Purchasing power parity, or PPP, is the oldest idea in currency valuation: in the long run, exchange rates should adjust so that a basket of goods costs about the same in every country when priced in a common currency. It is elegant, widely quoted and frequently misused by traders who treat it as a timing tool. It is better understood as a slow-moving anchor.
The basic logic
The "law of one price" says an identical good should sell for the same price everywhere once you convert currencies. If it did not, arbitrageurs would buy where it is cheap and sell where it is expensive until the gap closed. PPP extends this to whole baskets of goods and services.
Absolute PPP says the exchange rate equals the ratio of price levels. Relative PPP is the more practical version: over time, a currency should depreciate by roughly the difference in inflation between two countries. If country A has 5% annual inflation and country B has 2%, A's currency should weaken about 3% a year against B's, all else equal.
Illustrative arithmetic
Imagine, purely for illustration, that a basket costs 100 units in country A and 80 units of country B's currency in country B, and the market rate is 1 A = 1.00 B. The basket is cheaper in B. Absolute PPP would suggest the rate should be 1 A = 0.80 B, which means A's currency looks overvalued by 25% against PPP. That gap is what analysts call misvaluation.
The Big Mac index is the famous informal version of the same calculation: a single product, a rough look at which currencies appear cheap or expensive.
Why PPP fails as a trading tool
- Not everything is tradable. Haircuts, rent and restaurant meals cannot be shipped across borders, so prices in rich countries stay structurally higher than in poorer ones. Currencies can sit far from PPP for decades.
- Baskets differ. Consumption patterns vary by country, so there is no single correct basket.
- Capital flows dominate in the short run. Interest-rate differentials, risk appetite and positioning move exchange rates far more than goods prices do over days and months. See interest rate expectations and the carry trade.
- Adjustment is slow. Academic studies commonly estimate that deviations from PPP close at a rate of only a few percent per year, implying half-lives measured in years. A currency can be "cheap" and get cheaper for a long time.
- Shocks and policy. Commodity booms, intervention and sanctions all push rates away from parity. Related reading: currency intervention and oil shocks and stagflation.
What PPP is good for
PPP is most informative at the extremes and over long horizons. If a currency has been depreciating persistently for years while its country's inflation is far higher than its trading partners', PPP explains much of the move. Likewise, a currency sitting very far above its long-run PPP estimate has historically tended to give back some of that over multi-year spans. It also helps businesses and investors think about competitiveness: an overvalued currency makes exports expensive.
For a self-directed retail trader holding positions for hours or weeks, PPP is context, not a trigger. It can inform bias for a very long-term view, and it can remind you why a high-inflation currency is structurally fragile. It cannot tell you whether next week's central bank meeting will lift or sink a pair.
Real exchange rates
You may see "real effective exchange rate" (REER) charts. These adjust a trade-weighted exchange rate for relative price levels, effectively showing deviation from a PPP-like path. A rising REER means the currency has become more expensive in real terms. Several central banks and the BIS publish such series. They are a convenient way to see long-run valuation without building your own basket.
Using it responsibly
- Use PPP to understand why a currency trends over years, not to enter trades.
- Pair it with the drivers that matter on your holding period: policy rates, data surprises, risk sentiment. See how central banks move FX and real yields.
- Remember that "undervalued" is a statement about the distant future, while your stop-loss sits close by. Size trades accordingly, using risk per trade.
- Check carry: holding a supposedly undervalued, low-yield currency may cost you rollover every night.
Takeaway
PPP gives a sensible long-run anchor: inflation differentials matter, and extreme misvaluations tend to correct eventually. But "eventually" can be years, and markets can stay far from parity far longer than a trading account can stay solvent. Use it for perspective, not for timing. This article is educational and uses illustrative numbers only.