How this instrument works
Purchasing power parity starts from the law of one price: an identical good should cost the same everywhere once you convert through the exchange rate, because arbitrage — buying where it is cheap, selling where it is dear — closes any gap. Divide the good's price in country A's currency by its price in country B's currency and the units of 'stuff' cancel out, leaving the exchange rate at which the two currencies would carry equal purchasing power for that basket. That ratio is the PPP-implied rate this instrument computes first, before comparing it against whatever rate the market actually quotes.
The comparison is the same logic behind The Economist's Big Mac Index, generalized to whatever basket you enter. Economists at the IMF and World Bank run a far larger version — the International Comparison Program prices thousands of goods and services across countries to build the official PPP figures used to compare GDP and living standards in terms that ignore exchange-rate swings. A long-horizon currency strategist, a student checking a textbook example, or a traveler wondering whether a trip abroad is cheap runs the same two-line arithmetic this page does, just with fewer goods in the basket.
A nonzero valuation percentage is not proof the exchange rate is wrong or about to move. Tariffs, shipping costs, retail markups, and services that cannot cross a border at all — a haircut, a restaurant meal — sit between the two prices this formula compares, and wages differ by country in ways that keep even a well-measured gap open for years; economists label part of that persistence the Balassa-Samuelson effect. Reading a single-good deviation as a signal for tomorrow's trade is the common misuse of this arithmetic; reading it as a snapshot of relative cost is the sound one.
- Enter Price of good, country A currency and Price of same good, country B currency — the identical basket, priced in each currency.
- Enter Actual exchange rate (A per B) — the market rate quoted for the pair today.
- Read PPP-implied exchange rate — the rate at which the two prices carry identical purchasing power.
- Check Over/undervaluation, % — positive means currency A costs more than the basket justifies, negative means less.
Worked example — an 8% overvalued currency
Take the default sheet: the same basket costs 5 units of currency A and 4 units of currency B. Dividing 5 by 4 gives an implied exchange rate of 1.25 — the rate at which both prices represent identical purchasing power. The actual market quotes 1.35 units of A per unit of B, so the valuation formula returns (1.35 − 1.25) ⁄ 1.25 × 100 = 8.0%. Currency A is trading about 8% richer than the basket alone would justify — the same reasoning The Economist applies each year in its Big Mac Index.
Hold the two prices steady and slide the actual rate down to 1.25 instead of 1.35, and the valuation reads exactly 0% — the market rate matches the basket's implied fair value with nothing left over. Push the actual rate lower still, to 1.10, and the sign flips: valuation lands at −12%, meaning currency A now looks cheap rather than dear against the identical fixed basket. The formula does not care which direction the gap runs — it only measures how far today's quote sits from what the two prices alone would predict.
Questions
What does an 8% overvaluation actually mean?
It means the market charges roughly 8% more for currency A than the two goods' prices alone justify — exchanging into A buys about 8% less of the basket than a pure basket-for-basket swap would. It says nothing about which way the rate moves next; PPP measures a price relationship at one moment, not a trading signal, and a gap like this can persist for years without closing.
How is this different from an interest-rate-parity calculator?
Interest rate parity backs out a currency's money-market rate from spot and forward FX quotes under a no-arbitrage assumption between financial markets. This instrument compares goods prices instead of interest rates — it asks whether a currency buys more or less of an identical basket than the exchange rate implies, a question about relative cost of living, not covered financial arbitrage.
Why use one good instead of a full price index?
A single good keeps the arithmetic transparent — it is the same logic behind The Economist's Big Mac Index, which prices one identical product worldwide. Real institutions such as the World Bank's International Comparison Program instead survey thousands of goods and services to build an official PPP figure; a one-good version here is a fast approximation, not a substitute for that broader survey.
Why don't exchange rates just converge to the PPP-implied rate?
Because plenty of costs sit between the two prices that this formula ignores — tariffs, shipping, retail markups, taxes, and services that never cross a border at all. Wages and productivity also differ by country, which keeps even a carefully measured gap open for years rather than closing it, so a persistent nonzero valuation percentage is ordinary rather than a sign of a mispriced rate.
Who actually relies on a comparison like this?
Economists at the IMF and World Bank use large-basket PPP to compare living standards and GDP across countries in terms unaffected by exchange-rate swings. Long-horizon currency strategists use similar comparisons to judge whether a currency looks cheap or dear over years rather than days, and students or analysts use single-good versions like this one as a quick, transparent check.
Can the valuation percentage come out negative?
Yes — a negative figure means the actual exchange rate sits below the PPP-implied rate, so currency A buys more of the basket than parity would predict, the opposite of overvaluation. The formula flips sign automatically depending on which side of the implied rate the actual quote falls; neither sign is an error, both are the same ratio read from the other direction.
References
- World Bank — PPP conversion factor data (International Comparison Program)
- Federal Reserve — H.10 Foreign Exchange Rates
Read this first: This instrument shows arithmetic, not advice. Real offers add fees, taxes and terms that vary by lender and place — verify the figures against your actual paperwork before deciding anything.