What is Purchasing Power Parity?
Purchasing power parity (PPP) is the theory that exchange rates should equalize the price of identical goods across countries. If a hamburger costs 400 yen in Japan and $4 in the US, the PPP exchange rate is 100 yen per dollar. When the actual rate is 150 yen per dollar, PPP suggests the yen is undervalued. While short-term rates deviate widely from PPP, currencies tend to converge toward PPP over very long periods.
The Big Mac Index
The Economist's Big Mac Index is the most accessible PPP measure. It compares burger prices worldwide to estimate currency over- or undervaluation. For international investors, the gap between PPP and market exchange rates provides a rough gauge of currency risk. Investing in countries whose currencies are significantly below PPP may offer additional returns if the currency appreciates toward fair value over time.
Limitations
PPP is a long-term anchor, not a short-term predictor. Exchange rates are driven by interest rate differentials, capital flows, and speculation, and can deviate from PPP for years or decades. Non-tradable services create structural gaps between PPP and market rates, especially between developed and emerging economies. Use PPP as one input among many when evaluating international investment opportunities.