The 28/36 rule
Lenders commonly look at two ratios. Your total housing payment — principal, interest, property tax, insurance and HOA — should stay under about 28% of gross monthly income. All debt payments together, housing included, should stay under about 36%. The calculator uses whichever limit is lower.
The "stretch" option uses 36/43, closer to the maximum many lenders will approve. Being approved for a loan isn't the same as being comfortable paying it, so most people do better near the conservative number.
Frequently asked questions
Does this include PMI?
No. With less than 20% down, expect private mortgage insurance on top, which lowers what you can afford.
What property tax rate should I use?
Look up your county's effective rate; in the US it ranges from well under 1% to over 2% of the home's value per year.
Does it include closing costs?
No. Set aside a few percent of the price for closing costs, separate from your down payment.