How debt-to-income ratio works

A debt-to-income ratio is one division:

$$\text{DTI} = \frac{\text{total monthly debt payments}}{\text{gross monthly income}}$$

Above the line goes every payment you are required to make each month. Below it goes gross pay, before tax and deductions. The division gives a decimal, so multiply by 100 for the percentage a lender quotes.

On this sheet the housing payment is $1,580.17, the car loan takes $450, student loans take $220 and card minimums take $150. That is $2,400.17 of monthly debt. Gross pay is $7,500. Divide and the back-end DTI is 32.00 percent.

The ratio counts payments, not balances. A card with a small minimum and a large balance barely moves DTI. Finishing a car loan takes its $450 straight off the top line. That is why the fastest way down is often clearing a debt whose payment is large relative to its remaining balance, not chipping at the biggest number on the statement.

The debt-to-income calculator on this page returns the back-end ratio, the front-end housing ratio underneath it, and how much monthly debt you could add before you reach the limit you picked.

The DTI explorer is the same ratio as a bar: drag the debts and watch them eat the ceiling.

Two different ratios share the name DTI, and most pages quote one without saying which.

The front-end ratio counts housing only: mortgage principal and interest, plus property tax, insurance and any association dues, whether or not they are escrowed with the loan payment. On this sheet that is $1,580.17 against $7,500, or 21.07 percent.

The back-end ratio counts housing plus every other required monthly payment. Same income, $2,400.17 of debt, 32.00 percent. When a lender says your DTI without qualifying it, this is the one meant, and it is the one the limits below apply to.

A rule of thumb still quoted in underwriting is 28 and 36: housing under 28 percent, everything under 36 percent. Plenty of approvals sit above both, but the pair is a useful check because it separates a housing problem from a consumer-debt problem. If the front-end ratio is comfortable and the back-end ratio is not, the mortgage is not the thing standing in the way.

How mortgage affordability works is the loan-size ceiling that sits on top of this ratio. Front-end against back-end DTI holds the two formulas on one sheet.

Counted: rent or the proposed mortgage payment with tax and insurance, car loans and leases, student loans, personal loans, card minimums, court-ordered support, and any loan you co-signed, because you are liable for it.

Not counted: utilities, groceries, phone bills, car and health insurance, tax withheld from pay, pension contributions and subscriptions. They are real costs and they are not debt service, so a lender's ratio ignores them.

Student loans in deferment are the line people leave out most often. A payment of zero today is not a payment of zero in underwriting: a lender will use the documented payment, or a set percentage of the balance when there is no payment to document. Dropping the $220 student-loan line from this sheet is the difference between a number you like and the number the lender computes.

Lenders work to a ceiling. The ceilings on this page are United States ones. Pick 36 percent for a conservative rule of thumb, 43 percent for the figure most often quoted as the edge of a mainstream mortgage, or 50 percent for what automated underwriting sometimes allows when the rest of the file is strong. Other countries set the limit somewhere else or test what you can afford a different way, so the ratio travels but the ceiling does not.

At 43 percent of $7,500 the ceiling is $3,225.00, and $2,400.17 of it is already committed, so $824.83 a month is free. That is the figure to check before signing up for anything new, because a car payment is never judged on its own: it is judged on what it does to this ratio.

A card with a $150 minimum and a large balance barely moves DTI. A car loan with a $450 payment and a small remaining balance moves it a lot. The lender is asking whether the next month's contracted payments fit inside the next month's gross pay, not how many years of debt remain.

That is why finishing the car loan takes $450 straight off the top line, and why paying a card down to a still-nonzero balance can leave the minimum, and the ratio, almost unchanged. The fastest way down is often clearing a debt whose payment is large relative to what is left on it, not chipping at the biggest number on the statement.

Co-signed loans sit on the same line. You are liable for the payment, so it counts even if somebody else has been sending it. A student loan in deferment with a payment of zero today is not a payment of zero in underwriting: a lender will use the documented payment, or a set percentage of the balance when there is no payment to document. Dropping the $220 student-loan line from this sheet is the difference between 32.00 percent and a number you like that the lender will not reproduce.

A credit score predicts the chance of a missed payment from the file. DTI measures whether the payments already contracted leave room for one more. A strong score with a 42 percent DTI and a weaker score with a 28 percent DTI are two different files. Mixing the two numbers is how a good score gets treated as spare capacity it does not grant.

The ratio also is not a budget. Utilities, groceries, tax withheld from pay and pension contributions are real costs and they do not appear above the line. A household can pass a 32.00 percent DTI and still have nothing left after those bills. How budgeting works is that leftover. This page is the contracted-payment test a lender runs, which is a narrower question.

The DTI explorer is the back-end ratio as a bar you can drag. Front-end against back-end DTI is the two names on one sheet. How mortgage affordability works is the loan-size ceiling that sits on top of this ratio.

Gross monthly income is the bottom line of this ratio: pay before tax, pension contributions and everything else deducted at source. Use what actually lands in your account and the same debts read far worse. On this sheet, $2,400.17 against $5,700 of take-home pay is 42.11 percent, while against $7,500 of gross pay it is 32.00 percent, and room left goes from $824.83 to $50.83.

Nothing about the file changed. Only the line you divided by. That gap is enough to talk someone out of an application a lender would have taken, or the other way: leaving out a student loan in deferment, a card whose balance you usually clear but which still reports a minimum, or a loan you co-signed, gives you a number the lender will not reproduce.

A credit score is a different object. It predicts the chance of a missed payment. DTI measures whether the payments already contracted leave room for one more. A strong score with a 42 percent DTI and a weaker score with a 28 percent DTI are two different files, and mixing the two numbers is how a good score gets treated as spare capacity it does not grant.

This page is the one division, the two names that share it, and the mistake of dividing by take-home pay. It is not an approval, not a budget, and not a ranking of lenders. The three sheets are $2,400.17 against $7,500 (32.00 percent, $824.83 of room), housing only at $1,580.17 (21.07 percent), and the same debts against $5,700 of take-home pay (42.11 percent). This is educational material, not financial advice.