Rates bounce around daily on news and sentiment — but is there any sense in which a currency has a “fair value”? The oldest answer is purchasing power parity.
The core idea
Purchasing power parity (PPP) says that, over the long run, exchange rates should drift toward the level where identical goods cost the same in different countries once you convert the prices. If a basket of goods costs $100 in the US and £80 in the UK, PPP implies GBP/USD should be around 1.25 ($100 ÷ £80). If it isn’t, the cheaper country’s currency looks undervalued.
The logic: if something is much cheaper abroad, people and businesses buy it there, demanding that currency, nudging the rate back toward parity.
The Big Mac index
The Economist turned this into a famous, half-serious gauge: the Big Mac index. A Big Mac is roughly the same product everywhere, so comparing its price across countries gives a quick read on which currencies look cheap or dear versus the dollar. It’s a teaching tool, not a trading signal.
Why it’s only a long-run anchor
PPP is useless in the short run. Rates can stray far from “fair value” for years, pushed by interest rates, capital flows, and sentiment — all of which swamp slow-moving price levels. Real goods also aren’t perfectly tradable (you can’t import a haircut). So treat PPP as a gravitational pull over decades, not a forecast for next month.
The takeaway
Purchasing power parity is the long-run idea that exchange rates should settle where the same goods cost the same across countries — the intuition behind the Big Mac index. It’s a useful anchor for “fair value” but a poor short-run guide, since rates can wander for years. (This is education, not investment advice.)