1. What the calculators are designed to do
Check The Damage is a planning tool. Its purpose is to let a reader change a mortgage amount, interest rate, term, deposit or overpayment and see the mathematical effect. The output is intended to answer questions such as “what would this repayment look like?” or “how much difference could an extra payment make?”
The calculators are not designed to predict an approval. A lender may use information that is not available to a public calculator, including its own affordability model, credit assessment, product criteria and underwriting judgement.
2. Standard mortgage repayment formula
For a standard capital-and-interest mortgage with a constant rate, the monthly payment is calculated from the principal, monthly interest rate and number of monthly payments.
M is the monthly repayment, P is the starting mortgage balance, r is the monthly interest rate and n is the number of monthly payments. The annual percentage rate used for an illustration is divided by 12 for this calculation.
This is a mathematical illustration. Real mortgages can include product-specific features, fees, rate changes, payment dates or other terms that are not represented by the basic formula.
3. Income-based borrowing illustrations
Salary pages use the current Central Bank mortgage-measures framework as a simple starting point. The standard loan-to-income limit is 4 times gross income for first-time buyers and 3.5 times gross income for second and subsequent buyers. The Central Bank also permits a proportion of lending above the standard limits.
These figures are regulatory limits, not guarantees of borrowing capacity. A lender can offer less after assessing income, existing commitments, expenditure, dependants, employment and other factors. Some mortgages fall outside or have specific treatment under the mortgage measures, so readers should check the current Central Bank framework for their circumstances.
4. Loan-to-value and deposit assumptions
For standard principal-home lending, first-time buyers and second/subsequent buyers generally have a 90% maximum LTV, meaning a 10% minimum deposit under the Central Bank framework. Buy-to-let lending has different treatment, including a 70% maximum LTV and therefore a 30% minimum deposit.
A deposit is not the same as the full amount of cash needed to buy a home. Buyers may also need money for Stamp Duty, solicitor and valuation costs, mortgage protection, insurance, moving costs and other expenses.
5. Interest-rate assumptions
Some worked examples on the site use a 3.53% rate as an illustrative assumption (based on the Central Bank of Ireland weighted average for new Irish mortgage agreements in August 2026). Where the main calculator uses an official average, the page labels the rate and its publication period. Published lender rates are shown as snapshots with a “last checked” date where applicable.
An average rate is not a personal quote. A lender's actual rate can vary according to product, LTV, borrower circumstances, BER, fixed period and other conditions. A rate can also change after the date a public source was published.
6. Overpayments
Overpayment calculations model the effect of reducing the outstanding principal earlier. The saving comes from paying interest on a smaller balance and, where the normal payment is maintained, potentially clearing the mortgage sooner.
Actual lender rules matter. Variable-rate mortgages often permit more flexibility, while fixed-rate mortgages can have annual limits, breakage charges or other restrictions. Extra money should be confirmed with the lender as being applied to capital in the way the borrower intends.
7. Buying costs
Where a guide includes Stamp Duty, the site uses the current Revenue rates as a reference. The standard residential rates are 1% up to €1 million, 2% on the portion over €1 million up to €1.5 million, and 6% on the portion over €1.5 million. Certain acquisitions have different treatment, so the calculator should not be treated as a tax determination.
8. Source hierarchy
When information is regulatory or tax-related, primary sources are preferred. The normal hierarchy is:
- Central Bank of Ireland for mortgage-measure rules and related regulatory information.
- Revenue for Stamp Duty and other tax information used by the site.
- CCPC for practical consumer guidance on mortgages, switching and overpayments.
- Lender publications for product rates and conditions, treated as dated snapshots rather than universal market facts.
If two sources appear to conflict, the relevant primary authority should take precedence and the site should not present an uncertain point as settled fact.
9. Review dates and corrections
Important pages carry a review date when practical. Time-sensitive figures such as lender rates are paired with a “last checked” date. The site is intended to be updated when a material official rule or assumption changes.
If a calculation or explanation is found to be wrong, the correction should address the underlying figure or wording rather than simply adding more search-oriented text. Readers should always check the live source before acting on a rule that may have changed.
10. What is deliberately excluded
Unless explicitly stated, calculations do not include mortgage protection, home insurance, property tax, maintenance, utilities, legal costs, valuation costs, lender fees, early repayment charges or a household's other personal spending. These can materially affect whether a mortgage is affordable.
11. Bottom line
The site's figures are best treated as a first pass: calculate a scenario, understand the assumptions, compare alternatives, and then verify the current position with the lender or official authority. The goal is not to make a decision for the reader; it is to make the numbers and trade-offs easier to understand.