The rules we apply
- Central Bank of Ireland lending rules: loan-to-income caps of 4× gross income for first-time buyers and 3.5× for second-time and subsequent buyers, with a minimum 10% deposit. These are the standard limits — lenders may grant a limited share of exceptions each year, which we deliberately do not model.
- Residential stamp duty: 1% on the portion of the price up to €1,000,000 and 2% on the balance above it, per current Revenue rates.
- Repayments: the standard annuity formula used by Irish lenders — M = P × r(1+r)n / ((1+r)n − 1) — with interest compounded monthly at your annual rate ÷ 12.
- Overpayments: modelled as a constant extra monthly payment applied to the balance with the term reducing, which is the arrangement that produces the interest savings shown. Fixed-rate overpayment caps and breakage fees are flagged but not modelled.
- Rent vs buy: a cash-cost comparison. Buying = stamp duty + one-off fees + mortgage interest + running costs over your horizon. Principal repayments build equity, so they are treated as savings, not costs. Renting = rent over the same horizon with your chosen annual growth rate. House price appreciation and investment returns on your deposit are deliberately excluded.
What we deliberately leave out
Lender-specific stress tests, exception lending, cashback offers, green-rate discounts, mortgage protection insurance, and tax reliefs are not yet modelled. Where a simplification could flatter one option over another (for example, rent vs buy), we say so on the page rather than hiding it in fine print.
When these numbers were last reviewed
Rules and rates on this site were last reviewed in July 2026. Central Bank measures and stamp duty bands can change — particularly in the annual Budget. If you spot something out of date, the figures on centralbank.ie and revenue.ie always win.
Your privacy
Every calculator runs entirely in your browser. The numbers you enter are never sent to a server, logged, or stored — the moment you close the tab, they're gone. Shareable links encode your inputs in the URL itself, so only the people you send them to see them.
How we're funded
MortgageMath is currently free and carries no advertising. If that changes — for example, if we add a clearly-labelled option to speak to a regulated mortgage broker — it will be disclosed plainly next to the feature itself, never buried here. We will never gate calculator results behind giving us your contact details.
Verified math
Every financial function on this site is covered by an automated test suite that checks the annuity formula, amortisation schedules, overpayment savings, Central Bank limits, stamp duty bands and the rent-vs-buy comparison against worked examples.