Put a hypothetical $10,000 into an investment that earns 7% a year and leave it alone. After the first year you have $10,700. The $700 is simple interest on your original deposit. In year two, you earn 7% on $10,700, which is $749. The extra $49 is interest on your interest, and that is compounding.
Forty-nine dollars is not exciting. But the effect grows every year, because each year’s gain becomes part of the base for the next. By year 10, the $10,000 has become about $19,700. By year 20, about $38,700. By year 30, about $76,100. And by year 40, roughly $149,700, nearly fifteen times the original deposit, without adding another cent.
Notice the pattern: the balance roughly doubles every decade at 7%. The first decade adds about $9,700. The fourth decade adds about $73,600. Same rate, same money, very different results, depending only on how long the money has been compounding.
Growth by decade
The table below shows two hypothetical savers. One deposits $10,000 once. The other invests $300 a month and never makes a lump-sum deposit. Both scenarios are shown at a 4% and a 7% annual return, compounded monthly for the monthly saver. These are illustrations, not predictions; real returns vary year to year and are never guaranteed.
| Years | $10,000 once at 4% | $10,000 once at 7% | $300/month at 4% | $300/month at 7% | Total put in at $300/month |
|---|---|---|---|---|---|
| 10 | $14,802 | $19,672 | $44,175 | $51,925 | $36,000 |
| 20 | $21,911 | $38,697 | $110,032 | $156,278 | $72,000 |
| 30 | $32,434 | $76,123 | $208,215 | $365,991 | $108,000 |
| 40 | $48,010 | $149,745 | $354,588 | $787,444 | $144,000 |
Two things stand out. First, after 40 years at 7%, the monthly saver has put in $144,000 and has about $787,000. More than four out of every five dollars in the account came from growth, not deposits. Second, the gap between 4% and 7% looks modest in year 10 and enormous by year 40. Small differences in rate matter far more over long periods than short ones.
You can run your own numbers with the SEC’s free compound interest calculator on Investor.gov.
The Rule of 72
A quick mental shortcut: divide 72 by your annual rate of return to estimate how many years it takes money to double.
- At 4%, money doubles in about 18 years.
- At 7%, about 10 years.
- At 10%, about 7 years.
The rule is an approximation, but a close one for typical rates. It also works in reverse on costs. A credit card charging 24% doubles an unpaid balance in about three years if no payments are made, which is why compounding works against you on debt as reliably as it works for you on savings. Our article on paying only the minimum shows what that looks like on a real statement.
Why starting at 25 beats starting at 35
Time is the variable you control least once it has passed. Take two hypothetical savers who both invest $200 a month at 7% until age 65:
- One starts at 25 and contributes for 40 years, a total of $96,000. She ends with about $525,000.
- The other starts at 35 and contributes for 30 years, a total of $72,000. He ends with about $244,000.
The early starter put in only $24,000 more but finished with more than twice as much. Those first ten years of contributions had an extra decade to compound, and that decade happened at the end, when the balance was largest.
The lesson is not that a 35-year-old has missed out. It is that the best time to start is whenever you can, and that small amounts invested early can matter more than larger amounts invested late. If money is tight, starting with a modest monthly amount is far better than waiting; our guide to investing $100 a month lays out how.
What quietly slows compounding down
Fees
A fund’s expense ratio comes out of your return every year, so it compounds too. A hypothetical $10,000 growing at 7% for 30 years becomes about $75,000 in a fund charging 0.05% a year, but only about $57,400 in one charging 1%. The fee looks small; the compounded cost is more than $17,000 on a single deposit.
Taxes
Paying tax on gains every year shrinks the base that compounds. Tax-advantaged accounts such as 401(k)s and IRAs let the full balance grow without annual tax drag, which is a big part of why they are so effective over decades.
Interruptions
Withdrawing money early, or selling after a market drop and sitting in cash, resets part of the clock. The most powerful part of compounding happens late, so cutting it short costs more than it seems.
Inflation
The table shows nominal dollars. If prices rise about 3% a year, $787,000 in 40 years will buy much less than $787,000 does today. A common approach is to subtract expected inflation from the return to get a rough “real” figure; at 7% nominal and 3% inflation, the 4% columns above give a closer sense of purchasing power.
APY versus APR: how often interest compounds
On savings accounts and CDs, compounding frequency matters slightly. A rate quoted as APY (annual percentage yield) already includes the effect of compounding, so it is the number to compare between accounts. A rate quoted as APR does not. For a savings account, daily versus monthly compounding makes only a small difference; the rate itself matters much more. For investments like stock funds, there is no fixed compounding schedule at all. Growth comes from rising prices and reinvested dividends, and the “7%” in these examples is an average that hides plenty of up and down years.
What this means for you
Compounding rewards three things: time, consistency and low costs. You cannot control market returns, but you can control when you start, whether you keep contributing through bad years, and how much you pay in fees. A modest automatic contribution, invested in low-cost diversified funds and left alone for decades, uses all three. The numbers in the table are hypothetical, but the shape of the curve is not: the slow early years are the price of the steep later ones, and the only way to reach those later years is to stay invested long enough to see them.
