Purchasing Power Parity
PPP exchange rates equalize the cost of identical baskets of goods across countries. By PPP measures, China's economy is larger than the US ($35T vs $29T); by market-rate measures, the US is much larger ($29T vs $19T). PPP gives more honest living-standards comparisons; market rates are right for trade and finance.
Key insights
One Big Mac, many currencies
The Economist's Big Mac index is a popular but crude PPP proxy. ICP (International Comparison Program) uses formal price surveys of ~3,000 items across 190+ countries every 6 years. Latest ICP round: 2021, published 2024. The figures recalibrate PPP exchange rates and update GDP-per-capita rankings — sometimes dramatically (Indonesia's economy is ~15% larger by ICP 2021 than ICP 2017 implied).
China crossed US in PPP terms around 2014
By PPP measures, China's economy passed the US in 2014 according to IMF estimates. In nominal terms it remains substantially smaller (~$19T vs $29T) — and the gap may not close for decades due to slow Chinese growth and dollar strength. Whether 'biggest economy' means PPP or nominal depends on the question: PPP for living standards and population-weighted comparisons; nominal for trade, finance, sovereign borrowing capacity.
Service-heavy PPP differences are large
PPP differences are largest in non-tradable services — haircuts, healthcare, housing, restaurant meals. A haircut costs $40 in Manhattan, $4 in Mumbai. These services dominate GDP composition (60-70% in most economies) but vary 10× in price. PPP rates incorporate these gaps; market rates don't. This is why China's PPP-to-nominal multiplier is ~1.9× while the US ratio is 1.0× (by definition).
GDP — PPP vs nominal (2024)
USD trillions
Key Finding: PPP and nominal rankings differ substantially for major economies, especially China and India.
Price level index — selected countries (USA=100)
ICP 2021 price level relative to USA
Key Finding: Switzerland, Norway, Iceland trade above US prices; major emerging markets at 30-60% of US prices.
Methodology & caveats
Geary-Khamis vs EKS aggregation
ICP uses Geary-Khamis aggregation for the global comparison; bilateral comparisons use EKS (Éltető-Köves-Szulc). The two methods give slightly different answers for the same underlying prices. Most published international GDP-PPP figures use Geary-Khamis. Differences typically under 5%, but matter for country rankings.
Why PPP rates are estimated, not observed
Market exchange rates are observed continuously in FX markets. PPP rates are estimated by comparing prices of comparable goods across countries. Some goods are easier to compare (Big Macs, iPhones) than others (housing, healthcare, education). ICP uses elaborate item-level matching; the residual cross-country price-level comparisons carry ±5-10% uncertainty even with rigorous methodology.
Use cases
PPP-adjusted GDP for: international comparisons of living standards, population-weighted regional aggregates, poverty thresholds ($2.15 and $3.65/day are PPP-defined). Market-rate GDP for: trade balance analysis, sovereign debt service capacity, foreign currency reserves, foreign aid comparison. World Bank classifications use both depending on the metric.