What it means
Mortgage underwriting asks two related questions, and this ratio answers the narrower one. It looks only at what the house will cost each month, deliberately ignoring car loans, credit cards and student debt, which are captured by the back-end ratio instead.
The costs included are usually bundled under the shorthand PITI, meaning principal, interest, taxes and insurance, plus association or service charge fees where they apply. Income is measured gross, before tax, which is why the ratio always looks more comfortable than the borrower's actual bank statement feels.
It matters because it is often the first hurdle in an application. A borrower can have a spotless credit record and still be declined or asked for a larger deposit because the property they chose pushes housing costs above the lender's threshold.
The usual guideline is 28%, though this varies. Government-backed schemes may allow more, some lenders will stretch for borrowers with large reserves or very high incomes, and others apply a stress test at a higher interest rate than the one being offered, which effectively tightens the limit.
The most useful thing a buyer can do with the ratio is run it backwards. Multiply your gross monthly income by 0.28 to get the maximum housing payment a typical lender will accept, then work back to a purchase price, and you have a realistic budget before you ever visit a property.
In practice
Real-world examples.
Example
A nurse earning $6,000 a month gross applies for a loan with total housing costs of $1,560, giving a front-end ratio of $1,560 / $6,000 x 100 = 26%. She is approved comfortably even though her back-end ratio is higher because of a car loan.
Example
A self-employed designer with income averaged at $7,500 a month wants a property with housing costs of $2,400, a front-end ratio of $2,400 / $7,500 x 100 = 32%. The lender declines at that level and suggests a larger deposit to bring the monthly payment down.
Example
A dual-income couple earning $11,000 a month gross is quoted housing costs of $2,970, a ratio of $2,970 / $11,000 x 100 = 27%. They pass the front-end test, but the lender still stress tests the payment at a higher rate before issuing the offer.
Formula
Calculation
Front-End DTI = Monthly Housing Costs / Gross Monthly Income x 100
Monthly Housing Costs = Principal + Interest + Property Tax + Insurance + Association Fees
A couple earns $9,000 a month gross between them. The mortgage they are considering has principal and interest of $1,850, property tax of $350 a month, home insurance of $110 and an association fee of $90.
Monthly housing costs = $1,850 + $350 + $110 + $90 = $2,400. Front-end DTI = $2,400 / $9,000 x 100 = 26.7%.
At a 28% cap, the maximum housing payment this lender would accept is $9,000 x 0.28 = $2,520, so the couple has $2,520 - $2,400 = $120 a month of headroom. That headroom is small enough that a rise in the insurance premium or the association fee could push them over the line, which is worth knowing before they commit.Case study
Seen in the real world.
Rowanbrook Credit Union is a fictional lender used here to illustrate how the test is applied. An applicant earning $7,200 a month gross came in with a proposed housing cost of $2,300, giving a front-end ratio of $2,300 / $7,200 x 100 = 31.9%, comfortably above the 28% policy cap of $7,200 x 0.28 = $2,016.
Rather than declining outright, the loan officer showed the applicant the arithmetic and the three ways to close the gap: a larger deposit, a longer term, or a cheaper property. The applicant chose to add to the deposit, which reduced the monthly payment by $290 to $2,010.
The revised ratio was $2,010 / $7,200 x 100 = 27.9%, just inside policy, and the loan was approved. In this illustrative case the useful part was not the decision but the transparency, because the applicant left understanding precisely which number had to move and by how much.
Watch out
Common mistakes.
- Calculating the ratio on take-home pay instead of gross income, which makes the result look far worse than the one the lender will compute.
- Including only principal and interest and forgetting taxes, insurance and association fees, which can easily add several hundred dollars a month.
- Treating the lender's cap as a budget, when a payment at the maximum leaves nothing for maintenance, furniture or a change in circumstances.
Questions
People also ask.
What is the difference between front-end and back-end DTI?
Front-end counts housing costs only, while back-end adds every other recurring debt payment such as cars, cards and student loans.
Is 28% a hard rule?
No, it is a common guideline, and some schemes or lenders will go higher for borrowers with strong reserves, high income or excellent credit.
Does rent count in the front-end ratio?
Not for the property being purchased, since the calculation uses the proposed housing cost, though current rent is often used as evidence that the payment is affordable.
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