Debt-to-income ratio, explained: the two numbers your lender actually runs

The two ratios, in one minute

Every mortgage lender computes both, from the same gross (pre-tax) monthly income:

• Front-end (housing) ratio: the full housing payment — principal, interest, property tax, homeowner insurance, plus HOA dues and mortgage insurance if they apply — divided by gross monthly income.
• Back-end (total debt) ratio: that same housing payment plus car loans, student loans, personal loans, credit card minimum payments, and court-ordered support — divided by gross monthly income.

What does not count: groceries, utilities, phone bills, insurance premiums, childcare, and 401(k) loan repayments. Lenders count debts that show up as recurring obligations — mostly what is on your credit report — not your actual cost of living. That gap is why an approval can be larger than a budget can bear.

A worked example: $72,000 of income, four debts

Gross income $72,000/year = $6,000/month. The file:

• Housing (PITI + HOA): $1,650/month
• Car loan: $420/month
• Student loan: $280/month
• Credit card minimums: $90/month

The math:

• Front-end DTI: $1,650 ÷ $6,000 = 27.5% — inside the traditional 28% guideline.
• Back-end DTI: ($1,650 + $420 + $280 + $90) ÷ $6,000 = $2,440 ÷ $6,000 = 40.7% — over the traditional 36%, comfortably inside modern program limits.

This is the normal modern case, not an edge case: a borrower who passes the old front-end test, fails the old back-end test, and still qualifies — because the 28/36 rule is a guideline from the manual-underwriting era, not the ceiling automated systems use today.

The real limits, by program

• Conventional (Fannie Mae): maximum total DTI of 50% for loans submitted through Desktop Underwriter, its automated system (Selling Guide topic B3-6-02). Manually underwritten loans cap at 36%, stretching to 45% when the borrower meets additional credit-score and reserve requirements. A 50% DTI does not guarantee approval — the system weighs the whole file.
• FHA: the manual-underwriting benchmark is 31% front-end / 43% back-end. Through FHA automated underwriting (the TOTAL Mortgage Scorecard), ratios can reach 46.9% front-end and 56.9% back-end (HUD Single Family Housing Policy Handbook 4000.1).
• VA: no maximum DTI. The VA underwrites on residual income — what is left after debts and living costs — with 41% used as a guideline above which the file needs closer review.
• The 43% you still see quoted: that was the original Consumer Financial Protection Bureau Qualified Mortgage cap. The CFPB replaced the hard DTI cap for general Qualified Mortgages with a price-based test (APR relative to the average prime offer rate), mandatory for loans from October 2022 — so 43% survives as folklore and as some lenders' own overlay, not as the law of the land.

One more layer: lender overlays. Any individual lender can set a lower ceiling than the program allows. Two lenders can give the same borrower two different answers at the same DTI, and both are following the rules.

Three traps that move the number

• Deferred student loans still count. A $0 reported payment is not treated as $0 everywhere: FHA uses 0.5% of the outstanding balance as the monthly payment — $200/month on a $40,000 balance — which adds 3.3 points of back-end DTI on $6,000/month of income.
• Credit cards count at the minimum payment, not the balance. A $9,000 balance with a $90 minimum counts as $90/month. That flatters DTI — and it is exactly why our minimum-payment trap math matters more to your wallet than the ratio does to your lender.
• Gross income flatters everything. A 40.7% back-end DTI on $6,000 gross is a much bigger share of take-home pay — on roughly $4,600/month of net income, that same $2,440 of debt is about 53% of the money that actually lands. Run your real paycheck through the take-home pay calculator before treating a lender ceiling as a personal budget.

Lowering DTI: payments, not balances

DTI moves by payment ÷ gross monthly income. On $6,000/month, every $100/month of payment eliminated cuts back-end DTI by 1.7 points. In priority order:

• Kill an installment payment entirely. Paying off the $420/month car loan in the example drops back-end DTI from 40.7% to 33.7% — 7 points — even if the payoff costs several thousand dollars, because the whole payment leaves the numerator.
• Pay down cards only where the minimum falls. Card minimums are usually a percentage of the balance, so a large paydown helps some — but a small paydown that barely moves the minimum barely moves DTI.
• Do not open new debt mid-application, and ask your lender before paying off, opening, or closing anything — a payoff that drains your reserves can hurt a file that the lower DTI was supposed to help.

DTI is also the hinge inside house affordability: in our $100K salary worked example, $900/month of car and student debt cuts the maximum home price by about $38,000 — and the full salary-by-salary version is in the house affordability table.

Sources

• Fannie Mae Selling Guide, topic B3-6-02 (Debt-to-Income Ratios): 50% maximum total DTI for Desktop Underwriter casefiles; 36% manual maximum, up to 45% with credit and reserve requirements.
• U.S. Department of Housing and Urban Development, Single Family Housing Policy Handbook 4000.1 (FHA): 31%/43% manual benchmark ratios; 46.9%/56.9% maximum ratios through automated underwriting; 0.5%-of-balance payment for student loans without a reported payment.
• U.S. Department of Veterans Affairs, VA Lenders Handbook: no maximum DTI; residual-income underwriting with a 41% guideline.
• Consumer Financial Protection Bureau, Ability-to-Repay / Qualified Mortgage rule (12 CFR 1026.43, including Appendix Q): the original 43% general QM cap and its replacement with a price-based threshold.

The bottom line

Compute both ratios before a lender does: housing ÷ gross income, then everything ÷ gross income. Compare them against the program limit — 50% conventional, up to 56.9% FHA, residual income for VA — not the 28/36 guideline or the retired 43% cap. Then do the second, more honest test: the same payments against your take-home pay. The first test decides whether you can borrow; the second decides whether you should.

Test it on your numbers

Our free mortgage payoff calculator shows what your housing payment does over time and what extra principal saves, and take-home pay shows the gross-to-net gap every DTI ratio hides.

Open the Take-Home Pay Calculator →

Related research

• Credit utilization explained — per-card vs overall utilization, and why under 30% is a ceiling, not a target.
• How much house can you afford on $100K salary — the 28/36 rule worked out to a purchase price.
• How much house can you afford on any salary — the full table from $50K to $250K.
• PMI: what it costs and when it drops off — mortgage insurance is part of the housing payment in both ratios.

Related calculators

• Mortgage Payoff — how extra payments shorten your loan and cut interest.
• Take-Home Pay — what your debts really cost against your actual paycheck.
• Rent vs Buy — the full cost of buying vs. renting the same home.