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.
The rule of 72: a quick mental shortcut
A handy way to grasp compounding without a calculator is the rule of 72. Divide 72 by your annual interest rate, and the answer is roughly how many years it takes your money to double. At 8% a year, for example, your money doubles in about nine years (72 ÷ 8 = 9); at 6%, it takes about twelve years. This simple trick shows why even small differences in rate have such a large effect over a lifetime of saving.
How compounding frequency changes your returns
Compound interest can be calculated yearly, quarterly, monthly or even daily, and the more often it compounds, the slightly faster your money grows — because each round of interest starts earning its own interest sooner. The difference between annual and monthly compounding is modest over one year, but across decades it adds up. When comparing savings accounts or investments, it's worth checking not just the rate but how often interest is compounded.
Why starting early matters more than starting big
The single most powerful factor in compounding is time. Because interest builds on interest, the earliest contributions have the longest runway to grow and end up contributing the most to your final total. Someone who invests a modest amount in their twenties can easily finish ahead of someone who invests far more but starts in their forties. If you take one thing from compounding, let it be this: the best time to start was years ago, and the second-best time is now.
Frequently asked questions
What is the difference between simple and compound interest?
Simple interest is calculated only on your original amount, while compound interest is calculated on your original amount plus all the interest already earned. Compounding is what makes long-term growth accelerate over time.
How often does compound interest get added?
It depends on the account or investment — it can compound annually, quarterly, monthly or daily. More frequent compounding grows your money slightly faster because interest starts earning its own interest sooner.
How can I calculate compound interest on my savings?
You can use our compound interest calculator to see year-by-year growth for your own starting amount, contributions and rate.
This article is general information, not financial, tax, or medical advice. See our disclaimer.