≈ 1,502 PLN
- Net future value after tax: ≈ 16,406 PLN
Individual Retirement Accounts (IKE and IKZE) allow you to shelter returns from the Belka tax — worth considering if you are saving for the long term.
Start Calculating Your Savings Growth
Use our free compound interest calculator to model different deposit amounts, rates, and time horizons — with and without the Belka tax effect.
Try it now: Compound Interest Calculator Poland →