Compound Interest: How Your Money Grows
Understand compound interest in plain English — how it works, why time matters more than timing, and how regular contributions turn small savings into large balances.
Albert Einstein supposedly called compound interest the eighth wonder of the world. Whether or not he really said it, the idea is worth understanding, because it quietly powers everything from your savings account to your retirement fund. This guide explains what compound interest is, why it accelerates over time, and how to make it work for you.
Simple vs. compound interest
Simple interest is earned only on your original deposit. Put $1,000 in an account paying 10% simple interest and you earn $100 every year — forever.
Compound interest is earned on your deposit and on the interest you have already earned. That same $1,000 at 10% compounded annually earns $100 in year one, but $110 in year two (because you now have $1,100), then $121, and so on. The balance does not grow in a straight line — it curves upward.
Why time beats timing
The longer your money compounds, the more dramatic the effect, because the interest-on-interest snowball has more time to roll. This is why starting early is so powerful: someone who invests modestly in their twenties often ends up ahead of someone who invests far more starting in their forties.
Comparing two rates honestly
Two accounts quoting different rates on different schedules cannot be compared as printed. 6% compounded monthly and 6.16778% compounded annually grow money at exactly the same speed — which is the only reason a card’s APR and a savings account’s APY can ever be set beside each other. The calculator below converts a rate from the period it compounds on to any other, so you can put two offers in the same terms before you judge them.
Example
This is a sample result, not your calculation. Enter your own values to replace it.
Enter an interest rate and choose the two compounding periods, then press Calculate to see the equivalent rate.
Equivalent rate
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What it earns
- Effective annual rate
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Bars show what each period earns above annual compounding — the exact rate is beside each one. More frequent compounding always earns more, but the gain shrinks fast: most of it is already won by the time you reach monthly.
Check your entry
Enter an interest rate of zero or more, then choose the compounding period it is quoted on and the one to convert it to.
Beneath the answer it lists what your rate earns in a year at every compounding period. The bars measure each period against annual compounding rather than against zero, because plotted from zero the nine rates look identical — the difference is real but small, and the exact figure is printed beside every bar.
The quiet power of contributions
A lump sum compounds, but adding a little every month supercharges the result, because each contribution starts compounding too. To watch that happen, use the investment calculator: change the monthly amount and see Total Interest shift against Total Contributions — over long periods, the growth often exceeds everything you deposited.
Compounding frequency
Interest can compound annually, monthly, or even daily. More frequent compounding means interest is added to your balance sooner, so it starts earning itself sooner. The difference is small at low rates and grows at higher rates and longer time horizons.
The rule of 72
Want a quick estimate of how long it takes to double your money? Divide 72 by the annual rate. At 8%, that is roughly 9 years to double; at 6%, about 12 years. It is an approximation, but a handy one.
Put it to work
- For a savings or investment goal, use the compound interest calculator.
- To model a portfolio with regular contributions, try the investment calculator.
- To project compounding all the way to retirement, use the retirement calculator.
Projections assume a constant rate of return. Real returns vary and can be negative. This is educational information, not investment advice.