Cost of Living Index Explained: What That "100 = Average" Number Actually Means
Last updated · Methodology
Every cost of living comparison you'll see uses some version of an index — typically with the national average set to 100. New York City might be 187, Memphis 89, San Francisco 195. These numbers are everywhere, but most people don't know exactly what they measure, how they're calculated, or which sources are trustworthy. This guide explains the math behind the index, the components, and how to read it accurately so you don't get tricked by misleading numbers.
What the index actually represents
A cost of living index is a single number that summarizes the relative cost of a standard "basket of goods and services" in different locations. The reference point is set to 100 — usually the US national average. Locations more expensive than average get numbers above 100; cheaper locations get numbers below 100.
An index of 130 means the location is 30 percent more expensive than the national average for the same basket. Index of 80 means 20 percent cheaper. The math is simple: divide the cost of the basket in your target location by the cost in the reference location, multiply by 100.
The challenge is the "basket" — what to include and how to weight it. Different sources use different baskets, which is why two indexes can give different numbers for the same city.
The major sources and their methodologies
Five major COL data sources, each with its own approach:
- BLS Regional Price Parities (RPP): the federal government's official measure. Based on Consumer Price Index (CPI) data with regional adjustments. National average = 100. Most authoritative; updated annually.
- Council for Community and Economic Research (C2ER): the original "100 = average" index, used by relocation services and chambers of commerce. Surveys ~300 cities quarterly. Different basket from BLS.
- Numbeo: crowd-sourced from user submissions. Largest coverage globally. Less consistent than official sources because it relies on volunteer data quality.
- NerdWallet, Bankrate, Bestplaces: aggregators that combine multiple sources. Useful for quick lookup but less precise than primary sources.
- MIT Living Wage Calculator: not exactly a COL index, but calculates a minimum income threshold to cover basic needs by region and family size. Useful complement.
For US comparisons, BLS RPP is the gold standard because it uses official federal data and consistent methodology. For global comparisons, Numbeo is broader but less reliable.
Why two indexes can disagree by 30 percent
If you look up Austin, Texas in different sources, you might see indices ranging from 95 to 115 — a 20 percent gap for the same city. The reasons:
- Different baskets. Index A includes housing-heavy weighting (40% housing, 15% transport, 15% food, etc.). Index B uses different proportions. A city with cheap housing but expensive food will rank differently in each.
- Different reference periods. Some indexes update annually, some quarterly, some monthly. Austin's housing market in early 2024 looks dramatically different from late 2024.
- Different geography. "Austin" can mean the city limits, the Austin-Round Rock MSA, or Travis County. Each has different costs.
- Different data sources. BLS uses CPI surveys; C2ER uses local price collectors; Numbeo uses crowdsourced submissions. Each has selection bias.
When comparing cities, always use the SAME source for both. Comparing BLS Austin to Numbeo San Francisco is meaningless because the methodologies differ.
What 100 actually represents
The "100 = national average" reference is a synthetic construct, not an actual city. It's calculated as a population-weighted average of all US metropolitan areas. The closest real cities to 100 are typically mid-sized inland metros: Indianapolis, Kansas City, Columbus, Nashville (varies year to year).
This means: if you currently live in Indianapolis (close to 100) and move to San Francisco (around 195), your cost of living roughly doubles. If you move from Indianapolis to Memphis (89), it drops about 11 percent.
The 100 reference is not "the cheapest possible" — there are cities below 100. It's the average. Cities can be substantially cheaper (some Mississippi and Alabama metros at 81-85) or substantially more expensive (Manhattan often above 230).
Component breakdown: what makes up the index
Most COL indexes weight categories approximately like this:
- Housing: 40-45% (largest single component)
- Transportation: 12-15%
- Food (grocery + restaurant): 12-15%
- Healthcare: 8-10%
- Utilities: 6-8%
- Goods and services: 10-15%
Notice that housing is dominant. This means a city with low housing but expensive food will look much cheaper than a city with expensive housing but cheap food, even if the total spending is similar. The index's housing weight effectively magnifies housing differences.
This is why San Francisco (high housing, moderate other costs) ranks dramatically higher than Detroit (low housing, mid-range other costs) — even though groceries and gas are roughly comparable in both.
How to use COL data correctly
Three rules:
- Compare cities using the same source. Always use BLS RPP or C2ER consistently. Never mix indexes.
- Apply the index to your actual spending pattern. A retiree (low transportation, high healthcare) and a young commuter (high transportation, low healthcare) face different real costs even at the same nominal index.
- Cross-check with rent data. Housing dominates the index, so the index is mostly a function of rent. If your specific rent number differs from the city average, your personal cost of living will differ from the index proportionally.
Use our city pages for current data and the compare tool for side-by-side analysis.
Frequently Asked Questions
What is a cost of living index?+
A single number summarizing the relative cost of goods and services in a location, with the national average set to 100. An index of 130 means the location is 30 percent more expensive than average; 80 means 20 percent cheaper.
Is BLS or Numbeo more accurate for cost of living?+
For US comparisons, BLS Regional Price Parity (RPP) is more authoritative because it uses official federal CPI data with consistent methodology. Numbeo is more useful for global comparisons because it covers far more international cities, but data quality varies based on volunteer submissions.
Why do different sources show different cost of living for the same city?+
Different baskets, different weighting (especially housing), different geographies, different data periods, and different data sources. Always use the same source when comparing two cities.
What does an index of 100 mean?+
100 represents the national average for the standard basket of goods and services. Cities at 100 cost about average; cities at 130 cost 30 percent more than average; cities at 80 cost 20 percent less. The 100 reference is a population-weighted average, not a specific city.
Why is housing such a large part of the cost of living index?+
Because housing is the largest single household expense, typically 25-40 percent of income. COL indexes weight housing at 40-45% to reflect its real impact. This means cities with cheap housing rank lower on the index even if other costs are similar.
How do I use a cost of living index for relocation?+
Compare your current city's index to the target city. If you make $100,000 in a city with index 110 and move to a city with index 90, you maintain the same purchasing power on $81,800. But always cross-check with actual rent and real local prices for your specific lifestyle.