How Compound Interest Works

Simple interest is paid only on your original amount: $1,000 at 5% earns $50 every year, forever. Compound interest is paid on your balance including previously earned interest, so each year's interest is slightly larger than the last. The gap starts small and becomes enormous: $1,000 at 5% simple is $2,000 after 20 years, but compounded annually it's about $2,653 — and the divergence keeps widening the longer you wait.

The engine is that interest earns interest. Year one you earn $50; year two you earn 5% of $1,050 = $52.50; year three, 5% of $1,102.50, and so on. Each step is tiny, but stacked over decades the curve bends sharply upward. This is why the same rate produces wildly different outcomes depending on how long it runs.

How often interest compounds matters: the same annual rate compounded monthly beats compounded yearly, because interest starts earning interest sooner. The difference between annual and monthly compounding is modest at low rates but real over long horizons. Still, the biggest lever by far is time. A dollar invested at 7% doubles about every 10 years, so starting a decade earlier can mean two or three extra doublings — often more impactful than a higher rate or bigger contributions.

A handy shortcut is the Rule of 72: divide 72 by the annual rate to estimate the years to double. At 6%, money doubles in ~12 years; at 8%, ~9 years; at 9%, ~8. It's an approximation, but it makes the power of rate and time intuitive without a calculator — and it shows why small rate differences compound into large end-balance gaps.

The same math runs in reverse on debt. Credit-card balances compound against you, often at 20%+ APR — by the Rule of 72, that doubles what you owe in under four years if left unpaid. Minimum payments are structured so most of your money goes to interest, which is why balances feel stuck. Understanding compounding is what makes paying down high-interest debt an obviously good 'investment': eliminating a 22% debt is a guaranteed 22% return.

Two numbers to watch when comparing accounts or loans: APR (the stated annual rate) versus APY/effective rate (what you actually earn or pay once compounding is included). A 12% rate compounded monthly has an effective rate closer to 12.68%. Always compare the effective figures, since that's the real cost or return after compounding does its work.