What Is Purchasing Power Parity (PPP)?
Purchasing power parity is built on the law of one price: absent transport costs and trade barriers, an identical good should sell for the same price everywhere once converted into a common currency. If a basket of goods costs $100 in the U.S. and 80 in the eurozone denominated in euros, PPP implies an exchange rate of $1.25 per euro. When the market rate deviates from that level, one currency is considered overvalued or undervalued in PPP terms.
Economists distinguish absolute PPP, which compares price levels directly, from relative PPP, which says exchange rates should move to offset differences in inflation rates over time. Relative PPP holds up better empirically: a country with persistently higher inflation tends to see its currency depreciate roughly in line with the inflation gap over long horizons.
How PPP Is Measured and Used
The most famous informal gauge is The Economist's Big Mac Index, published since 1986, which compares the price of a McDonald's Big Mac across countries as a rough single-good PPP test. Serious measurement is done by organizations such as the World Bank's International Comparison Program, which prices enormous baskets of goods and services across economies to compute PPP conversion factors.
PPP-adjusted figures change how the world economy looks. Measured at market exchange rates, the U.S. has the largest GDP, but on a PPP basis China overtook the U.S. in the mid-2010s because goods and services are cheaper in China, so each unit of output stretches further. Analysts use PPP-adjusted GDP to compare living standards and real economic size, while market exchange rates remain more relevant for cross-border financial flows.
Why PPP Matters and Where It Breaks Down
PPP works poorly in the short run. Exchange rates are driven by capital flows, interest rate differentials, and risk sentiment, and can stray from PPP fair value for years. Non-traded goods such as haircuts and rent never face international arbitrage, and the Balassa-Samuelson effect predicts that richer countries will have systematically higher price levels because of productivity differences in traded sectors. Deviations from PPP are therefore the norm rather than the exception.
Even so, PPP is a workhorse concept in finance. Macro investors use it as a long-run anchor when judging currency valuations, multinational companies use PPP-based measures for cost-of-living adjustments and market sizing, and economists rely on it whenever comparing incomes across borders. In interviews for macro, FX, or research roles, being able to explain both the logic of PPP and its limitations demonstrates genuine fluency.
