Compound Interest Calculator
This calculator shows how regular saving and compound interest multiply your capital over time. Enter your starting amount, regular contributions, expected return rate, and inflation to see both nominal and real portfolio value. You can also choose how Polish capital gains tax is charged: when you sell (ETFs, funds), on interest as it comes (deposits, savings accounts) or not at all (IKE).
Updated:
Polish avg. inflation 2010–2025: ~3.5%
Final value
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nominal
Real value
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after inflation
Total invested
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Interest earned
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after tax
Growth multiplier
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Capital gains tax
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Portfolio composition
Growth over time
Values after capital gains tax — what you would keep if you cashed out that year.
Example: PLN 10,000 upfront and PLN 500 a month
We assume a starting amount of PLN 10,000, contributions of PLN 500 a month, a 7% annual return compounded monthly and 3.5% annual inflation. The profit is taxed at 19% when you sell, as with ETFs.
| After (years) | Paid in | Before tax | After tax | Real, after tax |
|---|---|---|---|---|
| 10 | PLN 70,000 | PLN 106,639 | PLN 99,678 | PLN 70,663 |
| 20 | PLN 130,000 | PLN 300,851 | PLN 268,389 | PLN 134,883 |
| 30 | PLN 190,000 | PLN 691,150 | PLN 595,932 | PLN 212,318 |
After 20 years you will have paid in PLN 130,000 and the portfolio will be worth PLN 300,851 — PLN 170,851 of which is compound growth. After PLN 32,462 of capital gains tax you keep PLN 268,389, which is PLN 134,883 in today's money.
The same portfolio after 20 years, depending on tax: PLN 300,851 without tax (IKE), PLN 268,389 taxed when you sell and PLN 253,196 taxed on interest as it comes, as in a savings account.
These numbers come from the same calculator you see above, using its default inputs. Enter your own to see your result.
How we calculate
Each month the balance grows by balance × (annual return ÷ 12), then the contribution is added. For monthly contributions this gives FV = P × (1 + r)ⁿ + W × ((1 + r)ⁿ − 1) / r, where P is the starting amount, W the monthly contribution, r the monthly return and n the number of months.
The real value is the nominal value divided by (1 + inflation)^years. It shows what the result would be worth in today's money.
Polish capital gains tax (19%) follows the option you choose. “When you sell” — once, on the whole gain (value minus contributions), as if the investments were sold that year. “On interest as it comes” — 19% of the interest is deducted every month, so only the rest keeps compounding. “No tax” shows the gross result.
Assumptions: the same return every year, monthly compounding, no fees or commissions. The “no tax” option matches an IKE withdrawn after age 60 (or 55 with early-retirement rights) once the contribution-history condition is met — an earlier IKE withdrawal is taxed on the profit. The calculator does not check the annual contribution limit.
Frequently Asked Questions
What is compound interest?
Compound interest is a mechanism where earned interest is added to the principal and itself starts generating further interest. This causes investment growth to accelerate over time — the longer you invest, the greater the compounding effect.
How do monthly contributions affect investment growth?
Regular contributions significantly accelerate portfolio growth. Even a small monthly amount, invested consistently over years, can turn into a substantial sum thanks to compound interest. The earlier you start, the more you earn.
What annual return rate should I assume for long-term investing?
For long-term stock investments, the historical average annual return is about 7–10% nominally. However, individual year results can vary significantly. A cautious assumption is 5–7% nominal — with inflation around 3.5%, that is roughly 1.5–3.5% real return.
Why does inflation matter for investment calculations?
Inflation gradually erodes the purchasing power of money. If an investment earns 7% per year and inflation is 3.5%, the real growth is only about 3.5%. Ignoring inflation gives a misleading picture of wealth — the nominal value rises, but real purchasing power may be much lower.
What is the difference between nominal and real investment value?
Nominal value is the amount in currency you see in your account. Real value is the same amount adjusted for inflation — it shows how many goods and services you can actually buy with it. At 3.5% inflation, the real value after 20 years will be about half the nominal value.
How much does Polish capital gains tax take from compound interest?
The tax (podatek Belki) is 19% of the profit, but how much it really takes depends on when you pay it. On a savings account or deposit the bank withholds it from every interest payment, so less money keeps compounding. With accumulating ETFs and funds you only pay it when you sell, so the whole amount compounds until then. For example, PLN 10,000 upfront and PLN 500 a month at a 7% annual return give PLN 300,851 after 20 years without tax, PLN 268,389 taxed when you sell and PLN 253,196 taxed on interest. In an IKE you avoid the tax if you withdraw after age 60 (or 55 with early-retirement rights) and meet the statutory conditions.
Guides
Compound interest — how it works and why time matters
Compound interest in practice: the formula, examples over 10, 20 and 30 years, the rule of 72 and the effect of fees, the Belka tax and inflation.
IKE or IKZE — which one to choose?
IKE or IKZE? Contribution limits, the tax relief, the tax on withdrawal and a comparison in numbers for different incomes — and when to have both.
Is PPK worth it?
PPK in numbers: what your employer and the state add, the tax you pay on it and what's left if you withdraw early. When opting out makes sense.
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