WIBOR impact calculator — mortgage instalment

WIBOR (Warsaw Interbank Offered Rate) is the reference interest rate at which banks lend money to each other on the Polish interbank market. Banks use it as the base rate for variable-rate loans — your instalment = WIBOR + bank margin. When WIBOR rises, so does the interest rate and monthly payment; when it falls, the payment decreases.

WIBOR is based on banks' declarations of willingness to lend funds (an offered rate), while WIRON (Warsaw Interest Rate Overnight) is calculated from actual overnight deposit transactions in the market. WIRON is considered more transparent and less susceptible to manipulation, and Poland is gradually transitioning from WIBOR to WIRON as the benchmark for mortgage loans.

The increase depends on the loan amount and term. For example, for a PLN 400,000 loan over 25 years (300 instalments), a 1 pp rise in WIBOR increases the payment by roughly PLN 200–230 per month. Use the calculator with your own figures for an exact value.

The calculator applies the annuity payment formula: R = P × [r(1+r)^n] / [(1+r)^n − 1], where P is the remaining principal, r is the monthly rate (WIBOR + margin) / 12, and n is the number of remaining instalments. Enter the current and new WIBOR, loan amount, margin and repayment period — the calculator automatically shows the difference.

The bank margin is the fixed addition to the interest rate, set in the loan agreement. Unlike WIBOR/WIRON, the margin does not change automatically — it is fixed for the entire loan term (unless the bank changes the contract terms). Total interest rate = WIBOR + bank margin.

Most banks use WIBOR 3M (three-month) or WIBOR 6M (six-month). This means your loan interest rate is updated every 3 or 6 months based on the WIBOR rate on a specific date. Check your agreement to see which applies — after each update, the new instalment will apply for the next period.

Not directly. The National Bank of Poland (NBP) sets the reference rate, which strongly influences WIBOR/WIRON, but it is not the same rate. An NBP rate cut usually leads to a drop in WIBOR within the following days or weeks, which then translates into a lower loan interest rate at the next update date.

In Poland between 2021 and 2022 WIBOR rose from about 0.2% to over 7%, which for a PLN 400,000 loan over 30 years meant a payment increase of about PLN 1,500–2,000 per month. The calculator lets you simulate any rate-change scenario, so you can check whether your budget can handle a rise of 1, 2 or 3 percentage points.

Refinancing is worthwhile when the new terms (lower WIBOR + margin or fixed rate) generate savings that exceed the refinancing costs (fees, new property valuation). A fixed rate protects against rate rises but is usually higher than the current WIBOR + margin. Compare both options using the calculator, simulating a pessimistic rate-rise scenario.

An overpayment is an amount paid above the required instalment, which reduces the remaining principal. A smaller principal means a 1 pp WIBOR rise has a smaller impact on the payment. For example, with a remaining principal of PLN 200,000 instead of PLN 400,000, a 1 pp WIBOR rise increases the payment by half as much. Enter the current remaining principal (after overpayments) to get accurate calculations.