Rules of thumb help people make fast, defensible choices when detailed planning isn’t practical. One classic housing guideline that appears across financial advice for decades is the “30% rule” — the idea that housing costs (rent or mortgage principal, interest, taxes, insurance) should ideally be no more than about 30% of gross monthly income. This rule traces back to mid-20th-century housing policy and consumer guidance and remains a quick filter for affordability, though it’s not universally optimal: high-cost cities, dual-income households, or those with other heavy obligations may need to adjust the target. Critics point out that using gross (pre-tax) income misses tax differences and that true affordability depends on many variables: local rents, commuting costs, household size, and non-housing obligations. Still, the 30% marker is useful when people need a simple starting point for budget conversations, mortgage pre-qualification, or rent searches. The next question checks your familiarity with that common housing affordability guideline.