The short version
Your debt-to-income ratio is your monthly debt payments, including the new house payment, divided by your gross monthly income. That's it. Most loan programs want that number somewhere in the 43% to 50% range, and FHA can stretch past that on a strong file. Go over the line and the loan doesn't work, no matter how good your credit is.
In Sonoma County this matters more than most places. Prices are high, property taxes and insurance are real money, and a $600 car payment can knock $90,000 or more off what you can buy. I'll show you how the math works, what counts and what doesn't, and the moves that actually fix a DTI problem.
How do lenders calculate debt-to-income ratio?
Start with gross monthly income. That's before taxes, not your take-home pay. If you're salaried at $96,000, you're $8,000 a month. Hourly, overtime, bonus and commission get averaged, usually over two years, and they have to look likely to continue. Self-employed income is what's left after write-offs on your tax returns, which surprises a lot of business owners.
Then add up the monthly debts that show on your credit report, plus the full proposed housing payment. The housing piece isn't just principal and interest. It's PITI: principal, interest, property taxes, homeowners insurance, plus mortgage insurance and HOA dues if you have them. Lenders actually look at two ratios. The front-end ratio is housing alone divided by income. The back-end ratio is housing plus everything else. The back-end number is the one that usually decides the loan.
- Counts: car loans and leases, student loans, minimum credit-card payments, personal loans, child support and alimony, other mortgages.
- Usually doesn't count: utilities, phone, groceries, car insurance, subscriptions, childcare.
- Credit cards count at the minimum payment on your report, even if you pay in full every month.
- Installment debts with fewer than 10 payments left can sometimes be excluded on conventional loans (rules vary by program).
What's the maximum DTI by loan program?
Here's where the internet gets it wrong. You'll read '43%' everywhere like it's a law. It isn't. It's a rough benchmark. The real ceiling depends on the program and on what the automated underwriting system thinks of your whole file. Strong credit, cash reserves and a bigger down payment buy you room.
These are approximate and change with guidelines and lender overlays, so verify for your scenario. But they're the ranges I'm working with day to day.
- Conventional: up to about 50% back-end with an automated approval; manually underwritten files are tighter, closer to 36%–45%.
- FHA: the classic guide is 31% front / 43% back, but automated approvals regularly go into the mid-50s on solid files.
- VA: 41% is a guideline, not a hard cap. VA leans heavily on residual income, which is the cash left over each month after all your obligations.
- USDA: roughly 29% front / 41% back, with some flexibility on an automated approval.
- Jumbo: typically 43% or lower, sometimes up to 45%–50% with big reserves. Less forgiving across the board.
What does this look like on a real Sonoma County purchase?
Let's use a household making $150,000 combined. That's $12,500 a month gross. At a 45% back-end limit, total debts can't go past $5,625 a month.
Now say they've got a $650 truck payment, $250 in student loans and $100 in card minimums. That's $1,000 gone before the house. So the housing payment has to fit under about $4,625. In Sonoma County, property tax alone runs somewhere around 1.1% to 1.25% of the purchase price a year, and insurance in a wildfire-adjacent area can be $250 to $500 a month or more. By the time you back those out, that $4,625 doesn't buy as much principal and interest as people expect.
Here's the part I always point out. Kill the truck payment and that $650 goes straight into the housing budget. Depending on rates, that's roughly another $90,000 to $110,000 of purchasing power. One debt. I've had buyers go from 'not quite Rohnert Park' to 'comfortably Windsor' on that move alone.
How can I lower my debt-to-income ratio before I apply?
You've got two levers: less debt or more counted income. Debt is usually the faster one.
Don't just pay down whatever balance is biggest. Go after the highest monthly payment relative to what it costs to make it go away. A $300 card minimum on a $6,000 balance is a great target. Paying $15,000 off a car loan to drop a $400 payment might be smart or might drain the down payment you need. I run it both ways before anyone writes a check.
And please, don't buy a car during escrow. I've seen this go sideways more times than I can count. Buyer gets approved, gets excited, finances a new SUV, and the lender re-pulls credit right before funding. Suddenly the DTI is 53% and the deal is in trouble.
- Pay off small installment loans or cards with high minimums. Biggest payment drop per dollar wins.
- Get a co-signer's or partner's income on the loan if they're on title and have stable income.
- Document all your income: bonus, overtime, part-time work with a two-year history, rental income.
- On student loans, make sure the lender uses your actual income-driven payment when the program allows it, not an estimated percentage of the balance.
- Put more down or buy down the rate to shrink the housing payment itself.
- Freeze all new credit from pre-approval through closing.
Is a high DTI a deal-breaker?
Not always. An automated approval looks at your whole file, not just the ratio. A 50% DTI with a 760 score, six months of reserves and 20% down can sail through. The same 50% with a 640 score and no savings probably won't. Lenders call those 'compensating factors,' and they're real.
This is where shopping lenders pays. Some lenders cap FHA at 50% as an overlay even when the program would allow more. Others will take the full automated approval. Same borrower, different answer. As a broker I can see that across lenders instead of hearing one bank's 'no' and stopping there.
One honest note, though. Just because you can get approved at 52% doesn't mean you should. I tell clients to look at their take-home pay, not their gross. If the payment makes your monthly budget feel like a tightrope, it's too much house, whatever the software says.