Advanced Financial Management · International investment and financing decisions

Purchasing Power Parity and Interest Rate Parity in AFM

Updated 11 October 2026 · Fact-checked

Purchasing power parity (PPP) forecasts a future spot rate from expected inflation differences between two countries. Interest rate parity (IRP) gives the forward rate from interest rate differences. Multiply the current rate by the ratio of (1 + the rate in the quoted currency) to (1 + the rate in the base currency).

Understand Foreign Exchange Rate Forecasting and Parity Theories

A spot rate is the price today of one currency in terms of another. A forward rate is a rate fixed today for exchange on a future date. Both parity theories explain how these should relate to price levels and interest rates in the two countries.

Purchasing power parity says exchange rates move to offset differences in inflation. If prices rise faster in one country, its currency should weaken by about the same proportion. PPP is used to forecast the future spot rate. It is a long-run idea and often fails over short periods.

Interest rate parity says the forward rate is set so that you cannot make a risk-free profit by borrowing in one currency, converting, investing in the other and locking in the forward rate. The country with the higher interest rate has the weaker forward currency. IRP is used to calculate the forward rate. Banks quote forwards on this basis, so it holds closely in practice.

The international Fisher effect links the two sides. It says the difference in nominal interest rates between two countries reflects the expected difference in inflation. Together with PPP, it implies the expected future spot rate is close to the forward rate.

The key difference: PPP uses inflation and forecasts a spot rate. IRP uses interest rates and calculates a forward rate. In AFM you use these rates to convert foreign cash flows into your home currency for project appraisal and hedging decisions.

Key rules to remember

Purchasing power parity (forecast spot): S1 = S0 × (1 + hc) ÷ (1 + hb); S0 and S1 are quoted as units of currency c per 1 unit of currency b (the base). hc is the inflation rate in the quoted currency country and hb is the inflation rate in the base currency country. Check which currency is the base before you start.
Interest rate parity (forward rate): F0 = S0 × (1 + ic) ÷ (1 + ib); Same quotation rule as PPP. ic and ib are the nominal interest rates for the period to the forward date, not necessarily annual rates. Adjust annual rates for periods under a year.
Interest rate for part of a year: Period rate = annual rate × months ÷ 12; Use the simple pro-rata rate unless the question gives compound rates or tells you otherwise.
International Fisher effect: (1 + ic) ÷ (1 + ib) = (1 + hc) ÷ (1 + hb); Links interest rate differences to expected inflation differences. You can use it to find an unknown inflation rate from interest rates, or the reverse.
Fisher effect (single country): (1 + i) = (1 + r) × (1 + h); Nominal rate = real rate combined with inflation. Use it to separate real and money rates.
Expectations theory: Expected future spot ≈ forward rate; The forward rate is treated as an unbiased predictor of the future spot rate. This is a theory and does not always hold.

How to solve Foreign Exchange Rate Forecasting and Parity Theories questions

Use this method for any question asking you to forecast an exchange rate or convert future foreign cash flows.

1Read the quote carefully. Identify the base currency (the 1 unit) and the quoted currency (the units per 1).
2Decide which theory the question needs. Inflation rates and a forecast spot rate point to PPP. Interest rates and a forward rate point to IRP.
3Write the formula with the quoted-currency country's rate on top and the base-currency country's rate below.
4Adjust rates to the correct time period. For a forward of 6 months, use half of the annual interest rate unless told otherwise.
5Calculate the rate. For several years, apply the ratio each year to the previous year's rate, or raise the ratio to the power of the number of years if the inflation rates are constant.
6Sense-check. The higher-inflation or higher-interest country must have the weaker currency. If your answer goes the wrong way, you have inverted the ratio.
7Use the rate to convert cash flows. Divide when you are converting from the quoted currency into the base currency and multiply when going the other way. State your assumptions.

Quickest way: Higher rate goes on top, check direction

When to use it: Use this when time is short and you need a rate in a few lines.

Write the quote as 'units of A per 1 B'. A is the quoted currency and B is the base.
Put (1 + rate of A) over (1 + rate of B) and multiply by the current rate.
Use inflation for PPP and interest rates for IRP. Do nothing else differently.
Check direction: if A has the higher rate, the number of A per 1 B must rise.
For multi-year PPP, repeat the factor each year and keep working rates to at least four decimals.

