Compound Interest Explained: Why Time Beats Rate

Last updated: September 2026 · Written by Hugo Cardozo

What compound interest actually means

Simple interest only ever pays you on your original deposit. Compound interest pays you on your original deposit plus every bit of interest you've already earned — so your interest starts earning its own interest. That's the entire idea, and it's also why the effect looks small at first and enormous later.

A worked example by hand

Put $1,000 in an account at 10% annual interest, compounded once a year. Year 1: you earn $100, balance is $1,100. Year 2: you earn 10% of $1,100, which is $110 — not $100 — so your balance is $1,210. Year 3: 10% of $1,210 is $121, balance is $1,331. Notice the yearly gain keeps growing even though the rate never changed.

Why time matters more than the rate

Someone who invests $200/month starting at age 25 and stops at 35 (10 years of contributions, then leaves it alone) will typically end up with more money by retirement than someone who starts at 35 and contributes the same $200/month all the way to retirement — purely because of the extra decade of compounding on those early contributions. Rate matters, but an extra 10 years almost always beats an extra 1-2% return.

The rule of 72

A quick mental shortcut: divide 72 by your interest rate to estimate how many years it takes your money to double. At 6%, that's 12 years. At 9%, about 8 years. It's an approximation, not exact, but it's useful for a gut-check without opening a calculator.

Try it with your own numbers

The example above uses annual compounding with no ongoing contributions to keep the math simple enough to follow by hand. Real savings almost always include monthly contributions and monthly compounding — use the calculator below to see the actual numbers for your situation.

Frequently asked questions

Does compounding frequency (daily vs. monthly vs. annual) matter a lot?

Less than most people expect. Moving from annual to monthly compounding makes a real difference; moving from monthly to daily makes almost none. The contribution amount and the number of years invested matter far more than compounding frequency.

Is compound interest only relevant to investing?

No — it works in both directions. It's what grows savings and investments over time, but it's also exactly how credit card and other high-interest debt grows if only minimum payments are made, which is why paying down high-rate debt fast matters just as much as investing early.

What's a realistic rate to assume for long-term projections?

For a stock-heavy long-term portfolio, 6-7% after inflation is a commonly used conservative historical average. For a savings account or CD, use the actual stated APY instead — those two situations shouldn't use the same assumed rate.