A loan-to-income ratio, or LTI, divides the mortgage by your gross annual income. Borrow €320,000 on a household income of €80,000 and the LTI is 4. It says nothing about the interest rate or the monthly payment: it is a ceiling on the size of the debt, set against pay before tax.
Ireland uses it as its main rule. The Central Bank of Ireland limits first-time buyers to 4 times gross income and second and later buyers to 3.5 times, and lets 15% of each group's lending go above those multiples. On €80,000 that is €320,000 or €280,000, before the bank has looked at your other debts at all.
Britain caps the lender's book rather than the borrower. The Financial Policy Committee wants no more than 15% of new mortgages at 4.5 times income or above. In July 2025 the Prudential Regulation Authority let lenders set that limit aside for themselves while it reviewed the rule, and in April 2026 the regulators proposed keeping 15% for the market as a whole and dropping it for each firm.
New Zealand's version goes by another name. The Reserve Bank calls it debt-to-income and counts all your debts, the new loan included, against gross income, so borrowing above 6 times income is high-DTI for an owner-occupier and above 7 times for an investor. Elsewhere debt-to-income usually means the share of monthly pay spent on repayments, which is a separate measure.
The debt-to-income calculator switches to this kind of rule in countries that use one: in place of a ratio verdict it shows the maximum borrowing, your annual income times the multiple, and names the regulator that sets it.