Understanding DTI: how lenders actually calculate your debt-to-income ratio

DTI is the single biggest math problem in your mortgage application. Your credit score gets you the rate. Your down payment changes the loan structure. But DTI is what decides whether the loan happens at all — and it’s the number most buyers misunderstand.

The short version: lenders add up your fixed monthly debts (including the new mortgage) and divide by your gross monthly income. If the result is below the program’s threshold, you qualify. If it’s above, you don’t. Everything in this guide is about the inputs to that division.

Front-end vs back-end ratio

There are two DTI numbers, and lenders care about both.

Front-end ratio (also called the housing ratio) is just your proposed housing payment divided by gross monthly income. The “housing payment” here is PITI: principal, interest, taxes, insurance — plus HOA dues and PMI/MIP if applicable. Rent on your current place doesn’t count; only the new mortgage does.

Back-end ratio is PITI plus all your other monthly debt payments, divided by gross monthly income. This is the number that actually gates most loan approvals.

When you see DTI thresholds written as a pair like 28/36 or 43/50, the first number is front-end and the second is back-end.

The thresholds you’ll actually encounter

These bend a little by lender and by automated underwriting findings, but the typical ceilings:

• 28/36 — conservative. What CFPs and old-school personal finance writers recommend. You’ll have real margin to save, invest, and absorb a bad month.
• 36/43 — conventional sweet spot. Fannie Mae and Freddie Mac approve plenty of loans here without manual review.
• 43/50 — the stretch. Both DU (Fannie) and LP/LPA (Freddie) will approve up to 50% with the right compensating factors (big reserves, high credit score, low LTV, strong housing history). FHA’s TOTAL Scorecard will go higher.
• Up to 56.99% back-end — FHA max via TOTAL Scorecard. This is the practical ceiling on any major program. Above 50% the file needs strong comp factors (cash reserves, residual income, minimal payment shock vs current rent). At 56.99% you’re living paycheck-to-paycheck.

VA loans technically don’t have a hard DTI cap, but lenders use the residual income test — how much cash you have left over after paying everything — as a parallel guardrail. A high-income veteran with a 55% DTI but $3,000 of monthly residual income can still qualify.

What income counts

This is where it gets tricky. Lenders are skeptical of any income that isn’t boring and predictable.

• W-2 base salary: counts at face value. Easy.
• Bonus and commission: 2-year average required. If you earned $20k in 2024 and $40k in 2025, lenders use $30k/yr ($2,500/mo). If the trend is declining, they often use the lower year only.
• Overtime: same 2-year average rule, and only if your employer confirms it’s likely to continue.
• RSU income: counts only if there’s a 2-year vesting history AND projected continued vesting for at least 3 years past closing. Lenders typically use the lower of the 2-year average or the projected next-12-month vest at a haircut on stock price.
• Rental income: 75% of gross rents (the 75% rule) — the other 25% is lender’s assumed vacancy and maintenance. New rental with no history? Use the appraiser’s market rent schedule (Form 1007).
• Self-employment: 2-year average of net income from Schedule C or K-1, with add-backs for non-cash deductions (depreciation, amortization, business use of home, depletion). See the self-employed mortgage guide for the full breakdown.
• Social Security, pensions, disability: count, often with a “gross-up” of 25% if non-taxable.
• Child support / alimony received: counts if there’s a court order AND a 6–12 month receipt history AND it continues 3+ years past closing.

What income doesn’t count

• Gig work without a 2-year history. If you started driving for Uber 8 months ago and it’s a side hustle, it doesn’t count. If you’ve done it 2+ years AND it’s on your tax return, it counts as self-employment.
• Future income. A signed offer letter for a higher-paying job starting next month? Most lenders will use the new salary only if you start before closing and provide a first pay stub. Some allow “offer letter loans” with conditions.
• Expected raises. Even if guaranteed in writing.
• Tax refunds, one-time bonuses, inheritance. Not income for DTI purposes — though they may count as assets or reserves.
• Cash income that’s not on your tax return. If you didn’t pay tax on it, the lender pretends it doesn’t exist.

What debts count

Anything that shows up on your credit report as a minimum monthly payment, plus a few off-report items:

• Credit card minimums — the actual minimum from the statement, not what you typically pay. If your minimum is $35, the lender uses $35 even if you pay it off in full every month.
• Auto loans and leases — full monthly payment. Lease buyouts at term don’t exempt you.
• Student loans — complicated; see below.
• Personal loans, BNPL with reported terms — full monthly payment.
• Other mortgages (rentals, second homes, cosigned loans you didn’t escape).
• Alimony, child support, separate maintenance that you pay.
• Installment debts with 10 or fewer payments remaining can sometimes be excluded. If you’ve got 8 payments left on a furniture loan and the monthly payment doesn’t cause undue hardship, Fannie/ Freddie usually let you omit it. Auto leases are the exception — they always count regardless of remaining term, because you’ll re-lease at the end.
• Co-signed debts. If you co-signed a loan for your kid or parent, it counts against your DTI unless you can document 12 months of on-time payments made by the primary borrower (canceled checks or bank statements showing their account paid it).
• Business debts paid by your business. If you’re self-employed and your business pays a debt that shows on your personal credit (common with sole props), you can exclude it with 12 months of business-account payment history. This is the “business debt in borrower’s name” rule and saves a lot of self-employed deals.

