Rent as % of income, or (rent + debt) ≤ % of income
Many landlords and budgets use a front-end rule: rent should not exceed a share of your gross (pre-tax) income—often 25% to 33%. The 30% rule is common: if you earn $5,000 per month, cap rent at $1,500. Lower percentages (25%, 28%) leave more for other expenses and savings; 33% or 40% pushes the limit.
If you have significant monthly debt (car, student loans, cards), a rent+debt rule can be safer: for example, rent plus other debt stays under 36%, 40%, or 43% of income. The calculator then sets affordable rent = (income × that %) minus your debt. That keeps your total obligations in check.