What Is Debt-to-Income Ratio and How Lenders Use It to Size Your Loan

Ask a lender why your mortgage pre-approval came in at $300,000 when the house costs $400,000 and the answer will involve three letters: DTI. So what is debt-to-income ratio, how do lenders calculate it, and what can you do about it? This guide explains the ratio, shows the arithmetic an underwriter runs on your pay stubs, and walks through an $8,000 monthly income in the how much can I borrow calculator. By the end you will be able to estimate your own borrowing power within a few percent before you apply.

The definition

Debt-to-income ratio is the share of your gross monthly income that goes to debt payments. Lenders cap this share, and the gap between the cap and your existing payments is the payment they will let a new loan take.

DTI = (existing monthly debt payments + proposed payment) ÷ gross monthly income × 100

Mortgage lenders use two versions. The front-end ratio counts only the housing payment: principal, interest, taxes, insurance, PMI and any HOA dues. The back-end ratio adds every other debt. The classic guideline is 28/36: housing under 28% of gross income, all debts under 36%. Conventional loans sold to Fannie Mae and Freddie Mac allow a back-end ratio up to 45%, and up to 50% with strong credit and reserves. FHA loans go to 43% as standard and higher with compensating factors. The “qualified mortgage” rule that long set 43% as the ceiling was replaced in 2021 by a price-based test, so the 43% figure now lives on as a guideline rather than a rule.

What counts as income

For a salaried applicant, lenders take gross monthly pay before tax, from the last two pay stubs and the last two years’ W-2s. Overtime, bonuses and commissions count if they have a two-year history and are likely to continue, usually averaged over those two years. Rental income from a property you own is typically counted at 75% of the lease. A co-borrower’s income is added in full.

For the self-employed, income is the average of the last two years’ net profit from the tax returns, with depreciation and some other non-cash deductions added back. Income that is not on the returns does not count, no matter how real it is.

What counts as a debt

Every monthly payment on the credit report: mortgages, auto loans, student loans, personal loans. Credit card minimum payments, from the report, not the balance you plan to pay. Child support and alimony you pay. Student loans in deferment still count, at a percentage of the balance set by the loan program if no payment is reported. A loan with fewer than ten payments left is often excluded. Utilities, insurance premiums, phone bills and groceries are not debts for DTI purposes; the cap exists because lenders assume the income left over covers them.

A worked example

Take a gross income of $8,000 a month, no existing debts, a back-end cap of 36%, a rate of 6.5% and a 30-year term.

1. Maximum total debt payments: 36% of $8,000 = $2,880.
2. Existing debts: none, so the whole $2,880 is available for the new payment.
3. Loan that a $2,880 payment supports at 6.5% over 360 months: $455,647.

That is principal and interest only. A real mortgage payment includes taxes, insurance and possibly PMI, say $750 a month together. Take that off first and the loan the remaining $2,130 supports is $336,989. Now add a car loan of $700 and card minimums of $300. Available payment falls to $1,130 and the loan to under $180,000. One car loan and some card debt have cost about $160,000 of mortgage. That is the single most useful thing to know about DTI: existing payments are expensive at application time, and paying off a small loan before you apply can add more borrowing power than a year of raises.

The loan payment calculator does the reverse of this table: give it the loan, and it tells you the payment.

Front-end versus back-end in practice

Most lenders test both ratios and the loan must pass the tighter one. On the $8,000 income, the 28% front-end cap allows a housing payment of $2,240 including escrow. With $750 of taxes and insurance that leaves $1,490 for principal and interest, which supports about $235,700. The back-end cap, after the $1,000 of other debts, allows $1,130 and supports $178,800. Here the back-end ratio binds, which is the usual case for borrowers with a car loan or student debt. A borrower with no other debts is usually bound by the front-end ratio instead. Knowing which one binds tells you which move helps: paying off debts for the back-end, a bigger down payment or cheaper taxes and insurance for the front-end.

DTI is not the only gate

An underwriter runs at least three other checks, and the loan is the lowest of them.

Loan-to-value. Conventional loans go to 97% of the appraised value for first-time buyers with PMI, 80% without it. Jumbo loans above the conforming limit usually need 20% down or more. The appraisal, not the contract price, sets the value, and a low appraisal cuts the loan regardless of income.

Credit score. Below 620 most conventional lenders decline; FHA goes to 580 with 3.5% down. Above 740 you get the best pricing, and the lower rate raises what a given payment supports. Fannie Mae’s pricing grid combines score and down payment, so a weaker score costs more on a small down payment.

Reserves and history. Jumbo and investment-property loans often require several months of payments in the bank after closing. Two years of steady employment is the norm, with explanations for gaps.

So in the $400,000 example, an $8,000 income supports $455,647 by DTI before escrow, but with $750 of taxes and insurance and 10% down, the loan is capped by the payment at about $337,000, and the buyer will be asked to bring more cash or shop a cheaper house.

Why your figure differs from the calculator’s

The how much can I borrow calculator applies the DTI formula with the cap, rate and term you choose, to the income and debts you enter. A lender’s figure can be lower because it counted student loans in deferment or a card balance you forgot, subtracted taxes and insurance from the payment, used a lower cap for your credit tier, or applied loan-to-value. It can be higher because the lender allows 45 or 50% on your file or counted a co-borrower.

If the gap is large, ask the loan officer for the DTI worksheet and the debts it counted. Credit reports often list paid-off loans as open, and a single corrected line can change the answer. Then check the figure again with the tool and, if paying down a debt would help, size it with the extra payment calculator. Other tools are on the homepage.

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