Compound interest and inflation: why your real return is smaller than it looks

Compound interest grows your savings, but inflation compounds too, and it works against you. The growth your money shows on a statement is the nominal rate; the growth in what that money can actually buy is the real rate. Only the real rate feeds you. If your savings compound at 5% a year while prices compound at 2% a year, your real return is closer to 3% a year, not 5%.

Two kinds of growth

The nominal rate is the percentage your balance grows by, before accounting for prices rising. The real rate is what’s left after inflation has taken its share, and it’s the number that tells you whether your purchasing power actually increased.

A savings account advertising 5% a year is quoting a nominal rate. Whether that 5% leaves you better off depends entirely on what’s happening to prices at the same time.

The same maths, working against you

Compounding is just repeated multiplication: a balance grows by the same percentage each year, and each year’s growth builds on the year before. That’s true of savings compounding upward at 5%. It’s equally true of prices compounding upward at 2%. A loaf of bread, a rent bill, or a weekly shop that rises 2% a year is following the exact same mathematical pattern as a savings account earning 5% a year, just running in the opposite direction from your perspective. Your balance compounds up. The cost of everything you’d spend it on compounds up too. The real return is what survives once both effects are netted against each other.

This is why a rate that looks generous on paper can still leave a saver treading water. Two compounding processes are running at once, and the one you don’t see on your bank statement is the one that decides what your money is actually worth.

A worked example

Take €10,000 saved at 5% a year for 20 years. Left to compound, it grows to €26,533 on paper. That’s the nominal result, and it’s the figure your bank statement would show.

Now bring inflation into it. If prices rise at 2% a year over the same 20 years, that €26,533 doesn’t buy what €26,533 buys today. It buys what €17,856 buys today. The difference between those two figures, €26,533 against €17,856, is entirely down to prices compounding upward while your savings were compounding upward too.

Expressed as a rate, the real return in this example works out to about 2.94% a year, not the 5% nominal rate the account advertised. That 2.94% is the number that actually reflects how much better off the saver is at the end of 20 years, in terms of what their money can buy.

The quick approximation

There’s a fast way to estimate a real rate without working through the full compounding: subtract the inflation rate from the nominal rate. Real return is approximately equal to nominal return minus inflation. In the example above, 5% minus 2% gives 3%, which is close to the more precisely calculated 2.94%.

This shortcut is genuinely useful for a rough, in-your-head estimate. But it’s an approximation, and the gap between the approximation and the true figure widens as the rates involved get larger. At the modest rates typical of savings accounts, the shortcut holds up well. At higher nominal rates or higher inflation, it loosens, and the simple subtraction starts understating or overstating the real picture by more than a rounding error.

What this means for savers

Cash sitting in an account can compound every year and still lose buying power, if the nominal rate it earns doesn’t keep pace with inflation. A saver can watch their balance rise, year after year, and genuinely be getting poorer in terms of what that money can purchase. This isn’t a flaw in compound interest. It’s simply that compounding describes how a number grows, and it says nothing on its own about what that number is worth.

Central banks in many economies aim to keep inflation around 2% a year as a matter of policy, which is part of why 2% is such a common reference point when thinking about real returns. Naming this problem clearly, nominal growth against real growth, is the first step toward understanding what a savings rate is actually delivering. What to do about it is a personal decision that depends on individual circumstances, and it’s the kind of decision worth discussing with a professional financial adviser rather than working out from general principles alone.