The bottom line

A “cost of living index” is a single number that summarizes the relative cost of living in different places — usually with a reference city set to 100 (Numbeo uses New York; OECD PPP uses the United States). The number is useful for ballpark salary translation across cities but misleading at the household-decision level because the basket it measures rarely matches your basket: the index includes restaurant prices you may not eat at, transit costs that don't apply to drivers, rents averaged across a metro area when your specific neighborhood differs by 30–50%. Use COL indices as a sanity check, not as the primary input for relocation decisions.

What COL indices measure

The major published indices:

Numbeo — Cost of Living Plus Rent Index. Crowdsourced. Users submit prices for a basket of categories (groceries, restaurants, transport, utilities, sports, clothing, rent). Numbeo aggregates by city and produces an index with New York City = 100. The basket is fixed but city-specific weights aren't — meaning a 100-index city in the developing world isn't consuming the same basket as a 100-index city in the developed world. Numbeo is free, broad, and updated continuously, which makes it the most popular index for casual comparisons. It is also the most prone to noise from low submission counts in less-traveled cities.

Mercer — Cost of Living City Ranking. Used by HR departments to set expatriate compensation. Mercer surveys over 200 prices in each city — housing, food, transportation, household goods, clothing, recreation — weighted by an expat consumption basket (i.e., what international assignees buy). Mercer's ranking is consistent and stable but expensive (the data is sold as a B2B product).

OECD Purchasing Power Parities (PPP). Government-statistics-grade. The OECD-Eurostat PPP program runs full price surveys across thousands of items every three years and updates between cycles. PPP values let you convert from currency to currency at a real-purchasing-power exchange rate rather than at the market FX rate. PPP is what economists use; it's the most rigorous but the slowest to update and reported at country level rather than city level.

Economist Intelligence Unit — Worldwide Cost of Living. Closer in spirit to Mercer. Twice-yearly survey, ~160 cities. Often quoted in news.

Big Mac Index. A pop-PPP indicator from The Economist. Useful as a single-product sanity check on currency over- or under-valuation; not a serious COL methodology.

How indices are built

The general method:

  1. Define a basket of goods and services representative of household consumption.
  2. Survey prices in each city for each item.
  3. Convert all prices to a common currency at market FX.
  4. Compute a weighted average using the consumption basket weights.
  5. Report each city's value relative to a reference city (usually = 100).

Each step is a methodological choice. Whose basket? Whose weights? FX as of which date? How to treat housing (rent, mortgage equivalent, owner-equivalent rent)? Different organizations make different choices, which is why their rankings disagree on the margins.

Worked example: New York vs Mumbai

Numbeo (rough April 2026 figures):

  • New York City: index 100 (reference).
  • Mumbai: index ~28.
  • Tokyo: index ~55.
  • London: index ~75.

Naive read: NYC is 3.6x more expensive than Mumbai. Misleading because:

  1. Housing dominates. NYC rent is roughly 5–6x Mumbai rent in central districts. Restaurants are roughly 3x. Groceries are roughly 1.8x. Public transport is roughly 4x. The average weighted index hides huge category-level variation.

  2. Quality differences. A $20 Mumbai meal and a $50 NYC meal aren't the same meal. Cars, apartments, schools — quality differs across the basket.

  3. Income-driven habits. A middle-class American in NYC eats out more, drives more, spends on entertainment more than a middle-class Mumbaikar. The basket the index measures isn't how either actually lives.

  4. Tax differences. COL indices are typically pre-tax. NYC gross-to-net is much harsher than Mumbai gross-to-net (US federal + NY state + NYC city taxes vs Indian income tax). Tax-adjusted, the gap shrinks.

The 3.6x ratio is a defensible ballpark, not a household decision tool.

Using indices the right way

The right way to use a COL index:

  1. Sanity-check a relocation offer. “Is $120k in Austin really equivalent to $140k in San Francisco?” The Numbeo / Mercer index gives you a rough yes/no.

  2. Translate cost across major categories you care about. Don't use the headline index. Pull the rent sub-index (rent in city X / rent in city Y) for housing comparison; pull the transport sub-index if you commute.

  3. Compare cost rankings within similar income tiers. Comparing San Francisco to Mumbai for a Western salary doesn't tell you much about lifestyle; comparing San Francisco to Austin or NYC to Chicago is more directly useful because the consumption basket is similar.

  4. Combine with local salary surveys. “How much do software engineers make in Austin vs SF?” Levels.fyi, Glassdoor, BLS data. Pair the salary distribution with the COL index for a real comparison.

The wrong way to use a COL index:

  1. As the deciding factor in a move. Climate, family, career trajectory, public services, healthcare, schools all dwarf marginal COL differences for most decisions.

  2. As a precise ratio. “Salary should scale exactly 1.4x because COL is 1.4x” ignores that some costs are fixed (childcare, healthcare premiums) and others scale more than linearly with salary.

  3. For specific neighborhoods within a city. Manhattan Upper East Side and Bushwick Brooklyn are very different cost environments, both labeled NYC.

Worked example: salary translation with PPP

The infoz Salary Negotiation Calculator with PPP implements the standard PPP-style translation:

  • Offer in San Francisco: $150,000.
  • COL index SF: 165 (NYC = 100 reference).
  • COL index Austin: 95.
  • Comparison location: Austin.
  • COL ratio: 95 / 165 = 0.576.
  • PPP-equivalent in Austin: $150,000 × 0.576 = $86,400.

The calculator says: $86,400 in Austin gives you the same purchasing power as $150,000 in San Francisco, on average across the basket. That's a starting number for negotiation; reality involves your specific neighborhood, household composition, lifestyle preferences, and tax differences (Texas has no state income tax — a meaningful additional benefit).

A counter-offer band of -5% / mid / +10% gives you negotiation room around the PPP midpoint.

Common mistakes

1. Comparing crowdsourced and surveyed indices directly. Numbeo (crowdsourced) and Mercer (surveyed expat basket) won't produce the same ranking for the same cities. Pick one source and stick with it for the comparison.

2. Forgetting tax differences. A COL-equivalent dollar isn't a take-home-equivalent dollar. Use the 4-country take-home compare or build the math by hand for cross-border moves.

3. Treating “the city” as homogeneous. NYC is a 100; Park Slope is 130, Bronx is 75. Rent dominates the variation; if you're moving, the neighborhood matters more than the city.

4. Ignoring the FX year. Indices use the FX as of the snapshot date. Currency moves 5–15% per year, which can wipe out a year's worth of stated COL differences.

5. Using consumption-basket-mismatched indices. An index built for expats living in serviced apartments is not the right index for a local middle-class household.

Use the calculators

What this guide does NOT cover

  • Specific city-by-city COL breakdowns (consult Numbeo / Mercer / EIU directly)
  • BEA Regional Price Parities (US-only, US states / metros)
  • Big Mac Index methodology and limitations in detail
  • Hedonic adjustments for quality differences across countries
  • Healthcare cost translation (US vs rest of world is a separate problem)
  • Education / private school cost comparison
  • Currency hedging strategies for cross-border income

For a relocation decision involving real money, supplement COL data with local salary surveys, neighborhood-specific rent comps, and a tax estimate that reflects your actual situation.