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What is compound interest, and why does starting early matter so much?
by financebrain · October 20, 2026
Compound interest, or compound growth, is what happens when your returns are reinvested and start earning returns of their own. Instead of growing in a straight line, your money grows along a curve that gets steeper over time.
Starting early matters because time is the biggest lever in that curve. Money invested at 25 has 40 years to compound before 65. Money invested at 55 has ten. Each extra year multiplies everything that came before it.
A simple example
Invest $10,000 at a 7% annual return and reinvest everything:
| After | Value |
|---|---|
| 1 year | $10,700 |
| 10 years | $19,672 |
| 20 years | $38,697 |
| 30 years | $76,123 |
You never add another dollar. The first ten years add about $9,700. The third decade alone adds more than $37,000, because by then 7% is being earned on a much bigger balance.
These are illustrative numbers. Real investment returns go up and down from year to year rather than arriving as a steady 7%.
The formula
The formula is A = P(1 + r)^t:
- P is what you start with.
- r is the annual return.
- t is the number of years.
- A is what you end up with.
Time sits in the exponent. That's why adding years has a much bigger effect than you'd expect from a straight-line view.
The Rule of 72
A quick shortcut: divide 72 by the annual return to estimate how many years it takes your money to double.
- At 3%, roughly 24 years.
- At 6%, roughly 12 years.
- At 9%, roughly 8 years.
Early start vs late start
This is the example that surprises most people. Two investors both earn 7% a year:
- Investor A invests $5,000 a year from age 25 to 34, then stops completely. That's ten payments, $50,000 in total.
- Investor B waits, then invests $5,000 a year from age 35 to 64. That's thirty payments, $150,000 in total.
At 65, Investor A has about $563,000 and Investor B has about $505,000. That assumes each payment is made at the start of the year. A put in a third as much money and still ends up ahead, because A's money had an extra decade to compound.
The point isn't that you should stop at 35. It's that the early years are the most valuable ones you have, so the cost of waiting is higher than it looks.
It works against you too
Compounding doesn't care which side you're on. Interest on a credit card balance compounds the same way, which is why high-interest debt grows so quickly when only the minimum is paid. Fees compound as well: a fund that charges 1% a year more takes a growing slice of a growing balance.
Learn it properly
Finance Brain's The Power of Compound Growth lesson, in the Investment Classes course, goes further. It covers John Bogle's research on long-run stock market returns and why reinvested dividends account for so much of them. It ends with a short quiz.
Finance Brain is educational content, not financial advice. Examples use a fixed 7% return for illustration only; real returns vary and aren't guaranteed.