Housing Affordability Index: What It Measures & How to Interpret It

I spent a Tuesday afternoon last spring sitting in a real-estate office in suburban Charlotte, watching a young couple look at their preapproval letter. They'd saved for six years, had solid jobs, and felt ready. The number on the page said they qualified to borrow $380,000. Their reaction? Deflation. Their realtor opened her laptop to a map showing homes in their price range and started scrolling through listings that felt, according to them, "wrong for the neighborhood we actually live in." That moment crystallized something I'd been hearing for years: people sense housing costs have broken away from their earnings, but they struggle to name exactly what's changed or how bad it's gotten. That's where the housing affordability index comes in.

Why Housing Affordability Matters More Than You Think

The housing affordability index is, at its core, a measuring stick. It compares what people earn to what homes cost in their market. Economists, policymakers, and real-estate professionals use it to answer one deceptively simple question: Can people in this region actually afford to buy houses here? The reason it matters is that housing costs ripple through everything else—your retirement savings, whether you can afford children, your career mobility. When housing eats 35% or 45% or 55% of a household's gross income, the math stops being abstract.

The index emerged partly because raw home prices lie. A $500,000 house sounds expensive until you realize the median income in that area is $200,000 a year. Same house, same price, feels radically different in a market where median income is $60,000. The affordability index normalizes these numbers so we can actually compare apples to apples.

The Core Formula: Breaking Down the Affordability Index

Here's the thing: the formula is almost embarrassingly straightforward, but that simplicity is both its strength and its weakness.

The basic National Association of Realtors (NAR) affordability index works like this:

Affordability Index = (Median Household Income ÷ Median Home Price) × 100

Wait, that doesn't look right. Let me correct that: actually, the standard formula is typically rearranged as:

Affordability Index = (Qualifying Income ÷ Actual Median Income) × 100

Where "qualifying income" is the annual income you'd need to afford the median home price with a standard 20% down payment, current mortgage rates, property taxes, insurance, and HOA fees (if applicable) on a 30-year fixed mortgage.

Let me walk through a concrete example. Suppose the median home price in a market is $350,000. You put 20% down ($70,000), so you're borrowing $280,000. With a 6.5% mortgage rate over 30 years, that monthly payment is roughly $1,770. Add property taxes of maybe $250 per month, homeowners insurance at $120, and your total housing payment sits around $2,140 monthly. Lenders typically want housing costs to stay below 28% of gross monthly income. So you'd need about $7,650 in gross monthly income—or roughly $91,800 annually—to qualify. That's the "qualifying income."

If the actual median household income in that market is $85,000, then:

Affordability Index = ($91,800 ÷ $85,000) × 100 = 108

That 108 index number tells you something concrete: you'd need to earn about 8% more than the median household earns in order to qualify for a mortgage on the median-priced home. It's a small gap, manageable.

Interpreting Index Scores: What Numbers 80, 100, and 150 Really Mean

Here's where it gets interesting—and where most people get confused. The number 100 is your reference point: it's supposed to mean "fair" or "at parity." But what do other values actually signal?

An index of 100: Median income and qualifying income line up. Theoretically, the average household in the market has just enough income to qualify for a mortgage on the median-priced home. Reality is slightly messier (down-payment savings, credit scores, debt matter), but 100 is your baseline.

An index above 100 (say, 140): This is the good news scenario. The qualifying income is lower than the median. Homes are relatively affordable. You don't need above-average income to buy. In fact, an index of 140 means you'd need only 71% ($100,000 ÷ $140,000) of the median income to qualify. That's breathing room.

An index below 100 (say, 75): The warning zone. You'd need to earn significantly more than the median to qualify. An index of 75 means qualifying income is 133% of median income ($100,000 ÷ 75,000 = 1.33). You'd need above-average income just to reach the baseline. In markets like San Francisco or New York, index values in the 40–60 range are not uncommon, meaning qualifying income can be 160–250% of median income. The median household can't afford the median home.

