Purchasing Power Parity (PPP): The Long-Run Exchange Rate Theory Behind the Big Mac Index

Is There a Level Where the Exchange Rate "Should" Be?

Exchange Rates and Foreign Investor Flows explained why the exchange rate moves the way it does right now, through a feedback loop of capital flows. Currency-Hedged vs. Unhedged ETFs covered covered interest rate parity, the principle that sets forward FX contract prices from the gap between two countries' interest rates. Both of those are theories about why the exchange rate is moving the way it is moving right now. There's a different kind of question, though: however much an exchange rate bounces around in the short run, is there some long-run equilibrium level it's supposed to be gravitating toward? Purchasing Power Parity (PPP), systematized by the Swedish economist Gustav Cassel in the 1920s, is the theory that tries to answer exactly that. This lesson covers what PPP uses to calculate an exchange rate's "fair level," and why the theory so often — and for so long — diverges from the real exchange rate.

Starting From the Law of One Price

PPP's starting point is simple. If there were no shipping costs or tariffs, and goods could be freely bought and sold anywhere, the same good should cost the same amount everywhere once converted into a common currency — the law of one price. PPP extends this idea from a single good to an entire country's price level (the whole basket of goods and services that makes up a consumer price index). The logic runs like this: if one country's overall prices have risen faster than another's, the exchange rate between the two currencies should adjust by that same gap so that real purchasing power equalizes again. Written as a formula:

PPP exchange rate = Domestic price level ÷ Foreign price level

If Korean prices rise faster than US prices, for example, PPP theory says the won should weaken (the won-dollar rate should rise) by roughly that same gap, so that the two currencies' real purchasing power realigns. If that adjustment doesn't happen, PPP's underlying equilibrium logic implies there should still be an arbitrage incentive to buy goods cheaply in one country and sell them in the other.

The Big Mac Index: Making PPP Tangible With a Single Burger

Calculating PPP properly requires comparing an entire price index made up of hundreds of goods and services — not something anyone can eyeball. In 1986, the British magazine The Economist came up with a famously simple shortcut: the Big Mac Index. McDonald's Big Mac is sold in dozens of countries made from nearly identical ingredients and recipe, which makes it one of the few genuinely standardized products in the world — and that made it a convenient way to make PPP tangible.

The calculation is straightforward: divide Korea's Big Mac price by the US Big Mac price to get the "Big Mac PPP exchange rate," then compare that to the actual market exchange rate. Based on a January 2026 observation, Korea's Big Mac price was about 5,500 won while the US price was about $6.12. Dividing those gives roughly 5,500 ÷ 6.12 ≈ 899 won — the rate at which the won and the dollar would have equal Big-Mac-based purchasing power. The actual won-dollar rate at the time was around 1,470, which implies the won was undervalued by close to 40% relative to the Big Mac benchmark. In other words, converting money into dollars to buy a Big Mac in the US gets you noticeably less burger-buying power than spending the same amount of won on a Big Mac in Korea.

What the Big Mac Index Shows — and Doesn't

It's tempting to read a Big Mac Index gap as a forecast — "this rate should snap back toward 899" — but that's the wrong way to use it. A Big Mac's price bakes in far more than the cost of beef and a bun: store rent, labor costs, local taxes, even how the menu mix differs by country. Rent and labor are classic non-tradable goods — they can't be shipped across a border and arbitraged away — while PPP's law-of-one-price logic was built around tradable goods that can be. A single Big Mac price blends both tradable (raw ingredients) and non-tradable (rent, labor) costs together, which distorts the comparison more than a pure tradable-goods comparison would. That's exactly why institutions that take PPP seriously — the OECD, the World Bank — calculate it from a much broader basket: consumer goods, services, capital equipment, even construction projects. The Big Mac Index is best understood as an educational thermometer that makes that much more complicated exercise intuitive, not a substitute for it.

Real Effective Exchange Rate: Comparing Against Many Countries at Once

The Big Mac Index is a one-to-one comparison between the won and the dollar. But an economy like Korea's, which trades heavily with China, Japan, the eurozone, and more — not just the US — can't have its overall competitiveness judged by a single currency pair. That's where the Real Effective Exchange Rate (REER) comes in. REER takes the won's exchange rate against each major trading partner's currency, weights each by trade volume, adjusts for the price-level gap with each country, and compresses the whole thing into a single index. The Bank for International Settlements (BIS) publishes REER indices for major economies every month; a reading below its historical average (often normalized to 100) suggests the won is undervalued on average against its whole basket of trading partners, while a reading above suggests the opposite. Looking at REER alongside the plain won-dollar rate gives a fuller picture of export competitiveness than relying on the dollar rate alone, extending the earnings-translation mechanics covered in How Currency Moves Affect Corporate Earnings.

Why PPP So Often — and for So Long — Fails to Hold

PPP's biggest weakness is that it may hold up over the very long run but routinely fails over the short and medium run. Empirical research repeatedly shows real exchange rates sitting away from their PPP-implied level for years, sometimes more than a decade. There are a few reasons.

