Economics
Exchange Rates and Purchasing Power Parity
Quick fact
The Economist's Big Mac Index compares the price of a Big Mac across countries to estimate whether currencies are under- or over-valued relative to purchasing power parity.
Why this is interesting
You've likely seen that $10 can buy a hearty meal in some countries but just a coffee in others. Why does the money 'go further' in some places, and what does the exchange rate really tell us about value?
Read the full explanation
Understanding Exchange Rates and Purchasing Power Parity
Imagine you're planning a trip to another country. You exchange your dollars for local currency, and the exchange rate tells you how many units of that currency you get per dollar. But that rate only tells you the cost of holding currency, not what you can actually buy with it. Purchasing power parity (PPP) is a different, more practical comparison: it asks, 'How many units of currency would you need to buy the same basket of goods here as you could buy with one dollar at home?' For example, if a standard basket costs $10 in the US and 500 yen in Japan, then the PPP exchange rate would be 50 yen per dollar. If the actual market exchange rate is different, then one currency is relatively undervalued or overvalued compared to the other. PPP doesn't tell you what you'll pay at a currency kiosk; it tells you how much purchasing power you get in each country.
A deeper explanation
The underlying principle is the 'law of one price': in a world without barriers, identical goods should sell for the same price when expressed in a common currency. But because of trade costs, tariffs, and non-tradable goods, this rarely holds exactly. PPP adjusts for these differences by comparing price levels, not just currency prices. When economists compare living standards across countries, they often use GDP per capita adjusted for PPP, because using market exchange rates can distort comparisons due to volatile currency markets. Exchange rates are influenced by supply and demand for currencies, which in turn depend on trade flows, capital investment, interest rates, and speculation. Over time, PPP provides a long-run benchmark: if a currency is significantly undervalued or overvalued according to PPP, it may eventually move toward that balance as trade and investment respond. Understanding this distinction helps you interpret news about exchange rates, evaluate global economic data, and even anticipate policy decisions.