People fret endlessly about credit scores and then get blindsided by a different number entirely: their debt-to-income ratio, or DTI. The credit score tells a lender how reliably you've paid in the past. DTI tells them something more practical: how much room is left in your monthly income for a new payment. A perfect score doesn't help if there's no room.
Two ratios, not one
Lenders actually look at two versions of DTI, and they have unglamorous names.
The front-end ratio is your housing cost divided by your gross monthly income. Gross means before tax, which trips people up because nobody thinks of their income that way. Housing cost means the full monthly payment on the home: for a mortgage that's principal, interest, tax and insurance.
The back-end ratio starts with the same housing cost and adds all your other monthly debt payments on top: car loans, student loans, credit card minimums, personal loans, that sort of thing. Then it divides the whole lot by the same gross income.
Front-end asks "can you afford the house?" Back-end asks "can you afford the house given everything else you owe?" The second question is the one that catches people out.
A worked example
Say your gross monthly income is $6,500. Your housing cost would be $1,800 a month, and you've got $850 a month going to other debt payments.
Front-end: $1,800 divided by $6,500 is 27.7%.
Back-end: $1,800 plus $850 is $2,650, and $2,650 divided by $6,500 is 40.8%.
Notice how different those two numbers are. The housing itself looks fine. It's the car payment and the credit cards that push the overall picture into questionable territory. That's the pattern I see most often: people who could comfortably afford the house they want, if only they weren't already carrying so many other payments.
If you want your own numbers without doing the division by hand, the DTI calculator works out both ratios from your income and payments.
The thresholds lenders use
The traditional benchmarks are 28% front-end and 36% back-end. Stay under both and you look like a comfortable borrower. Some programmes stretch to 43% or beyond on the back end, so exceeding 36% doesn't automatically mean rejection, but you're moving from "easy yes" towards "depends".
Our example borrower is under the 28% front-end line but over the 36% back-end line at 40.8%. Some lenders would take them, some wouldn't, and the ones that would might charge for the privilege. That's what living in the stretch zone looks like.
Here's the thing I'd want you to take from those thresholds: they're limits, not targets. A lender approving you at 43% is not a lender telling you that 43% is comfortable. It's telling you that's the most they'll risk. Their line and your line are not the same line.
What counts as debt, and what doesn't
This confuses almost everyone the first time. DTI only counts debt payments, meaning obligations you owe to a lender. Utilities don't count. Groceries don't count. Subscriptions, petrol, childcare, insurance premiums, none of it counts. You could spend every remaining dollar of your income on streaming services and takeaways and your DTI wouldn't move.
That's precisely why the thresholds aren't targets. DTI ignores huge chunks of real life. Two households with identical 36% ratios can be in wildly different shape, because DTI can't see that one of them pays for full-time childcare and the other doesn't. The ratio measures what the lender can verify, not what you actually have left at the end of the month.
Using DTI in your favour
Since lenders run this maths on you anyway, run it on yourself first. Two useful moves fall out of it.
First, if you're planning to buy, work backwards from the thresholds to a realistic price range before you fall in love with anything. The home affordability calculator does exactly this: it takes your income and existing debts and shows what house price keeps you inside sensible ratios.
Second, notice that clearing a monthly payment moves your DTI immediately. Paying off a car loan with a $400 payment does more for your borrowing power than months of tinkering with your credit score. If your back-end ratio is the problem, killing the smallest debts with the largest payments is usually the fastest fix.
Lenders obsess over DTI because it works. It's crude, it misses a lot, but it answers the only question they really care about: will this person's income actually stretch to another payment? It's worth asking yourself the same thing before they do.