I watched this play out in real estate markets over the past few years. Markets that saw index values drop from 95 to 65 in 18 months experienced visible buyer exhaustion—open houses got smaller crowds, bidding wars cooled, listings lingered longer. Conversely, markets where the index ticked up from 45 to 85 saw renewed activity, younger buyers re-entering the market, and a sense of "maybe now it's time."

A Tale of Two Markets: How Cities Compare

The index becomes truly powerful when you use it to compare regions. Let me show you what I mean with real-world scenarios.

In early 2026, Austin, Texas had a median home price around $425,000 with median household income near $95,000. That yields a qualifying income of roughly $108,000 (ballpark), so an affordability index around 113. Not bad—above 100, suggesting homes are reasonably accessible.

Meanwhile, the Bay Area's median home price hovered near $1.3 million, with median household income around $165,000. Qualifying income for that median home would be roughly $330,000. That's an affordability index around 50. Put differently: a household earning median income in the Bay Area needs to earn 6.6 times more than their actual income to afford the median-priced home. It's not a housing market anymore; it's a housing crisis.

Here's the thing I've learned from analyzing these numbers: the index is brutally honest about regional divergence, but it's also incomplete. Austin's index of 113 sounds better than the Bay Area's 50, and it is. But Austin's median income of $95,000 doesn't stretch as far as it once did, and newcomers priced out of coastal cities are now competing for homes, driving prices upward. The index tells you where we are; it doesn't tell you how fast we're moving or where we're heading.

What the Index Misses: Real Gaps in the Data

I'll be direct: the affordability index is a crude tool wrapped in a deceptively precise number. It measures one specific thing—can you qualify for a mortgage on the median home—and ignores almost everything else that matters.

For starters, the index assumes a 20% down payment. Most first-time buyers don't have 20%. They're working with 5% or 10%, which means higher monthly payments (PMI included), higher qualification thresholds, and a different reality than what the index depicts.

Second, the index ignores credit scores, existing debt, and savings. A household at median income with $60,000 in student loans and a 620 credit score faces a very different path to homeownership than a household at the same income with no debt and a 750 credit score. The index treats them identically.

Third, it focuses on income and price in isolation. A market with an index of 100 might have excellent job growth, meaning that qualifying income is within reach for ambitious younger workers. Another market at 100 might be hollowed out, where that qualifying income exists for 15% of the population but nowhere else. The index doesn't see opportunity structure.

Finally—and this is where I found myself most skeptical—the index doesn't account for neighborhood quality, school districts, or commute times. A $350,000 home in a high-crime neighborhood with poor schools is not the same as a $350,000 home in a suburban enclave with top-rated schools, even if they're in the same market. The index counts them the same.

Putting the Index to Work: Real Decisions, Real Stakes

Despite its limits, I've seen the affordability index change real behavior. A couple I know used it to make a decision to move from the Northeast to the Southeast—they ran the numbers, saw that index values in their target region were 40 points higher, did the math on their take-home pay, and realized that buying felt possible again. They closed on a home eight months later.

Real-estate investors use it to spot markets that are overheated (low index, prices rising faster than income) versus markets that have room to run (high index, underlying demand still building). A market with an index of 120 and 12% annual price appreciation is showing different fundamentals than a market with an index of 120 and 2% appreciation—the first is demand-driven, the second is cap-rate compression, which has different risk profiles.

Policymakers use it to justify zoning reform, affordable-housing mandates, or down-payment assistance programs. When a city's affordability index slides below 60, suddenly there's urgency around "we need to build more units" or "we need to help people save for down payments."

The key is using the index as one piece of a larger puzzle. Check the trend line—is your market's index improving or deteriorating? Look at regional variations—is your neighborhood's index better or worse than the broader metro area? And always, always cross-reference with actual payment calculators, because the index gives you direction, not precision.

The Charlotte couple I mentioned at the start? They ended up buying a home in a neighborhood that was more established and less trendy than they'd initially hoped, but the affordability index for that specific area—not the metro overall, but the actual neighborhood—was 117. They could breathe. That's what the index is really for: giving you permission to stop panicking and start making decisions based on numbers, not fear.