First, there's the non-tradable goods problem already mentioned: costs like rent and labor can't be arbitraged across borders, so the price convergence PPP assumes simply can't happen for a large share of what goes into any price index. Second, real-world transaction costs — tariffs, shipping — are far from the "zero cost" PPP assumes. Third, and most decisively, there's capital flow, the same force covered in The Yen Carry Trade and Currency-Hedged vs. Unhedged ETFs. The overwhelming majority of money that actually moves through FX markets today isn't trade settlement — it's capital chasing interest-rate differentials, investment returns, or risk appetite, and that capital responds far more sensitively to rate gaps and policy expectations than to price-level gaps between countries. The upshot is that short-run exchange rates are driven far more by capital flows and rate differentials than by the price-level logic PPP describes. That's why PPP is a weak tool for predicting where a rate is headed next, and a far more useful one for checking, over long stretches of decades, whether exchange rates and price-level gaps tend to move in the same direction on average.

A Numbers Check: Why the Gap Doesn't Just Close on Its Own

It's easy to assume a gap this large must eventually close. A simplified side-by-side makes clear why that assumption is risky.

Date	Korea Big Mac Price	US Big Mac Price	Big Mac PPP Rate	Actual Won-Dollar Rate	Gap
January 2024	5,500 won	$5.69	~966 won	~1,330 won	~27% undervalued
January 2026	5,500 won	$6.12	~899 won	~1,470 won	~39% undervalued

Something counterintuitive shows up here: over two years, the gap didn't close — it widened. As US prices (and Big Mac prices specifically) rose faster than Korea's, the PPP-implied "fair rate" actually dropped further, from 966 won to 899 won. Yet the real market rate moved in the opposite direction, rising instead. PPP logic would have predicted the won strengthening as the price gap widened; in practice, it did the reverse. This is consistent with the point above — whatever moved the exchange rate over this stretch (US rate policy, semiconductor-sector conditions, geopolitical risk) was a capital-flow force, not the price-level force PPP describes. That doesn't mean PPP is "wrong" so much as outweighed: other forces were simply stronger over this particular window. Academic research has repeatedly found that PPP deviations have a half-life of roughly three to five years on average, sometimes longer — a stretch of time long enough to swallow most individual investors' entire holding period.

So Why Does Anyone Still Use PPP?

A weak short-term forecasting tool isn't a useless one. When comparing real income or living standards across countries, PPP-adjusted exchange rates — not market rates — are the standard. That's why the IMF and World Bank report GDP figures on a "PPP basis" alongside market-rate figures: market exchange rates can be temporarily distorted in ways that have nothing to do with a country's actual living standards, so using market rates alone to compare GDP can inflate or deflate a country's apparent size for reasons unrelated to what its citizens can actually buy with their own currency. PPP-adjusted rates strip out that distortion and compare what each country's money can actually buy at home. For investors, PPP and REER also work as a reference point for whether a currency looks historically expensive or cheap relative to its own long-run average. Just remember the caveat from the top of this lesson: knowing a currency looks cheap by this measure is a completely different question from knowing when, or even whether, that gap will actually close.

Takeaway

• Purchasing Power Parity extends the law of one price to a country's entire price level, arguing that exchange rates should adjust by the gap in price levels between two countries to equalize real purchasing power.
• The Big Mac Index compares prices of a standardized product worldwide to make PPP tangible; as of early 2026, the won looked undervalued against the dollar by close to 40% on this measure.
• Non-tradable costs like rent and labor, real transaction costs, and above all capital flows chasing rate differentials and returns mean actual exchange rates routinely sit away from PPP-implied levels for years.
• The Real Effective Exchange Rate (REER) weights a currency's rate against many trading partners at once, giving a broader read on competitiveness than any single currency pair.
• PPP is a weak short-term forecasting tool but remains widely used for comparing real income and living standards across countries, and as a long-run reference for whether a currency looks historically cheap or expensive.

FAQ

If the Big Mac Index says a currency is undervalued, will it rise soon?

Not necessarily. The index captures a purchasing-power gap at a single point in time — it isn't a forecast of when, or whether, that gap will narrow. Undervaluation by this measure has persisted for years, and sometimes widened further, in real cases.

How is PPP different from covered interest rate parity?

Covered interest rate parity, covered in Currency-Hedged vs. Unhedged ETFs, is a short-term, near-instantaneous arbitrage relationship that sets a specific contract price (the forward FX rate) from the interest-rate gap between two countries. PPP is a much slower-moving theory about where the exchange rate should sit in the long run based on price-level differences — one that can take years to hold, or may not hold cleanly at all.

Where can I check the real effective exchange rate?

The Bank for International Settlements (BIS) publishes monthly REER indices for major economies on its website, and the Bank of Korea publishes its own calculated REER figures for the won.

⚠️ This article is for informational purposes only and is not investment advice. The Big Mac prices and exchange rates cited are illustrative figures from specific points in time. You are solely responsible for your own investment decisions.