What Is the Housing Affordability Index?

The Housing Affordability Index measures whether a family earning the median income can qualify for a mortgage on a median-priced home in a given market. A value of 100 means they exactly qualify; above 100 means more affordable, below 100 means the typical family cannot qualify without a bigger income or down payment.

When news headlines say "housing is the least affordable it's been in decades," they're usually pointing at some version of this index. The most widely cited version comes from the National Association of Realtors (NAR), published monthly, and it boils a huge amount of market data — home prices, mortgage rates, and household income — down into a single comparable number.

How the Index Is Built

The calculation compares the median family income in a region to the qualifying income needed to buy the median-priced home there, assuming a 20% down payment, a 30-year fixed mortgage at prevailing rates, and that no more than 25% of gross income goes toward principal and interest. The formula is essentially: (median family income ÷ qualifying income) × 100.

Index = (Median Family Income ÷ Qualifying Income) × 100

Worked Example

Say the median home price in a metro is $420,000. With 20% down ($84,000) and a 30-year mortgage at 6.5% on the remaining $336,000, the monthly principal and interest payment is roughly $2,124. To keep that at 25% of gross monthly income, a buyer needs to earn about $8,496/month, or $101,952/year. If the actual median family income in that metro is only $85,000, the index works out to (85,000 ÷ 101,952) × 100 ≈ 83.4 — meaning the typical family falls about 17% short of qualifying.

MetricValue
Median home price$420,000
Qualifying income needed$101,952
Actual median family income$85,000
Affordability Index83.4 (below 100 = unaffordable for the median family)

Why It Moves

The index falls when home prices rise faster than incomes, or when mortgage rates climb — a 2-point rate jump can knock 15-20% off the affordability index even with flat home prices. It's a national and regional snapshot, not a personal underwriting tool, so an individual buyer's actual qualification depends on their specific income, debts, and down payment.

Figures above are illustrative estimates only, not financial advice. Actual affordability depends on current rates, local prices, lender criteria, and personal finances — consult a mortgage professional for precise figures.

Frequently Asked Questions

What is the Housing Affordability Index in one sentence?

The Housing Affordability Index measures whether a family earning the median income can qualify for a mortgage on a median-priced home, with 100 meaning they exactly qualify.

How is the Housing Affordability Index calculated?

It compares median family income to the income required to qualify for a mortgage on the median-priced home, assuming a 20% down payment and a standard 25% of income going to principal and interest, then expresses the ratio as an index number times 100.

What does an index value below 100 mean?

A value below 100 means the median-income family does not earn enough to qualify for the median-priced home under standard lending assumptions, indicating an affordability crunch in that market.

What is a healthy Housing Affordability Index value?

Historically, values around 130-150 were considered comfortable, since NAR's national index has ranged from as high as 200+ during the 2012 housing bust to below 100 during the tightest affordability periods, like 2023-2024.

Does the index account for local cost differences?

The national index is an average and can mask huge regional variation — a metro like Austin or Miami may show far worse affordability than a national figure suggests, so NAR and other sources also publish regional and metro-level breakdowns.

How is affordability different from a debt-to-income ratio?

Debt-to-income ratio is a personal underwriting metric for one borrower's application, while the Housing Affordability Index is a market-wide statistic comparing typical income to typical home prices across an entire region or country.

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