Compound Interest Calculator

The human brain isn't built for exponential thinking — we expect linear change and consistently underestimate compounding. This explains why Investor A (invests for 10 years then stops) beats Investor B (invests for 30 years starting later), why panic-selling during a 30% decline is more costly than the math suggests, and why the difference between a 0.1% and 1.0% fund fee compounds into nearly double the original investment over 30 years.

Inflation is the invisible compound interest on your cost of living — 3% annual inflation reduces £1,000's purchasing power to £744 over 10 years. Combining this with the "real return" framework (nominal return minus inflation) reveals that a 7% return in a 3% inflation environment compounds your real wealth at 4%, not 7%. Here's why cash in inflationary periods silently destroys purchasing power, the historical real returns by asset class, and how sequence of returns risk amplifies this in retirement.

A 5.00% account compounding annually and a 4.95% account compounding daily aren't equivalent — the daily-compounding account actually pays more, once "effective annual rate" accounts for the compounding frequency. Here's the nominal-vs-effective rate formula, the continuous-compounding limit (e^r), why APR/APY terminology varies by jurisdiction, and how this same math determines the real cost of credit card debt.

Compound interest works as powerfully against you in credit card debt as it works for you in savings. Here's how daily compounding works on credit cards, the mathematics of minimum payment traps, how mortgage interest front-loads early payments, and the dramatic savings from overpaying.

Divide 72 by the interest rate and you get the approximate years for money to double. At 8%, that's 9 years. At 6% inflation, your purchasing power halves in 12 years. The Rule of 72 is the fastest compound interest mental shortcut — here's how it works, why it's accurate, and how to extend it to tripling, quadrupling, and GDP growth.