Why does my bank’s mortgage calculator show a different monthly repayment?

Differences can come from rounding, payment timing conventions, lender fees, product structure (split loans), and how the lender models stress tests or affordability. Treat online calculators as estimates.

Do small rounding differences matter?

They can add up over long terms, but most differences are small. Larger differences usually come from assumptions (fees, timing, or product structure) rather than rounding alone.

Can a split mortgage change repayment maths?

Yes. If part of the loan is fixed and part is variable, your total repayment is the combination of two sub-loans with different rates and terms.

Is my real repayment always exactly the amortisation formula?

The amortisation formula is the baseline, but product details like payment timing, fees, and lender rounding conventions can create small differences.

Why mortgage calculators give different results (Ireland)

If two calculators show different repayments for the same inputs, it does not always mean one is “wrong”. This guide explains the common causes and which assumptions are most likely to drive bigger differences. Use it to sanity-check estimates, then confirm the authoritative lender figures.

Mortgage calculator (estimate)Rate rise scenarios Stress test guide

1) Timing and payment conventions

Small differences can come from whether a calculator assumes payments occur at the start or end of a month, and how it treats the first partial period.

These differences are usually modest but can show up when you compare calculators side-by-side.

2) Fees and product structure

Some lender calculators include fees or model split loans (part fixed, part variable). If your loan is split, the repayment is the sum of two sub-loans, not a single-rate repayment.

If one calculator includes fees and another does not, the difference can look like a repayment difference even when the underlying loan is the same.

3) Stress test methods

Affordability and stress tests can vary by lender. Two tools can show the same baseline repayment but different “stress-tested” repayment if they use different scenario rates or affordability assumptions.

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