How does compound interest work?
Compound interest means you earn returns on your earlier returns, so a balance grows faster the longer it stays invested. For example, $180 deposited at the end of each month for 20 years at a constant 7% a year, compounded monthly, becomes about $93,767, of which $43,200 is money you put in.
Free, no account. Checked October 13, 2026.
Returns on returns
With simple interest, only your original deposit earns. With compounding, the interest you already earned also earns, so each period starts from a larger balance than the last.
That is why the final balance already includes the money you contributed. Adding the contributions on top of the balance counts them twice.
Rate, time and contributions
Three inputs set the result: the rate, the length of time and the amount you add each month. Changing one while holding the others fixed shows how much each matters.
Time and contribution size both matter. Starting early does not always beat saving more later, and the lab lets you test both with your own numbers.
What the rate means in the example
The 7% in the example is a nominal annual rate compounded monthly. Each month the balance grows by 7% divided by 12, which is about 0.58%, and then your next $180 goes in.
Growth is back-loaded. At 7%, the first ten years of $180 a month reach $31,155 and the full twenty reach $93,767, so the second decade adds about twice as much as the first.
Why a fixed rate is an illustration
Investment returns fluctuate and can be negative. A constant rate such as 7% compares scenarios. It is not a forecast of what an account will earn.
Fees and taxes reduce the result, and the figures here are before taxes and fees. Compare scenarios in future dollars carefully, because prices rise too, and $93,767 in twenty years buys less than $93,767 does today.
Money you will need soon stays out of investments that can lose value, because a short time gives compounding little room to recover from a drop.
Compounding works on debt too
A balance that you carry on a credit card compounds against you. A $3,000 balance at 22% APR with a fixed $90 payment each month takes 52 months to pay off and costs about $1,679 in interest.
A worked example
Sample, not your numbers
$180 a month for 20 years
Sample numbers, not yours. You deposit $180 at the end of each month for 20 years, which is $43,200 in total, at three constant annual rates compounded monthly.
- At 0%: you end with
- $43,200
- At 4%: you end with about
- $66,019
- At 7%: you end with about
- $93,767
- Growth at 7% (balance minus $43,200)
- $50,567
- Same $43,200 over 10 years at $360 a month, 7%
- $62,311
The last two rows contribute the same $43,200. Twenty years ends about $31,456 higher than ten, which is the extra time compounding had.
Run your own numbers
Compare 0%, 4% and 7% for an amount you could save each month.
Common questions
How often does interest compound?
That depends on the account. The example here compounds monthly. An account's terms say how often it compounds and what APY it pays.
Is 7% what I should expect?
No. 7% is a sample rate for illustration. Real returns change year to year and can be negative, so money you need soon stays out of investments that can lose value.
Does the balance include my deposits?
Yes. The ending balance is deposits plus growth. In the example, $43,200 of the $93,767 is deposits.
Go further in the course
Chapters 1 to 3 are free, no card needed. Chapter 1 works through this $180 example and the habits behind it.
Sources
Figures checked on October 13, 2026. All figures are before taxes and fees and assume a constant rate. The 4% and 7% rates are samples and not a prediction.
Education, not financial, tax or legal advice. The example uses sample numbers, and your own situation will differ.