Compound Interest vs Inflation: The Real Purchasing Power Formula
Compound interest has often been described as the eighth wonder of the world: those who understand it earn it, and those who do not pay it. However, calculating investment returns without factoring in inflation gives a misleading picture of future wealth.
Written by the Toolseza team · Updated 2026-10-18
The Mechanics of Compound Growth
Simple interest only pays a return on the initial principal. Compound interest pays returns on both the principal and previously accumulated interest. The mathematical formula is A = P(1 + r/n)^(nt), where P is principal, r is the nominal annual rate, n is compounding frequency, and t is time in years.
More frequent compounding (monthly or daily) increases the effective annual yield (APY). For example, £10,000 invested at 7% annual interest compounded monthly grows to £19,671 in 10 years, and £38,696 in 20 years, without adding a single extra pound of new capital.
The Rule of 72 for Quick Mental Estimates
To quickly estimate how many years it will take for your money to double at a constant annual return, divide 72 by the annual interest rate. At an 8% return, your money doubles approximately every 9 years (72 ÷ 8 = 9). At a 6% return, it takes 12 years.
Accounting for Inflation: Nominal vs Real Returns
Inflation is the steady increase in the general price level of goods and services. If your savings earn a 5% nominal annual return, but annual inflation averages 3%, your real purchasing power increases by only approximately 2%.
Over long horizons, cash sitting in non-interest-bearing current accounts loses substantial value. An annual inflation rate of 3% halves purchasing power in roughly 24 years. Sustainable long-term wealth preservation requires investment returns that comfortably outpace inflation.