What is an Inflation-Adjusted Exchange Rate?

Nominal exchange rates tell you how much currency you can buy. Real exchange rates tell you how much goods and services that currency actually purchases.

When planning cross-border investments, evaluating corporate asset histories, or analyzing historical pricing trends, relying purely on market exchange rates can lead to misleading conclusions. The reason is simple: inflation.

A currency's purchasing power declines over time. Therefore, comparing a currency pair's exchange rate from 1990 directly to today's rate fails to account for the changes in local price levels in both countries. To solve this, economists use the Real (Inflation-Adjusted) Exchange Rate.

Nominal vs. Real Exchange Rates

The Nominal Exchange Rate is the market price of one currency in terms of another. For example, if 1 USD = 35 THB today, that is the nominal rate. It tells you the immediate conversion ratio.

The Real Exchange Rate (RER) is the nominal rate adjusted for the relative price levels (CPI) between the two countries. It measures the rate at which you can trade the basket of goods of one country for a basket of goods of another.

The Calculation Formula

To adjust a historical exchange rate for modern inflation, we look at the Consumer Price Index (CPI) of both the base and target countries, normalized to a common base year. The formula is:

Real Rate = Nominal Rate × (CPI_target_current / CPI_target_past) 
                       ÷ (CPI_base_current / CPI_base_past)

A Real-World Example: 100 USD in 1990

Let's look at how inflation changes value over time.

1. USD domestic inflation:

• In 1990, the United States CPI was approximately 55.2 (on our normalized scale where 2020 = 100).
• In 2024, the US CPI rose to 121.2.
• This means 100 USD in 1990 is equivalent in domestic purchasing power to about $219.56 USD today (100 × 121.2 / 55.2).

2. Cross-Border Adjustment (THB):

What if you had converted 100 USD to Thai Baht in June 1990?

• The nominal exchange rate in June 1990 was 1 USD = 25.60 THB. Your 100 USD would have bought 2,560 THB.
• Thailand's CPI in 1990 was approximately 42.0, rising to 109.5 in 2024.
• If we calculate the real inflation-adjusted rate:
  Adjusted Rate = 25.60 × (109.5 / 42.0) ÷ (121.2 / 55.2) = 30.43 THB
• Your 100 USD from 1990 would buy the equivalent of 3,043 THB in today's terms.