Common mistakes in Foreign Exchange Rate Forecasting and Parity Theories

Inverting the ratio so the answer moves the wrong way
Students memorise the formula without linking it to the quote direction.
Fix: Always identify the base currency first. The quoted-currency rate goes on top. Then check that the higher-rate country's currency has weakened.
Using PPP to find a forward rate or IRP to forecast a spot rate
Both formulas look nearly identical and the two theories are easily confused.
Fix: Remember: inflation gives future spot (PPP), interest gives forward (IRP). Look at which rate data the question supplies.
Using the full annual interest rate for a forward of less than a year
Students rush and skip the time adjustment.
Fix: Scale the annual rate by months ÷ 12 before applying IRP, unless the question gives period rates.
Mixing real and nominal rates
Questions give nominal interest rates but some students treat them as inflation, or the reverse.
Fix: Use inflation rates for PPP and nominal interest rates for IRP. Use the Fisher effect only when you must convert between the two.
Multiplying when you should divide when converting cash flows
Students forget what the quote means.
Fix: If the rate is units of foreign currency per 1 home currency, divide foreign cash flows by it to get home currency. Do a quick check: a stronger home currency should make the converted value smaller.
Stating the forecast as certain and giving no commentary
Students focus on calculation marks only.
Fix: Add a line that parity theories are approximations. Mention that governments, speculation and trade barriers can make actual rates differ. This earns professional skills marks.

Worked examples

Example 1

The current spot rate is $1.60 per £1 (£ is the base). Expected inflation is 3% a year in the US and 5% a year in the UK. Forecast the spot rate in 3 years using PPP.

Show the solution

The quote is $ per £1, so $ is the quoted currency and £ is the base.
Annual factor = (1 + US inflation) ÷ (1 + UK inflation) = 1.03 ÷ 1.05 = 0.980952.
Raise the factor to the power 3: 0.980952³ = 0.980952 × 0.980952 = 0.962268; then × 0.980952 = 0.943939.
Spot in 3 years = 1.60 × 0.943939 = $1.5103 per £1.
Check direction: the UK has higher inflation, so the £ weakens against the $. The rate falls from 1.60 to about 1.51, which is correct.

Answer: Forecast spot rate in 3 years ≈ $1.510 per £1.

Example 2

The spot rate is ₹84.00 per $1. Annual interest rates are 7% in India and 3% in the US. Calculate the 6-month forward rate using interest rate parity.

Show the solution

The quote is ₹ per $1, so ₹ is the quoted currency and $ is the base.
Convert annual rates to 6-month rates: India = 7% × 6 ÷ 12 = 3.5%; US = 3% × 6 ÷ 12 = 1.5%.
Forward = 84.00 × 1.035 ÷ 1.015.
1.035 ÷ 1.015 = 1.019704.
Forward = 84.00 × 1.019704 = ₹85.655 per $1.
Check direction: India has the higher interest rate, so the rupee is weaker forward. The rate rises above 84.00, which is correct.

Answer: 6-month forward rate ≈ ₹85.66 per $1.

Exam tips

Write out the quote direction in one line at the top of your answer. It earns method marks even if you slip later.
State which theory you use and why, in one sentence. Examiners reward the link between the data given and the model chosen.
Show every year's forecast rate in a small list when cash flows span several years. You can then convert each year's flow cleanly.
Add a short comment on reliability. Note that PPP holds poorly in the short term and that forwards may differ from the future spot rate. Use this to support professional skills marks.
If the question gives both inflation and interest rates, check which one the requirement names. Do not use both unless asked to compare.

Practice questions from International investment and financing decisions

Foreign Exchange Rate Forecasting and Parity Theories in other exams

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

Foreign Exchange Rate Forecasting and Parity Theories: frequently asked questions

What is the difference between PPP and IRP?

PPP links exchange rate changes to differences in inflation and is used to forecast a future spot rate. IRP links the forward rate to differences in interest rates and is used to calculate a forward rate. IRP is usually accurate because arbitrage enforces it, while PPP is only a long-run tendency.

How do I know which currency goes on top in the formula?

Look at the quote. The currency that is the 'units' in 'units per 1 base' goes on top. For $1.60 per £1, the dollar rate is on top and the sterling rate is below. Then check that the higher-rate country ends up with the weaker currency.

What is the international Fisher effect?

It says that the difference in nominal interest rates between two countries equals the expected difference in inflation. In ratio form, (1 + ic) ÷ (1 + ib) = (1 + hc) ÷ (1 + hb). It lets you move between interest rate and inflation data, and explains why PPP and IRP give similar results.

Why does PPP often fail in practice?

Goods are not identical across countries, and trade barriers and transport costs stop prices from equalising. Capital flows, speculation and government intervention also move rates in the short term. For this reason you should treat PPP forecasts as estimates and say so in your answer.