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Purchasing Power Parity, and the Famous Big Mac Index

Macro Investing • Beginner Investing • 6 min

What this lesson is about

A serious economic theory about what exchange rates "should" be. Famously, and genuinely usefully, illustrated using the price of a hamburger.

2 parts · a quick check after each · then the quiz

Part 1 of 2

Purchasing power parity (PPP) is a real economic theory. Its core idea is simple. Exchange rates should adjust so that an identical basket of goods costs about the same amount in any country, once converted into a common currency. If a product costs significantly less in one country compared to another after conversion, PPP theory says that gap should close eventually. This happens through currency adjustments, trade flows, or both. In the end, purchasing power tends to equalize.

The most famous example of this is The Economist magazine's Big Mac Index, which launched in 1986. It compares the price of a McDonald's Big Mac, a standardized product sold in similar form across many countries. By converting these prices into a common currency, it helps estimate whether a country's currency is overvalued or undervalued according to PPP. While it's presented in a lighthearted way, it illustrates a serious economic concept using a product that most people understand. That's why it has remained a popular reference point for decades.

When the currency movesMove the rate. One side of every trade is pleased and the other is not.

Quick check

What is purchasing power parity (PPP)?

Part 2 of 2

Insider Angle: Here’s the important caveat: real-world currency exchange rates often diverge from PPP's "fair value." This can happen for years. It doesn’t mean PPP theory is wrong. Rather, short and medium-term currency movements are influenced more by capital flows and interest rate differences than by equalizing goods prices. Investors moving money across borders for higher interest rates or better opportunities can push a currency's value far from what a simple goods comparison would suggest. This can last a long time, often before any PPP-driven convergence takes place, if it happens at all. PPP is useful as a long-term theoretical anchor and a sanity check, but it’s not a precise short-term trading signal.
Try This: Look up the current Big Mac Index. The Economist updates it regularly, and there are free trackers available. Find one currency that the index suggests is significantly overvalued or undervalued. Then, research what other factors, like interest rates or capital flows, might explain that gap beyond just PPP theory.

Quick check

What is The Economist magazine's "Big Mac Index"?

Quiz

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