House Affordability Calculator Formula & How It Works
- 28% rule: housing costs (PITI) ≤ 28% of gross monthly income
- 36% rule: all debt payments ≤ 36% of gross monthly income
- PITI = Principal + Interest + Taxes + Insurance
- Max loan = solve mortgage formula backwards from max monthly payment
Lenders use the 28/36 qualifying ratios to assess loan eligibility. Your maximum mortgage payment is the lower of: 28% of gross income OR (36% of gross income minus all other debt payments). From that maximum payment, work backwards using the mortgage formula to find the maximum loan amount.