A borrower's monthly debt-to-income ratio is calculated by taking the:

The debt-to-income (DTI) ratio is a key metric used by lenders to assess a borrower’s ability to manage monthly payments and repay a mortgage. It is calculated by dividing the borrower’s total monthly debt obligations, including:

Monthly housing expenses (principal, interest, taxes, and insurance, also known as PITI).

Any other recurring debt obligations (car loans, student loans, credit card payments, etc.).

This total is divided by the borrower’s gross monthly income (before taxes and deductions). This calculation helps determine whether the borrower meets lending standards, with most lenders preferring a DTI ratio below 43% for qualified mortgages.

References:

Fannie Mae and Freddie Mac guidelines on debt-to-income ratio

CFPB Qualified Mortgage Rules