Lenders calculate your debt-to-income ratio by using these steps:
1) Add up the amount you pay each month for debt and recurring financial obligations (such as credit cards, car loans and leases, and student loans). Don’t include your current mortgage or rental payment, or other monthly expenses that aren’t debts (such as phone and electric bills).
2) Add your projected mortgage payment to your debt total from step 1.
3) Divide that total number by your monthly pre-tax income. The resulting percentage is your debt-to-income ratio.