Debt-to-income ratio, explained: how it's calculated, and why the '43% rule' isn't actually a rule anymore

"Keep your DTI under 43%" gets repeated almost as often as the 30% rule for credit utilization — and it has the same problem. It's a real number from a real regulation, quoted well past the point where the regulation actually said it.

What debt-to-income ratio actually measures

Per the CFPB, your debt-to-income ratio (DTI) is "all your monthly debt payments divided by your gross monthly income." The CFPB's own worked example: a $1,500 mortgage payment, a $100 auto loan payment, and $400 in other debt add up to $2,000 in monthly debt. Divide that by $6,000 in gross monthly income — income before taxes and other deductions — and the result is a 33% DTI.

Only debt payments count. A mortgage or rent-equivalent housing debt, auto loans, student loans, minimum credit card payments, and other loan obligations go into the numerator. Groceries, utilities, insurance premiums, and subscriptions are real monthly costs, but they aren't debt payments, so lenders don't fold them into this particular ratio.

Where the 43% number actually came from

The 43% figure isn't folklore — it was, for a while, a real legal threshold. Under the Ability-to-Repay/Qualified Mortgage rule that followed the 2008 financial crisis, a "General QM" loan originally had to keep the borrower's DTI at or below 43% to get the legal protections that come with QM status.

That changed. Per the CFPB's own final rule, issued December 10, 2020, the Bureau "removes the General QM loan definition's 43 percent DTI limit and replaces it with price-based thresholds" — meaning a loan qualifies (or doesn't) based on how its APR compares to benchmark mortgage rates, not a fixed DTI cutoff. The rule took effect March 1, 2021; per a follow-up CFPB release, the mandatory compliance date was later pushed from July 1, 2021 to October 1, 2022 to give lenders more transition time.

In other words: the specific "43% is the legal line" claim describes a rule that hasn't been the law for General QM loans since 2021. It's the same shape as the old "0% utilization is best" myth — a number that used to be precisely true, still gets repeated as if it is, past the point where the underlying rule changed.

What actually sets the limit now

Removing the nationwide 43% cap didn't remove DTI limits — it just moved them back to individual lenders and loan programs. Per Fannie Mae's Selling Guide, a loan run through its automated underwriting system (Desktop Underwriter) can qualify up to a 50% DTI. A manually underwritten loan is capped lower by default — 36% — though Fannie Mae's own guide allows that to stretch up to 45% if the borrower meets specific credit score and cash-reserve requirements on its Eligibility Matrix.

FHA, VA, USDA, and other loan programs each publish their own DTI guidelines separately, and they don't all match Fannie Mae's numbers. The practical takeaway isn't "43% doesn't matter" — plenty of lenders still land somewhere in that range — it's that there's no single DTI ceiling that applies to every loan. The number that matters is whatever the specific lender and loan program you're applying through actually uses, not a rule of thumb pulled from a regulation that changed years ago.

Why lenders look at DTI at all

The logic is straightforward: DTI is a lender's estimate of how much of a borrower's income is already spoken for before a new payment gets added. A high DTI means more of every incoming dollar is already committed to existing debt, which is exactly the kind of borrower a repayment-ability rule is designed to flag. That's also why DTI shows up specifically in loan underwriting, rather than in your credit score directly — it's a snapshot of capacity to take on a new payment, not a record of past payment behavior the way your credit report is.

What this isn't

This describes how the CFPB defines DTI and how the General QM rule's DTI requirement changed — not a guarantee about what any specific lender will approve. Loan programs, individual lenders, and compensating factors (credit score, reserves, down payment) all affect the actual number a given borrower needs to hit. Check directly with a lender or loan officer for the DTI limit that applies to a specific loan program.

Where this fits

Same pattern as the rest of this series: a widely repeated number, and a more precise answer sitting in the regulation once you actually read the current version of it. We covered the same gap for credit utilization — 30% isn't a cliff, and 0% isn't the goal — and for hard vs. soft credit inquiries, where the actual score impact is smaller and shorter-lived than assumed. DTI is the other number that shows up constantly in mortgage-qualifying conversations, and like those two, the rule of thumb everyone quotes is a rounded-off, out-of-date version of something more specific.