Purchasing Power Parity (PPP) Exchange Rate Calculator

The law of one price

Purchasing Power Parity (PPP) is built on a simple intuition: in an efficient global market, identical goods should cost the same everywhere when prices are expressed in a common currency. If an ounce of gold costs $2,000 in New York and €1,800 in Frankfurt, the implied exchange rate is €0.90 per dollar. Otherwise, you could arbitrage by buying low and selling high.

The theory dates to 16th-century Spain (Salamanca School scholars noted Spanish inflation from American silver depleting purchasing power) and was formalized by Gustav Cassel in 1918 to help post-WWI countries set realistic exchange rates.

Absolute PPP

The pure version: exchange rates should equal the ratio of price levels.

PPP exchange rate = Price level in Foreign Country ÷ Price level in Home Country

If a typical basket of goods costs $100 in the US and €85 in Germany, the PPP-implied rate is 0.85 EUR/USD. Suppose the market rate is 1.10 EUR/USD instead. At that rate the German basket costs 85 / 1.10 = $77.27 against $100 at home, so goods are cheaper in Germany and the euro is undervalued by about 23% relative to PPP. Equivalently, the dollar sits 29% above its PPP value.

Getting that direction right is the whole game, and it is easy to invert. Quote the rate as foreign units per one home unit, as this calculator does, and the rule is: a market rate higher than the PPP rate means it takes more foreign currency to buy a home unit than prices justify, so the foreign currency is the weak one, and it is undervalued.

Relative PPP

The more useful form: exchange rates should change in proportion to inflation differentials.

Quoted as foreign units per one home unit, the way this calculator takes it:

%Δ exchange rate ≈ Inflation (Foreign) − Inflation (Home)

If the US has 3% inflation and Japan has 1%, the dollar should depreciate about 2% per year against the yen, so the yen-per-dollar rate falls roughly 2% a year. This holds reasonably well as a long-run average; short-run deviations are large.

The Big Mac Index, the famous shortcut

The Economist magazine launched the Big Mac Index in 1986 as a casual PPP test. The thinking: McDonald’s sells essentially the same product (Big Mac) worldwide; comparing prices gives a quick read on currency misalignment.

Recent Big Mac Index data (2024):

The pattern: Big Macs are cheaper in lower-income countries because labor and non-tradeable inputs (rent, local food) are cheaper. Developed countries with strong currencies show “overvaluation” by this measure.

The Iced Latte Index, KFC Index, IKEA Index

Other casual PPP measures have emerged:

• Starbucks Tall Latte Index: similar pattern to Big Mac
• KFC Index (Africa-focused): designed for countries without McDonald’s
• IKEA Billy Bookshelf Index: focuses on durable goods rather than fast food
• iPad Index: more tradeable than fast food; closer to market exchange rates

These show the same broad pattern: tradeable goods (like iPads) follow PPP more closely; non-tradeable services (haircuts, rent) deviate dramatically.

Why PPP fails in the short run

Market exchange rates can sit well away from PPP for years at a time, and the reasons are not exotic.

The biggest one is that a lot of what you buy cannot cross a border. Housing, haircuts, a plumber’s hour, a restaurant meal: none of these can be arbitraged, so their prices never converge. This is the Balassa-Samuelson effect, named for two economists who worked it out independently in 1964. Rich countries are more productive in tradeable goods, that productivity pushes up wages across the whole economy, and the result is that a haircut in Zurich costs five times a haircut in Bangkok forever.

Then there is money that has nothing to do with goods. Foreign exchange markets turn over more than $7 trillion a day, and only a small slice of that is anyone buying anything. Capital chases yield and safety. The dollar has looked overvalued on PPP for most of the last two decades, and it has stayed strong anyway, because the world keeps buying Treasuries.

Tariffs, shipping and FX controls do the rest. A car is cheaper in Detroit than in Stockholm partly because Sweden is a long way away and taxes imports; the Argentine peso trades at two rates because the government says so.

The carry trade, which is PPP’s most persistent nemesis

High-interest currencies tend to appreciate even when PPP says they should fall, because investors borrow cheap and park the money where it pays. The Australian dollar did this for a decade from 2003, strengthening the whole time it was “overvalued.”

That pattern is worth internalising, because it is how PPP usually plays out: years of quiet drift away from fair value, then a violent correction over a few weeks when the trade unwinds. The 2008 crisis was the textbook version.

Where PPP earns its keep

Comparing economies. Market rates badly understate the real size of poorer countries. China’s GDP is around $18 trillion at market rates and roughly $33 trillion at PPP, which is larger than the US. The IMF and World Bank publish both, and the PPP figure is the one to use for living standards.

Comparing wages. Asking whether an Indian engineer is paid more or less than an American one is meaningless in nominal dollars and answerable at PPP.

Setting long-run expectations. A currency 50% away from PPP is more likely than not to close some of that gap over a decade. That is an anchor, not a trade.

The one case where PPP works almost perfectly

Hyperinflation. When Venezuela’s bolívar collapsed between 2017 and 2020, the market rate tracked the PPP-implied rate closely the whole way down. That is not a coincidence: when local prices are doubling every few weeks, nothing else is large enough to matter, and the currency has to fall at the same rate the price level rises. Argentina, Turkey and Zimbabwe have all run the same experiment.

Burgernomics, for the traveller

Before a trip, compare the local Big Mac price to the price at home and put the ratio against the market rate. If the market rate sits well above the burger ratio, the local currency is the cheap one and your money will stretch further than the headline number suggests. The Big Mac Index calculator does exactly this and uses the same Swiss figures as the table above.