Overpay the Mortgage or Invest? Calculator

Have a mortgage and some spare money every month? This calculator compares two paths: overpaying the loan and investing the same amount. The comparison is fair — you spend the same every month in both cases, and once the overpaid loan is gone, the whole former installment goes into investments too. At the end of the loan term we compare your wealth after Polish capital gains tax and show the investment return at which the answer flips.

Updated: October 20, 2026

Your mortgage

Remaining balance (PLN)

Annual interest rate (%)

Defaults to the NBP reference rate (3.75%, in effect from March 5, 2026) + 2 percentage points — roughly what a variable-rate mortgage costs today (WIBOR + the bank's margin). Enter your loan's current rate.

Remaining term (years)

Installments left

Spare money

Every month (PLN)

One-off, now (PLN)

You put this money either towards the mortgage or into investments — the calculator compares both.

Investing

Expected annual return (%)

Historical avg. for a global equity portfolio: ~7%

Inflation rate (%)

Only used to show the result in today's money

Tax on investment gains

In a standard brokerage account you pay 19% Polish capital gains tax on the profit when you sell. An IKE retirement account is tax-free if you withdraw after age 60 and meet the statutory conditions — an earlier withdrawal is taxed on the profit. An IKE also has an annual contribution limit.

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Break-even investment return

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You overpay

Wealth at the end

—

Mortgage repaid in: —
Sooner by: —
Interest paid: —
Interest saved: —
Portfolio before tax: —
Tax on gains: —

You invest

Wealth at the end

—

Mortgage repaid in: —
Interest paid: —
Portfolio before tax: —
Tax on gains: —

Over time

How much more wealth one option gives than the other, year by year.

Example: PLN 500 a month on a PLN 400,000 mortgage

A PLN 400,000 loan over 20 years at 5.75%, with installments of PLN 2,808. Every month you have an extra PLN 500. This is your wealth after 20 years for different investment returns (after 19% Polish capital gains tax):

PLN 400,000 loan, 5.75%, 20 years, PLN 500 a month: wealth after 20 years
Investment return	Overpay	Invest	Winner
5%	PLN 215,290	PLN 189,269	Overpaying by PLN 26,021
7%	PLN 224,497	PLN 233,775	Investing by PLN 9,278
9%	PLN 234,319	PLN 293,294	Investing by PLN 58,975

At a 7% annual return, investing leaves you PLN 9,278 better off. Overpaying wins if your investments return less than 6.54% a year on average.

These numbers come from the same calculator you see above, using its default inputs. Enter your own to see your result.

How we calculate

In both cases you spend the same every month: the mortgage installment plus your spare money. When you invest, the loan runs to schedule and the spare money goes into a portfolio. When you overpay, it goes to the loan — the installment stays the same, so the loan ends early (the “shorten period” strategy), and from then on everything that went to the loan goes into the portfolio.

The loan is calculated as in the mortgage overpayment calculator: equal installments, interest charged monthly on the remaining principal. The portfolio grows each month by the annual return ÷ 12 and then receives the contribution — as in the compound interest calculator. Tax on gains (19%, or 0% in an IKE) is deducted once, at the end, as if you sold the investments then.

We compare wealth — investments after tax minus the remaining debt — in the month the loan would end without overpaying. By then the loan is repaid in both cases. The break-even return is the investment return at which both options end with the same wealth. Without tax it equals the mortgage rate.

Assumptions: a constant mortgage rate and a constant investment return (in reality both change, and investments can lose money), equal installments, no prepayment fees, other fees or insurance. For an IKE we assume a tax-free withdrawal and ignore the annual contribution limit. The home is the same in both cases, so it is left out.

Frequently Asked Questions

Which is better: overpaying the mortgage or investing?

It mostly comes down to two numbers: your mortgage rate and the return you can realistically expect from investing after tax. Every złoty you overpay “earns” your mortgage rate — for certain, tax-free and without risk. Investments may earn more, but they may not. With a high mortgage rate overpaying usually wins; with a low one investing tends to, especially through an IKE.

What investment return do I need for investing to pay off?

It has to beat the break-even return the calculator shows. For example, on a PLN 400,000 mortgage at 5.75% with PLN 500 of spare money a month, a portfolio in a standard account has to return more than 6.54% a year on average. The break-even is higher than the mortgage rate because investment gains are taxed at 19% in Poland, while the “return” on overpaying is not taxed.

How do Polish capital gains tax and an IKE change the result?

The 19% capital gains tax (podatek Belki) takes a share of investment profits, so it tilts the result towards overpaying. In an IKE retirement account you avoid the tax if you withdraw after age 60 (or 55 if you are entitled to early retirement) and meet the contribution-history rule — for example, contributions in at least 5 different years. Then the break-even return drops to the mortgage rate. You can take the money out earlier too, but then the profit is taxed. An IKE also has an annual contribution limit. Choose “0% — IKE” in the calculator to see the difference.

What about risk and an emergency fund?

Overpaying gives a certain result, but money paid to the bank can only come back if you sell the home or borrow again. Investments can be sold at any time, but their value swings — in a bad year a stock portfolio can lose tens of percent. Before you overpay or invest, set aside an emergency fund covering a few months of expenses. The calculator assumes a constant return, so it does not show the risk of losses.

Can I split the money between overpaying and investing?

Yes — many people do, so as not to bet everything on one option. The result of a split lies between the two options: the more you overpay, the closer it gets to the overpaying result. It is a sensible choice when the return you expect is close to the break-even return and it is hard to tell which option will win.