The affordability index is an EconOS calculation, documented in our methodology notes: the mortgage payment on the median home (20% down, 30-year fixed at the prevailing rate, principal and interest only) is compared with 28% of median family income, and the ratio is scaled so 100 means exactly affordable. Historical values use each quarter’s average weekly mortgage rate and carry annual income across that year’s four quarters; quarters after the latest income vintage (2024) reuse that vintage and are revised when new data lands. The headline value mixes observation dates by necessity — quarterly prices, weekly rates, annual incomes — and each card above shows its own date.
What these measures do not capture: property taxes, homeowners insurance (a rapidly growing cost in much of the country), mortgage insurance for buyers below 20% down, maintenance, and closing costs — all excluded from the payment, so the index overstates affordability in absolute terms even as its movement over time remains informative. The median sales price is not quality-adjusted and shifts with the mix of homes sold, which is why Case-Shiller’s repeat-sales index is shown alongside it. National figures average over enormous regional variation — the affordability picture in Cleveland and coastal California are different markets entirely.
The index also describes only the buy side of housing. Renters — roughly a third of households, and disproportionately the households for whom affordability binds hardest — are covered by the rent measures on the Inflation page. And the 28% front-end convention is a lending rule of thumb, not a statement about what families can genuinely sustain at different income levels.