What debts don’t count

• Utilities, cell phone, internet, streaming. Even if late payments hit your credit.
• Insurance premiums (health, auto, life) paid directly — homeowners and PMI are different because they’re inside PITI.
• Gym memberships, subscriptions, daycare, groceries. Yes, daycare. Even though it might be your second-largest expense.
• 401(k) loan repayments. Officially you’re paying yourself.
• Medical collections under $500 (post-2023 credit reporting changes).

The student loan trap

Deferred student loans are the single most common reason a qualifying buyer suddenly doesn’t qualify. The rules vary by loan program in ways that don’t feel intuitive:

• Conventional (Fannie Mae): use the credit-report payment, including $0 on income-driven plans (SAVE/PAYE/IBR). If $0 is reported but the loan is NOT on an IDR plan (true deferment), use 1% of balance OR a fully-amortizing payment based on documented terms. Fannie updated this in 2023 to soften the old blanket 1% rule.
• Conventional (Freddie Mac): use the credit-report payment, including $0 on IDR. If $0 is reported and the loan is in deferment or forbearance, use 0.5% of balance.
• FHA: use the actual documented payment if it fully amortizes the loan. For deferred, IDR, or any non-amortizing plan, use 0.5% of balance (this was lowered from 1% in 2021).
• VA: use the actual payment from credit report. If deferred 12+ months past closing, the loan can be excluded entirely.

A $100k balance creates a $500/mo phantom debt under FHA’s 0.5% rule even if your real IDR payment is $0. Same balance under Fannie’s post-2023 rules, with documented IDR, counts as $0. Full breakdown in the student loans guide.

A worked example

Sarah earns $130,000 W-2 ($10,833/mo gross). She has:

• Car payment: $400/mo
• Student loans on SAVE plan, $50k balance, $0 actual payment
• Credit card minimums: $150/mo

She’s pursuing a conventional loan, so her student loans count at $0/mo (Fannie counts the documented IDR payment, even when $0). Total non-housing debt: $550/mo.

At a 43% back-end DTI ceiling, her max total monthly debt is $10,833 × 0.43 = $4,658. Subtract the $550 of existing debt, leaving $4,108/mo for PITI.

Assuming property tax + insurance + HOA of about $700/mo, her max principal-and-interest is roughly $3,400/mo. At a 7% 30-year fixed rate, that supports about $510,000 of mortgage. Add a 10% down payment and she’s shopping in the $565k range.

If those same student loans were under FHA at 0.5% of balance, her DTI hit jumps by $250/mo — cutting her affordability by roughly $37,500 in purchase price. Same person, same income, same loans, different program: $37,500 difference.

Run your own version in the DTI Scenario Planner and the Affordability calculator.

How to lower your DTI before applying

Roughly in order of bang-for-buck:

• Pay off small loans entirely. Eliminating a $300/mo car loan with $4,000 left raises your borrowing power by ~$45,000. Paying $4,000 down on a $40,000 loan does almost nothing for DTI.
• Pay down credit cards to lower the minimum. This is one of the few cases where paying down debt directly lowers your monthly obligation in a measurable way.
• Refinance high-balance student loans to a longer term to lower the minimum payment. Yes, you’ll pay more interest over time — but you’ll qualify for the house.
• Add a co-borrower. A spouse, parent, or partner whose income goes into the numerator AND whose debts go into the denominator. Net effect depends on their balance sheet.
• Lower the target price. Less satisfying, but the math always works.
• Switch loan programs. If FHA’s phantom student loan payment is killing you, conventional may save the deal.

What NOT to do

• Don’t pay off old collections during application — can re-age them on your credit report and lower your score.
• Don’t open new credit lines, even at 0% APR for the new sofa. A $2,000 store card with a $50 minimum costs you ~$7,500 of borrowing power.
• Don’t close old credit cards. Doesn’t lower DTI; hurts utilization and average age.
• Don’t cosign anything for anyone. The full payment hits your DTI even if they pay it.

Common mistakes

• Assuming “income” means take-home. DTI uses gross, not net. Don’t mentally convert.
• Forgetting bonuses are averaged, not annualized. Big year followed by small year doesn’t mean lenders use the big one.
• Not knowing your credit-report minimums. Pull your tri-merge before pre-approval and add up the minimums yourself.
• Mixing up front-end and back-end. A 28% front-end and a 28% back-end are wildly different financial pictures.
• Treating the program max as a target. A 49% DTI loan is approvable but life-ruining. Aim 8–12 points lower than your ceiling.

Tools you’ll use

• DTI Scenario Planner — what if you paid off this debt?
• Affordability calculator — max house at each DTI tier
• Credit Score Impact (LLPA) — how your FICO changes the math
• Mortgage Calculator — full PITI breakdown
• Closing Cost Estimator — total cash needs

For more on how lenders verify all this, see the pre-approval guide.