The definition, and the two versions of it
Your debt-to-income ratio is your required monthly debt payments divided by your gross monthly income — gross meaning before tax, not what lands in your account. If you earn $7,500 a month before tax and your required payments come to $2,400, your ratio is 32%.
Lenders usually look at two versions of it. The front-end ratio counts only housing: the mortgage principal and interest, property tax, homeowner's insurance, mortgage insurance and any HOA dues. The back-end ratio counts housing plus everything else with a monthly minimum — car loans, student loans, personal loans, credit card minimums, child support and alimony. When someone says "DTI" without qualifying it, they almost always mean the back-end number.
What counts, and what surprisingly does not
The rule underwriters apply is roughly: does this obligation appear on your credit report or in a court order, and will it still be there after closing? That produces some counter-intuitive results.
Counted: minimum credit card payments, even if you clear the balance every month. Auto and personal loan payments. Student loan payments — and if a loan is in deferment, most programmes still impute a payment rather than treating it as zero. Court-ordered support. The full payment on a co-signed loan, even if someone else pays it, unless you can document twelve months of someone else making the payments.
Not counted: utilities, phone bills, insurance premiums other than the ones escrowed with the mortgage, groceries, childcare, tuition paid out of pocket, income tax withholding, or 401(k) contributions. A household spending $2,000 a month on childcare and a household spending nothing look identical on this measure. That is a real limitation of the ratio, not a loophole — and it is worth remembering when a lender tells you what you can "afford".
Where the thresholds actually sit
The oldest guideline is 28/36: no more than 28% of gross income on housing, no more than 36% on total debt. It comes from a period of very different rates and house prices, and almost nobody underwrites to it today. It survives because it is a reasonable personal target, not because it is a rule.
The number people repeat most often is 43%, from the Consumer Financial Protection Bureau's Qualified Mortgage rule. That figure is out of date. In a final rule issued in December 2020, the CFPB removed the 43% DTI limit from the General QM definition and replaced it with a price-based test — broadly, how far the loan's APR sits above the average prime offer rate for a comparable loan. DTI still has to be considered and documented, but it is no longer a bright line drawn by regulation.
What actually binds now is investor and lender policy. Conventional loans sold to Fannie Mae or Freddie Mac commonly go to about 45%, and higher with strong compensating factors — significant reserves, a large down payment, a high credit score. FHA is more permissive still with those factors present. Individual lenders then impose their own overlays, which is why two lenders can look at the same file and give you different answers. Ask any lender for their limit rather than assuming the number you read online.
Why the ratio behaves the way it does
Two things about DTI catch people out.
The first is that it is driven by payments, not balances. A $30,000 car loan over 84 months hurts your ratio less than the same loan over 48 months, even though the longer term costs you far more in interest. Stretching a term to qualify is a real tactic, and a genuinely expensive one. The auto loan calculator shows the interest difference between terms.
The second is that paying a card down does nothing for your ratio until the minimum payment falls. Clearing a balance to zero and closing nothing removes the minimum entirely and is worth several points of DTI; halving a balance may barely move it. If you are trying to qualify, the highest-leverage move is usually to eliminate a small loan completely rather than to chip away at a large one.
Working out your own number
Add up the required monthly payments that would appear on your credit report, add the housing payment you are contemplating, and divide by your gross monthly income. Include the tax and insurance portion of the housing payment — leaving it out is the most common mistake, and on an escrowed loan it can be a fifth of the total.
The debt-to-income ratio calculator does the arithmetic and shows the front-end and back-end figures side by side. If you are working out what payment those ratios imply rather than testing one you already have, the home affordability calculator works backwards from your income and lets you set the ratios yourself instead of accepting a default.
A caveat worth keeping
A ratio a lender is happy with is not the same as a payment you will be happy with. Underwriting asks whether the loan is likely to be repaid; it does not ask whether you will still be able to save, replace a roof, or absorb a job change. Those are your questions, and the honest answer to them usually sits well below whatever the maximum turns out to be.