Compound interest is interest earning interest. It's the quiet engine behind every retirement account — and the reason starting early beats starting big.
Simple vs compound
With simple interest, you earn a fixed amount on your original deposit each year. With compound interest, each year's interest is added to your balance, so next year you earn interest on a larger amount. The growth isn't a straight line — it curves upward, gently at first, then steeply.
A quick example
Put $10,000 into an account earning 7% a year. After year one you have $10,700. In year two you earn 7% on $10,700, not just the original $10,000 — so you gain $749 instead of $700. That extra $49 is the compounding. It sounds trivial. Over 30 years, it turns $10,000 into roughly $76,000 — and you only deposited $10,000.
Why time beats amount
Because the curve steepens over time, the early years quietly do the heaviest lifting. Someone who invests $200/month from age 25 to 35 and then stops often ends up ahead of someone who starts at 35 and invests for thirty straight years — despite contributing far less. The first investor simply gave compounding more time to work.
What affects how much you get
- Rate of return — small differences compound into large gaps over decades.
- Time — the single most powerful lever, and the one you can't get back.
- Compounding frequency — monthly compounding edges out annual.
- Regular contributions — adding steadily supercharges the effect.
See it for your own savings
Our compound interest calculator plots the year-by-year curve so you can watch the snowball form and test different rates and contributions.
This article is general information, not financial, tax, or medical advice. See our disclaimer.