US mortgage DTI ratio: limits and how to calculate

On $90,000 gross annual income with a $380 car loan and $290 in student loan payments already on the books, a conventional lender applying the standard 28/36 limits will approve a maximum home price of about $296,000. Qualify for a 45% back-end ratio instead, and the front-end cap takes over: the ceiling rises to $306,000, about $10,000 more in buying power. That $10,000 gap is not luck or negotiation. It is the math of two separate ratios, and understanding which one controls your situation is the difference between shopping with a real budget and getting surprised at pre-approval. This article covers how both ratios work, what the limits are by loan type, and how to use the affordability calculator to find your own ceiling.

The scenario

One household, $90,000 gross annual income, $7,500 per month before tax. Two recurring debts are already in the picture before a mortgage payment enters the calculation:

DTI, or debt-to-income ratio, is the share of your gross monthly income that goes toward debt payments. Lenders check two versions of it. Lenders run two separate tests on every application.

The front-end ratio (also called the housing ratio) divides your proposed PITI payment alone by gross monthly income. PITI is principal, interest, property tax, and insurance. On a conventional loan, most lenders cap the front-end at 28%.

The back-end ratio divides all monthly debt obligations including the new mortgage payment by gross monthly income. On a conventional loan the standard limit is 36%, though many lenders will approve up to 45% through automated underwriting when credit score, reserves, and loan-to-value are strong.

Both ratios apply independently. Whichever ceiling is lower for your specific debt load is the one that actually controls how much you can borrow.

What the limits produce on this income

Here is what each loan type's thresholds produce for $7,500 gross monthly income and $670 in existing monthly debt, at 6.75% on a 30-year loan with the calculator's default cost assumptions (1.5% property tax, 0.5% insurance). Max home prices come from the affordability calculator; binding PITI rows are the simple ratio math lenders apply before taxes and insurance are folded in.

Three things in this table are worth reading carefully.

At 28/36, the back-end binds first. The $670 in existing debt has consumed enough of the back-end allowance that the back-end ceiling ($2,030 for housing) hits before the front-end ceiling ($2,100). Your car and student loans are costing you roughly $10,000 in home purchase price compared to a borrower with no existing debt.

At 28/45, the math reverses. The back-end loosens enough that the front-end cap at 28% becomes the controlling number. You cannot claim $2,705 for housing: lenders test both ratios independently, and the housing payment alone cannot exceed $2,100 (28% of $7,500) regardless of how clean the back-end looks. The practical ceiling becomes $306,000.

FHA's 31% front-end is more permissive than conventional's 28%. With only 3.5% required down, the FHA front-end ceiling on this income is $2,325, higher than either conventional scenario. But FHA adds 1.75% upfront mortgage insurance rolled into the loan and an annual MIP on top, which the affordability calculator does not yet fold into the qualifying payment. Factor MIP in before treating the FHA row as a head-to-head win over conventional at 28/45.

Use the affordability calculator

The table above shows the ceiling for one specific scenario. Your income, debts, down payment, and rate will produce a different number. The affordability calculator below runs the same math instantly for whatever inputs you enter.

For the scenario in this article, enter:

• Annual household income: $90,000
• Monthly debt payback: $670
• Down payment: 20%
• DTI ratio: Conventional loan (28/36)
• Interest rate: 6.75%
• Leave property tax at 1.5%, insurance at 0.5%

The calculator returns a max home price of approximately $296,000 with the front-end DTI at 27.1% and back-end at 36.0%. That back-end figure is your binding constraint here: the calculator labels it "binding constraint: back-end ratio."

Switch the DTI selector to Custom with a 28% front-end and 45% back-end (or use Custom to enter 28/45) and watch the max home price climb to $306,000. The binding constraint label switches to "front-end ratio," confirming that the 28% housing cap is now the tighter test.

The monthly breakdown in the right panel also shows what that home actually costs to carry: at $296,000, the total monthly cost including property tax, insurance, and maintenance comes to roughly $2,400 per month. That number is what you need to bring into your budget conversation, not just the P&I figure.

From affordability ceiling to loan comparison

The affordability calculator tells you the maximum purchase price your income and debts can support. That is the starting point, not the destination.

Once you have a target price, the next step is comparing the loans that get you there. On a $296,000 purchase with 20% down ($236,800 loan) at 6.75% for 30 years, the mortgage comparison calculator produces:

• Monthly P&I: $1,536
• Total interest over 30 years: $316,117

Run that same loan at a 15-year term and the monthly payment rises to around $2,095, but total interest drops by roughly $175,700. Run it as a 5/1 ARM at a lower opening rate and you can quantify exactly when the fixed-rate path breaks even. The affordability calculator sets the ceiling; the comparison calculator shows the cost of every path inside it.

US lender context

The CFPB's qualified mortgage (QM) framework used a 43% back-end DTI as its standard approval threshold for many years, a figure that became a de facto benchmark across the industry even for loans not covered by QM rules. The CFPB revised the QM standard in 2021 to a price-based approach, but lender guidelines from Fannie Mae, Freddie Mac, and FHA still publish specific DTI limits as primary underwriting criteria. The 43% back-end figure remains the most commonly cited ceiling in conventional lending conversations.

Source: Consumer Financial Protection Bureau, "What is a debt-to-income ratio? Why is the 43% debt-to-income ratio important?" accessed May 2026. URL: https://www.consumerfinance.gov/ask-cfpb/what-is-a-debt-to-income-ratio-why-is-the-43-debt-to-income-ratio-important-en-1791/

When this applies, and when it doesn't

This scenario fits when:

• Your income is straightforward to document: W-2 employment or a fixed salary with two years of history
• Your non-housing debts are fixed installment payments, car loans and student loans, not high revolving credit card balances
• You are targeting a conventional or FHA loan and want a concrete ceiling before talking to a lender

It doesn't fit when:

• Your income is commission-based, self-employed, or variable. Lenders typically use a two-year average from tax returns, which can push qualifying income well below your current year's earnings.
• Your debt is primarily revolving (credit cards). DTI uses minimum payments, not balances. A $12,000 card balance with a $240 minimum counts as $240. But high utilization suppresses your credit score, which raises your rate and changes the whole calculation.
• You have strong compensating factors. Fannie Mae's Desktop Underwriter can approve DTIs above 45% when cash reserves, residual income, or a very low loan-to-value ratio offset the risk. The table above reflects published guidelines, not the outer edge of what automated underwriting will approve.
• You have no non-housing debt at all. With $0 in existing monthly obligations, the back-end and front-end limits are equivalent on a conventional loan. The full front-end ceiling applies directly, and the binding constraint shifts immediately.

The two levers for improving DTI before you apply are paying a debt to zero (which removes the minimum payment from the calculation entirely) and documenting a higher gross income. Reducing a balance without eliminating the payment does not help: DTI uses scheduled minimums, not outstanding balances.

For a monthly-cost view of a target price once you know your ceiling, see our how much house can I afford guide and PITI breakdown guide.

Find your ceiling, then model the loan

Enter your income, debts, down payment, and rate into the affordability calculator to see your own maximum home price and the ratio binding your situation. Once you have that number, open the mortgage comparison calculator with your target purchase price to see exactly what a 30-year, 15-year, or ARM path costs in interest and monthly payment over the life of the loan.