How Mortgage Lenders Calculate Debt-to-Income

The definition

The CFPB defines your debt-to-income ratio as all your monthly debt payments divided by your gross monthly income, which is your income before taxes and other deductions. Its example: $1,500 for a mortgage, $100 for an auto loan and $400 for other debts total $2,000 a month; divided by $6,000 of gross monthly income, that is a DTI of 33%. The CFPB adds that different loan products and lenders have different DTI limits.

Two ratios, not one

Lenders often look at two versions. The front-end ratio is the housing payment alone divided by gross income. The back-end ratio, also called total DTI, adds every other monthly debt payment. The back-end number is the one that usually decides the outcome, because it captures everything you owe.

The housing payment in these ratios is more than principal and interest. It usually means the full monthly payment on the home: principal, interest, property taxes, homeowner's insurance, mortgage insurance and any HOA or condo dues. That is why a house that looks affordable on a rate quote can push the ratio higher than expected. The buying costs checklist shows how big those extra items can be.

What usually counts as a debt, and what usually does not

• Usually counted: the proposed housing payment, minimum payments on credit cards, auto loans, student loans, personal loans, other mortgages, and court-ordered payments such as child support.
• Usually not counted: utilities, groceries, phone, insurance premiums other than the home, subscriptions and everyday spending.

For debts, lenders generally use the required minimum payment shown on your credit report, not what you actually pay. Paying a card in full each month does not remove its minimum from many calculations if a balance shows on the report, so check what your lender will use. Rules differ for loans with only a few payments left, so ask.

The published limits

Fannie Mae's Selling Guide, which sets standards for many conventional loans, states that for manually underwritten loans the maximum total DTI is 36% of the borrower's stable monthly income. That can be exceeded up to 45% if the borrower meets the credit score and reserve requirements in its Eligibility Matrix. For loans underwritten through its automated system, Desktop Underwriter, the maximum allowable DTI is 50%. The guide lists exceptions, such as lower maximums for some cash-out refinances, and government-backed loans follow the rules of the relevant agency.

The practical reading is that 36% is a comfortable zone, the mid-40s is workable if the rest of your file is strong, and 50% is a ceiling that only some applications reach. These are guideline numbers for one major investor. Your lender may be stricter.

Worked example: how much payment fits?

Say your gross income is $7,000 a month and you have $500 a month in other debt payments. The table shows the housing payment each limit allows.

Now suppose property tax, insurance and HOA dues come to $450 a month. At 36%, that leaves $1,570 for principal and interest, which supports a loan of about $248,000 at 6.5% over 30 years. At 45% it leaves $2,200, which supports about $348,000. The difference between a comfortable and a stretched ratio is about $100,000 of borrowing power, which shows how sensitive buying power is to this one number. The rate matters too: the same payment supports a smaller loan as rates rise.

Ways to improve your DTI

1. Pay off a monthly payment, not just a balance. Removing a $300 car payment changes back-end DTI at $7,000 income from 32.9% to 28.6% when the housing payment is $1,800 and other debts are $500 before and $200 after. Clearing a small loan with a big payment helps the ratio more than paying a little extra on a large one.
2. Raise documented income. A raise, a second stable income or a co-borrower's income raises the denominator, but lenders typically want a history of it.
3. Choose a smaller payment. A larger down payment, a different price, a longer term or a lower-rate offer all reduce the housing payment. Check the effect on the PMI and the rent vs. buy result too.
4. Avoid new debt before applying. A new car loan during the process can change the ratio, and in some cases the loan itself, if the ratio is re-checked before closing.
5. Plan the order of payoff. If several debts could be cleared, the debt payoff calculator shows how to finish them fastest.

What DTI does not tell you

A ratio that qualifies is not the same as a payment you can live with. DTI uses gross income, ignores taxes, childcare, health costs and savings goals, and may leave out maintenance. Many people choose a lower ratio than the maximum for that reason. Use the debt-to-income calculator to see your own front-end and back-end numbers, then compare the payment against your actual take-home budget.

Methodology and sources

Front-end ratio = housing payment ÷ gross monthly income. Back-end ratio = (housing payment + other monthly debt payments) ÷ gross monthly income. The loan sizes in the example use the standard fixed-payment formula at 6.5% over 360 months, where each $1 of monthly principal and interest supports about $158 of loan.

Sources: CFPB, What is a debt-to-income ratio? (definition and the $2,000 ÷ $6,000 = 33% example); Fannie Mae Selling Guide, B3-6-02 Debt-to-Income Ratios (36%, up to 45% and 50% maximums and exceptions). Statements about which items lenders usually count are general descriptions and not quotes from those pages, so confirm them with your lender.

Limitations and assumptions

This is general education and not lending or financial advice. Underwriting depends on the loan program, your credit, your reserves and the lender's own overlays. The examples are illustrations and not an approval estimate.