Interest, Inflation, and Real Returns: More Money vs. More Buying Power

1.Nominal vs. Real: Numbers vs. Purchasing Power

In Lesson 1 we touched on inflation briefly while discussing the time value of money. This lesson tackles it head-on. Values stated as amounts of money, such as the balance in your account or a deposit's interest rate, are called nominal values. Values converted into how many goods and services that money can actually buy, that is, into purchasing power, are called real values.

As an example, if the money in your account grew 3% over a year while grocery prices also rose 3%, the number went up but what you can buy stayed the same. If prices rose 4%, the number went up yet you can buy less. So when you plan to save or grow money, ask not only "how much will it be?" but also "what will that money buy then?"

How much prices have risen is usually measured by the growth rate of the consumer price index (CPI). In Korea it is published by the National Data Office (formerly Statistics Korea). Keep in mind, though, that the CPI is an average across many items, so it can differ from the price changes of the things you buy most often (Lesson 10).

2.Real Return: The Approximate and Exact Formulas

To estimate real return, people often subtract the inflation rate from the nominal return. This is called the Fisher approximation: with a nominal return of 5% and inflation of 3%, the real return is about 2%. When both inflation and interest rates are low, this estimate is good enough.

The exact formula uses division, not subtraction. Your money grows by a factor of (1 + nominal) and prices by a factor of (1 + inflation), so what you can buy changes by a factor of (1 + nominal) ÷ (1 + inflation). It is the same idea as turning a series of changes into multipliers in Math Lesson 2. As the table below shows, the gap between the approximate and exact formulas grows as the numbers get bigger, so use the exact formula when inflation is high or the numbers are large.

3.After-Tax Return: Take Out Taxes First

Deposit interest is taxed. In Korea (as of 2026), interest and dividend income is usually subject to 15.4% withholding (14% income tax + 1.4% local income tax). Withholding means the financial institution deducts the tax in advance when it pays the interest and gives you the rest. How larger amounts of investment income are taxed is covered in Lesson 5. Tax laws and rules can change, so check official guidance such as the National Tax Service.

That is why the return you actually take home is lower than the advertised rate. After-tax interest is interest × (1 − tax rate), and the after-tax return shrinks the same way. To see the real return accurately, order matters: first take out taxes to get the after-tax nominal return, then divide by inflation to get the after-tax real return.

4.Purchasing Power: How Fast Inflation Shrinks Money

When prices rise, the same money buys less. This has the same shape as the present value formula from Lesson 1. Just as you divided future money by the discount rate, dividing money at some future point by how much prices have multiplied in the meantime gives its purchasing power in today's terms.

The rule of 72 works here too. As an example, if prices rise 3% every year, then after 72 ÷ 3 = 24 years prices roughly double, and the purchasing power of the same money roughly halves. In fact, 1.03^24 ≈ 2.03. Even an inflation rate that looks small makes a big difference when it builds up over a long time.

5.How Interest Rates and Inflation Are Linked

Deposit and loan rates differ by institution and product, but they share a reference point. In Korea, the Monetary Policy Board of the Bank of Korea sets the base rate (meeting 8 times a year), and many market rates are influenced by it. In general, when prices are rising quickly, central banks often respond by raising rates to make borrowing more expensive, and when the economy cools they often move the other way. Many factors go into these decisions, though, so you cannot say it always works that way.

Lenders want to be compensated at least for inflation, so nominal rates tend to be higher when inflation is expected to be high. A high interest rate alone, then, does not mean a good deal. You need to put the interest rate side by side with how much prices rose over the same period to see the real terms. How to read the base rate and inflation indicators in the news is covered in Lesson 10.

6.Five Questions for Reading Any Return

The same interest rate or return figure can mean very different things depending on its basis. When you see a return in an ad, an article, or an AI's answer, check the five points below in order. Miss even one, and you end up comparing different numbers as if they were the same.

• Annual return, or monthly or whole-period return? "1% a month" and "1% a year" are entirely different numbers (Lessons 4 and 9)
• Simple or compound? Even at the same annual rate, results differ over long periods (Lesson 1)
• Before or after tax? The number after taxes and fees is what you actually receive
• Nominal or real? Dividing by inflation shows the change in purchasing power
• Guaranteed or assumed? Distinguish a fixed value, like a deposit rate, from a past record or an example