Debt-to-Income Formula Explained
The math behind the Debt-to-Income Calculator: the equation, the variables, and the assumptions it makes.
By Onias Derilus, Mortgage Capital · NMLS# 1859012 · Last Updated: June 2026
It calculates your debt-to-income (DTI) ratio — the share of your gross monthly income that goes to debt payments — which lenders use to judge how much mortgage you can carry.
The Formula
DTI = total monthly debt payments / gross monthly income
The front-end ratio counts only the housing payment against income. The back-end ratio adds every other monthly debt — car loans, student loans, credit-card minimums, and the new mortgage — and is the number most lenders weigh most heavily.
Income is measured gross, before taxes. Because the ratio is a percentage, lowering either side of the fraction helps: paying down debt or documenting more qualifying income both reduce DTI.
Turn Your Debt-to-Income Estimate Into a Real Pre-Approval
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Rates are illustrative only. APR and payments vary by credit score, loan amount, and market conditions. Subject to credit approval. Not a commitment to lend. NMLS# 1859012. Equal Housing Lender.