Two ratios, because a lender looks at two
Debt-to-income is not one number. The front-end ratio is the housing payment measured against income. The back-end ratio adds every other debt payment to it: cards, car finance, student loans, maintenance. Underwriting looks at both, and a borrower can pass one comfortably and fail the other.
The pair matter in different ways. A high front-end ratio with nothing else outstanding says the house is expensive for the income. A modest front-end ratio inside a high back-end one says the house is fine and the rest of the balance sheet is not, which is a problem that can be fixed without moving.
Both are measured monthly, because the payments are monthly. Entering a yearly income divides it by twelve once, at one rounding point, rather than letting two different pages round the same salary differently.
Gross income, which is not what arrives
The base is income before tax. That is the convention lenders use and it is the opposite of what most people have in mind, so it is worth being blunt about the consequence: the share of your actual pay going to debt is a larger number than the one on this page, often by a third or more.
What counts as income is more generous than a payslip. Reliable overtime, bonuses with a history behind them, self-employment profit averaged over a couple of years, rent received, pensions and maintenance can all be included, usually with documentation and often with a haircut. What is generally excluded is anything that cannot be evidenced as likely to continue.
What counts as a debt payment
The test is roughly whether a contract obliges you to pay it. Minimum payments on cards count even if you clear the balance every month, because the minimum is what the obligation is. Instalment loans count. Court-ordered maintenance counts. Housing counts in full, which for a purchase means the loan payment plus property tax, insurance and any association fee, not the loan payment alone.
Groceries, utilities, childcare, subscriptions and savings do not count, however unavoidable they feel. They shape whether you can live with a payment, which is a real question, and it is the one the rent calculator asks instead.
Twenty-eight and thirty-six are conventions
The familiar pair of limits are underwriting rules of thumb rather than law, and lenders differ, product by product and country by country. The limit field takes whatever figure you have been given. There are regulated affordability tests in some places that also look at debt against income, and they define the terms their own way; a page that treated any single percentage as a legal ceiling would be wrong somewhere within a year.
What is certainly true is that the ratio is only one of several tests. Credit history, savings left after completion, employment stability and the property itself all feed the decision, and none of them are on this page.