How Lenders Calculate DTI

Introduction

Lenders use your debt-to-income ratio (DTI) to assess whether you can afford a mortgage. Understanding how they calculate it helps you estimate your DTI before applying and prepare the right documentation. This guide explains the formula, what counts as income, what counts as debt, and the difference between front-end and back-end DTI.

See What Is DTI, How DTI Affects Mortgage Approval, and the DTI Calculator to estimate your own ratios.

The DTI Formula

The basic formula is:

Lenders add all your monthly debt obligations—including your proposed mortgage payment (principal, interest, taxes, insurance)—and divide by your gross monthly income. Gross income means income before taxes and deductions.

During underwriting, the lender verifies your income and debts using pay stubs, tax returns, bank statements, and credit reports. They may adjust amounts if they find inconsistencies. Your Loan Estimate (TRID) reflects the terms offered based on that analysis.

What Counts as Income

Lenders use gross (pre-tax) income. Typical qualifying income includes:

• W-2 wages, salary, bonuses, overtime
• Self-employment income (from tax returns, Schedule C, or alternative documentation like bank statements—see How Self-Employed Income Is Verified)
• Rental income (often with a vacancy factor; rules vary)
• Alimony, child support (if you choose to include it)
• Retirement, pension, Social Security, disability
• Investment income, dividends (if stable and documented)

Income is typically averaged (e.g., 2 years of tax returns for self-employed). Overtime and bonus income may require a 2-year history. Guidelines vary by program. See How Income Is Verified for a Mortgage.

What Counts as Debt

Monthly debt payments included in DTI:

• Housing: Your current rent or mortgage (principal, interest, taxes, insurance). For a new mortgage, the lender uses your proposed PITI payment.
• Installment debts: Car loans, student loans, personal loans—the minimum monthly payment shown on the account.
• Revolving debts: Credit cards—the minimum monthly payment, not the balance. If a card has a $0 minimum, some lenders still use a percentage of the balance.
• Other: Alimony, child support you pay, co-signed loans, and other recurring obligations.

Not included: utilities, groceries, insurance (except mortgage insurance), cell phone, etc. These are not considered debt for DTI purposes.

Front-End vs Back-End DTI

Front-end DTI (housing ratio): housing payment ÷ gross income. It measures how much of your income goes to housing alone. Many programs prefer 28% or below.

Back-end DTI: (housing + all other debts) ÷ gross income. It measures total debt burden. Lenders typically focus on back-end DTI for approval. Qualified Mortgage (QM) rules often cap back-end at 43% for certain loans, with exceptions. Conventional programs may prefer 36% or below; FHA and VA have their own guidelines.

See How DTI Affects Mortgage Approval for typical limits and compensating factors.

Example Calculation

Alex earns $8,000 gross per month. Current debts: car loan $400, credit cards $150, student loan $300. Proposed mortgage PITI: $2,200.

• Front-end DTI: $2,200 ÷ $8,000 = 27.5%
• Back-end DTI: ($2,200 + $400 + $150 + $300) ÷ $8,000 = $3,050 ÷ $8,000 = 38.1%

Many conventional programs would view this favorably. If Alex had $1,200 in other debts instead, back-end would be 42.5%—closer to typical limits.

Frequently Asked Questions

What is the DTI formula?

DTI = (Total Monthly Debt Payments ÷ Gross Monthly Income) × 100. Lenders add your housing payment (or proposed mortgage payment) plus all other monthly debts, then divide by your gross income. The result is a percentage.

What counts as income for DTI?

Lenders typically use gross (pre-tax) income: W-2 wages, salary, bonuses, overtime, self-employment income (from tax returns or alternative docs), alimony, child support (if you choose to include it), rental income, retirement/pension, Social Security, and other qualifying income. Rules vary by program.

What counts as debt for DTI?

Monthly debt payments: housing (or proposed mortgage including PITI), car loans, credit card minimum payments, student loans, personal loans, alimony or child support you pay, and other recurring obligations. Some items (utilities, groceries) are not included.

What is the difference between front-end and back-end DTI?

Front-end DTI is housing payment ÷ gross income. Back-end DTI is (housing + all other debts) ÷ gross income. Lenders typically focus on back-end DTI for approval. Many conventional programs prefer back-end of 36% or below and front-end of 28% or below.

Does DTI use gross or net income?

Lenders use gross (pre-tax) monthly income, not net (take-home) income. This is standard across most mortgage programs. Your pay stubs and tax returns show gross income.

Sources

• Consumer Financial Protection Bureau (CFPB) – Debt-to-income ratio
• Fannie Mae – Selling Guide (income and debt calculation)
• Federal Housing Finance Agency – Qualified Mortgage standards