Purchasing power parity (PPP) is the idea that, once you convert the money, the same basket of goods should cost the same in any country. It is a long-run guide to whether a currency looks cheap or expensive — not a forecast of where the market exchange rate will move next.
For anyone converting a large sum — a property deposit, a business payment, a pension — PPP works as a compass, not a stopwatch. It can tell you the pound looks cheap against the dollar, but not when, or whether, the gap will close. This guide explains how PPP works, how the Big Mac index puts it in plain terms, and what it does and does not mean when you are timing a large transfer.
What is purchasing power parity?
Purchasing power parity is the exchange rate at which two currencies would buy the same quantity of goods and services in their home countries. If a basket costs £100 in the UK and the identical basket costs $130 in the United States, the PPP exchange rate is 1.30 dollars to the pound — the rate that makes the two prices equal.
It rests on the law of one price: in a market without barriers, an identical good should sell for the same price everywhere once expressed in a single currency. Extend that from one good to a whole basket and you have PPP.
The International Monetary Fund uses PPP to compare living standards and the real size of economies, because market exchange rates understate what a pound or a rupee actually buys at home. In other words, PPP is a measure of value, not a trading rate — and the live market exchange rate is a separate thing entirely.
Absolute vs relative purchasing power parity: what is the difference?
There are two versions. Absolute PPP says the exchange rate should equal the ratio of the two countries’ price levels at any moment. Relative PPP is less demanding: it says the exchange rate should move over time in line with the difference in inflation between the two countries. If UK prices rise 2% faster than US prices over a year, relative PPP expects the pound to lose roughly 2% against the dollar.
Relative PPP holds up better in the data, and it is the more useful version for a large transfer. It explains why a currency in a high-inflation economy tends to weaken over years, even though it says nothing about what next month’s rate will be. The OECD publishes official PPP conversion rates for its member countries, rebuilt from the actual prices of thousands of comparable goods and services.
What is the Big Mac index and how does it measure PPP?
The Big Mac index is a light-hearted PPP gauge published by The Economist since 1986. It compares the price of a McDonald’s Big Mac across countries to judge whether currencies are over- or undervalued against the dollar.
The logic is the law of one price applied to a single, near-identical product. Suppose a Big Mac costs £4.00 in the UK and $5.20 in the United States. The implied PPP rate is 5.20 ÷ 4.00 = 1.30 dollars to the pound. Compare that with the market rate: if the pound is trading at 1.25, sterling looks about 3.85% undervalued against the dollar on burger terms; at 1.35 it looks about 3.85% overvalued.
The index is deliberately rough — a Big Mac’s price includes local rent, wages and tax that never trade across borders — but it captures the core PPP idea in a way anyone can check. It is an illustration, not a trading signal.
Why do exchange rates rarely match purchasing power parity?
Market rates can sit far from PPP for years. Three forces pull them apart.
- Non-traded goods and services. Haircuts, rent and restaurant meals cannot be shipped, so their prices never equalise. Richer, higher-productivity economies tend to have higher price levels — the Balassa–Samuelson effect — which keeps their currencies looking “expensive” on PPP without being overvalued.
- Capital flows dwarf trade. Most daily currency turnover is investment and hedging, not buying goods. Interest-rate differentials, safe-haven demand and sentiment move rates far faster than shopping baskets do.
- Trade barriers and costs. Tariffs, transport, VAT and regulation stop the law of one price from ever fully holding.
Because of this, deviations from PPP correct only slowly — over many years, not weeks. That persistence is exactly why PPP is a long-run anchor rather than a timing tool, and why the question of whether one currency is “stronger” than another has more than one answer.
How can you tell if a currency is cheap or expensive?
