Financial Management · Causes of exchange rate differences and interest rate fluctuations

Purchasing Power Parity Formula and Forecasting Spot Rates

Updated 11 October 2026 · Fact-checked

Purchasing power parity (PPP) says exchange rates move to offset inflation differences between two countries. The currency of the higher-inflation country weakens. To forecast, multiply today's spot rate by (1 + inflation of the quoted currency's country) ÷ (1 + inflation of the base currency's country). Put the right country on top.

Understand Purchasing Power Parity

Start with a simple idea: the same basket of goods should cost the same in every country once you convert prices at the exchange rate. If it did not, traders would buy where it is cheap and sell where it is dear. That buying and selling would push the exchange rate until prices matched. This is the law of one price.

Absolute PPP applies this to price levels. It says the spot rate equals the ratio of the two countries' price levels. In practice this rarely holds. Transport costs, taxes, trade barriers and goods that cannot be traded all break it. So you will rarely calculate with absolute PPP in FM.

Relative PPP is the version you use. It says the *change* in the exchange rate over a period reflects the *difference in inflation rates*. If country A has 6% inflation and country B has 2%, A's goods get dearer faster. A's currency should lose value against B's by about the inflation gap.

The key skill is direction. The currency of the country with higher inflation depreciates. In a quote such as $/€ (dollars per one euro), the number is the price of one euro in dollars. If the US has higher inflation than the eurozone, the dollar weakens, so it takes more dollars to buy a euro and the quote rises.

PPP is a long-run theory. Over a few months, spot rates are driven by interest rates, speculation and capital flows. Exam questions still ask you to use it to forecast, and to comment that it is an imperfect predictor. PPP is closely linked to interest rate parity and the Fisher effect, which use the same structure with interest rates in place of inflation.

Key rules to remember

Relative PPP forecast of future spot rate: S1 = S0 × (1 + hc) ÷ (1 + hb); S0 = current spot, quoted as units of currency C per 1 unit of currency B. hc = inflation of the country of currency C (the quoted/top currency). hb = inflation of the country of currency B (the base/one-unit currency). Both are for the same period.
Multi-year forecast: S(n) = S0 × [(1 + hc) ÷ (1 + hb)]^n; Use when inflation is stable each year. If the rates differ by year, apply each year's ratio in turn.
Absolute PPP: S0 = Price level in country C ÷ Price level in country B; Theory only. It assumes identical goods, no trade costs and free trade. It rarely holds exactly.
Rule of direction: Higher inflation ⇒ that currency depreciates; Use this to check your answer. The top (quoted) currency's inflation sits in the numerator.

How to solve Purchasing Power Parity questions

Use this method for any PPP question, whatever the currencies or the number of years.

1Read the quote carefully. Identify the base currency (the one with 1 unit) and the quoted currency (the one with the number). For example, in 1.50 $/€ the euro is the base and the dollar is the quoted currency.
2Write down the inflation rate for each country and the time period. Convert percentages to decimals.
3Put the inflation of the quoted currency's country on top and the base currency's country on the bottom: (1 + h quoted) ÷ (1 + h base).
4Multiply the current spot rate by this ratio. For n years, raise the ratio to the power n, or apply each year's ratio in turn if inflation varies.
5Do a direction check. The higher-inflation currency must weaken. If the quoted currency has higher inflation, the quote number must rise. If the base currency has higher inflation, the number must fall.
6Answer in the same quote format as the question, with a sensible number of decimal places, and state the currency pair clearly.
7If asked, comment briefly: PPP is a long-run theory, and real rates are also affected by interest rates, speculation and government intervention.

Quickest way: Top-quoted-currency ratio

When to use it: Use in Section A and Section B objective questions where you must pick a forecast rate quickly from four options.

Find the currency with the number in the quote. Its inflation goes on top.
Compute the ratio (1 + top inflation) ÷ (1 + bottom inflation).
If the ratio is above 1, the quote rises. If below 1, it falls.
Eliminate options that move the wrong way. Then multiply and match the closest option.
For several years, compute one year's ratio, then use the power key. Do not round the ratio too early.

