Lenders use the 28/36 rule. Housing costs stay under 28 percent of gross monthly income, and all debt payments together stay under 36 percent. Housing here means the full payment including taxes and insurance, not just principal and interest.
What a lender will approve and what leaves you comfortable are different numbers. Approval is based on gross income, and you live on net.
Total housing cost stays at or below 28 percent of gross monthly income, and total debt payments including the mortgage stay at or below 36 percent. On $8,000 of gross monthly income that is $2,240 for housing and $2,880 for everything with a minimum payment.
It changes the price you can reach, not the monthly payment you can carry. A larger down payment also removes private mortgage insurance once you pass 20 percent equity, which is often worth more per month than the interest saved.
Lenders qualify you on gross income and only count debts that appear on a credit report. Childcare, retirement contributions, insurance and the actual cost of maintaining a house are invisible to that calculation and very visible to you.
A common planning figure is 1 to 2 percent of the home's value a year for maintenance and repairs, on top of property tax and insurance. On a $400,000 house that is $4,000 to $8,000 a year that no mortgage calculator shows you.