No single measure is definitive. Each answers a slightly different question, and only the market rate is the one you actually transact at.
| Measure | What it compares | Who publishes it | Best used for | Predicts short-term moves? |
|---|---|---|---|---|
| Market (spot) rate | Live supply and demand for the currency | The interbank FX market | The rate you actually transact at | It is the short-term rate |
| Big Mac index | The price of one identical product across countries | The Economist | A quick, intuitive read on over- or undervaluation | No |
| OECD / IMF PPP | The price of a full basket of goods and services | OECD and IMF | Comparing living standards and real GDP | No |
| Real effective exchange rate (REER) | A currency vs a trade-weighted basket, adjusted for inflation, vs its own history | Bank for International Settlements | Judging if a currency is cheap or dear vs its own past | No |
For a currency’s value against its own history, economists watch the real effective exchange rate, or REER, published by the Bank for International Settlements. It measures a currency against a trade-weighted basket of others, adjusted for inflation. A REER well above its long-run average suggests a currency is dear; well below suggests it is cheap.
What does purchasing power parity mean for a large money transfer?
Say you need to move £400,000 to the United States. On the Big Mac maths above, the PPP rate is an illustrative 1.30, but the market rate is 1.25. At 1.25 your £400k buys $500,000; at the PPP rate of 1.30 it would buy $520,000 — a $20,000 difference. Sterling looks cheap.
That is useful context, but it changes nothing about your payment date. PPP cannot tell you whether the pound will move towards 1.30 next month, next year, or at all — deviations can persist for years. A one-cent move on £400k is worth $4,000, so the GBP/USD rate can drift a long way before any PPP gap closes.
The practical use of PPP is to shape your risk appetite, not your entry minute. If your currency looks historically cheap and your payment is months away, that may argue for keeping some flexibility while you weigh whether now is a good time to exchange. If it looks expensive and your date is fixed, a forward contract can fix the rate today and remove the guesswork. Either way, the decision to lock in or wait rests on your deadline and how much movement you can absorb — not on the burger price. For forward-looking ranges on the major pairs, our currency forecasts set out the near-term drivers.
Common mistakes when using purchasing power parity
- Treating PPP as a forecast. “The pound is undervalued, so it must rise” ignores that gaps can last years.
- Reading a single-product index as precise. The Big Mac index is an illustration of the idea, not a trading signal.
- Confusing “expensive” with “overvalued”. A country can look dear simply because it is richer and more productive (Balassa–Samuelson), not because its currency is mispriced.
- Letting PPP override your deadline. A cheap-looking currency is no help if you must pay on a fixed date and the rate moves against you first.
- Confusing PPP with the rate you receive. Your actual cost is the market rate plus any margin — always check it against the live mid-market rate before you transact.
Frequently asked questions
What is purchasing power parity in simple terms?
It is the exchange rate that would make a basket of goods cost the same in two countries. If the same basket is £100 in the UK and $130 in the US, the PPP rate is 1.30 dollars to the pound. It is a benchmark for value, not the rate you trade at.
Does purchasing power parity predict exchange rates?
Not in the short run. PPP is a long-run anchor: currencies tend to drift towards it over many years, but they can stay well above or below it for a long time because capital flows, interest rates and sentiment move the market far faster than prices of goods.
What is the Big Mac index?
It is a PPP gauge published by The Economist since 1986. It compares the local price of a Big Mac across countries to estimate whether each currency is over- or undervalued against the dollar. It is intended as an accessible illustration of purchasing power parity rather than a precise measure.
Is the pound overvalued or undervalued right now?
That changes daily and depends on which measure you use. You can compare the current market rate with the mid-market benchmark on our currency converter and live exchange rates pages, and read the near-term drivers in our currency forecasts.
How is PPP different from the market exchange rate?
The market rate is set second by second by everyone buying and selling the currency — for investment, trade and hedging. PPP is a theoretical rate based only on the relative price of goods. The two can differ by 20% or more and stay apart for years.
Should purchasing power parity affect when I make a large transfer?
It is useful context rather than a trigger. PPP can tell you whether a currency looks historically cheap or dear, which may shape how much risk you are comfortable carrying. The timing decision itself should rest on your payment date and how much of a move you can absorb — a specialist can talk you through the options for your specific US or Spanish transfer.
Speak to a specialist about your transfer
Planning a large conversion and want to understand the level you are trading at? Cambridge Currencies is a UK specialist currency broker working with FCA-authorised partners Currencycloud and ScioPay, with client funds safeguarded at a credit institution. Request a live quote and a dedicated specialist will call you back with your trading rate and the margin disclosed upfront — every transfer is completed by phone, so you always speak to a real person before you commit.