Common mistakes in Purchasing Power Parity

Putting the inflation rates the wrong way round.
Students memorise a formula without tying it to the quote direction.
Fix: Always put the inflation of the country whose currency is the number in the quote on top. Then do the direction check.
Subtracting inflation rates instead of using the ratio.
The approximation 'change ≈ inflation gap' is easy to remember.
Fix: Use (1 + h) ÷ (1 + h) as the exam does. Use the approximation only for a quick sense check.
Confusing PPP with interest rate parity.
Both formulas look alike and use the same ratio structure.
Fix: PPP uses inflation and forecasts the future spot rate. Interest rate parity uses interest rates and gives the forward rate. Check which data the question gives.
Forgetting to compound over several years.
Students apply a one-year ratio to a three-year question.
Fix: Raise the ratio to the power n, or apply each year's ratio in turn when inflation changes.
Saying PPP always holds or predicts short-term rates.
Students treat the formula as a law rather than a theory.
Fix: In discussion answers, say PPP tends to hold only in the long run. Mention trade barriers, non-traded goods, capital flows and speculation.

Worked examples

Example 1

The current spot rate is 1.40 $/£ (dollars per pound). Expected annual inflation is 5% in the US and 2% in the UK. Using PPP, estimate the spot rate in one year and in three years.

Show the solution

The quote is $ per £. The dollar is the quoted (top) currency, so US inflation goes on top. The pound is the base.
One-year ratio = 1.05 ÷ 1.02 = 1.029412.
One-year spot = 1.40 × 1.029412 = 1.4412 $/£.
Direction check: US inflation is higher, so the dollar weakens and it takes more dollars per pound. The rate rises. Correct.
Three-year ratio = 1.029412³. 1.029412² = 1.059689. Times 1.029412 = 1.090858.
Three-year spot = 1.40 × 1.090858 = 1.5272 $/£.

Answer: One year: about 1.4412 $/£. Three years: about 1.5272 $/£. The dollar weakens because US inflation is higher.

Example 2

A company in Country X (currency X) trades with Country Y (currency Y). The spot rate is 8.00 Y per 1 X. Inflation is expected to be 3% a year in Country X and 9% a year in Country Y. Estimate the spot rate in two years using PPP, and say which currency strengthens.

Show the solution

The quote is Y per X. Y is the quoted (top) currency, so Y's inflation goes on top. X is the base.
Annual ratio = 1.09 ÷ 1.03 = 1.058252.
Two-year ratio = 1.058252² = 1.119896.
Two-year spot = 8.00 × 1.119896 = 8.959 Y per X.
Direction check: Y has higher inflation, so Y weakens. More Y is needed per X, so the number rises. Correct.

Answer: The forecast spot rate is about 8.96 Y per 1 X. Currency X strengthens and currency Y weakens, because Y has the higher inflation.

Exam tips

Circle the quote format in the question before doing any maths. Most lost marks come from putting inflation the wrong way round.
In objective tests, use the direction check to remove two options at once. An answer that moves the wrong way is always wrong, since marking is all or nothing.
Read whether inflation is given per year or for the whole period. Do not compound a figure that is already a total.
In Section C, set out the formula, the substitution and the result in lines, so you earn method marks even if a number slips.
If the question asks you to comment, give one point on the theory (inflation differentials drive rates) and one limitation (long-run only, other influences). Link the forecast to the company's exposure.

Practice questions from Causes of exchange rate differences and interest rate fluctuations

Purchasing Power Parity in other exams

The same ground in other exams, if you are preparing for more than one or want another angle on it.

Purchasing Power Parity: frequently asked questions

What is the difference between absolute and relative PPP?

Absolute PPP says the exchange rate equals the ratio of the two countries' price levels, so the same basket costs the same everywhere. Relative PPP says the rate changes in line with the inflation difference. Relative PPP is the one used in FM forecasts, because it needs only inflation rates.

How do I calculate a future spot rate using inflation rates?

Multiply today's spot rate by (1 + inflation of the quoted currency's country) ÷ (1 + inflation of the base currency's country). For more than one year, raise the ratio to the power of the number of years. Check that the higher-inflation currency ends up weaker.

Which currency weakens under PPP?

The currency of the country with the higher inflation weakens. Its goods become dearer, so demand for the currency falls. In a quote, this means more of that currency is needed to buy one unit of the other.

Why does PPP often fail in practice?

Transport costs, taxes, trade barriers and non-traded goods stop prices equalising. Exchange rates are also moved by interest rates, capital flows, speculation and government action. PPP is best seen as a long-run tendency, not a short-term predictor.