How Compound Interest Works, With Real Numbers

What is compound interest, in one sentence?#

The U.S. Securities and Exchange Commission's investor education site defines compound interest as interest paid on principal and on accumulated interest^[1]. Principal is the money you start with. Accumulated interest is everything the money has already earned and kept.

The Texas State Securities Board describes the same idea from the investor's side: earnings are added to principal, forming a larger base on which earnings may accumulate^[2]. That larger base is the whole trick. Nothing magical happens in any single year; the effect comes from repeating a small step many times.

How does compounding work year by year?#

Take $1,000 earning 6% a year, with the interest left in the account. The first year earns $60. The second year earns 6% of $1,060, which is $63.60. The third year earns 6% of $1,123.60, which is $67.42. The rate never changed — the base did.

Worked example

$1,000 at 6% a year, interest reinvested

Each row adds that year's interest to the balance, and the next year's interest is calculated on the new balance.

Year	Starting balance	Interest (6%)	Ending balance
1	$1,000.00	$60.00	$1,060.00
2	$1,060.00	$63.60	$1,123.60
3	$1,123.60	$67.42	$1,191.02

Figures computed in code from the stated inputs; rounded to the nearest cent or tenth.

The compounding loop

The same four steps repeat every period. The only thing that changes is the size of the base.

How is compound interest different from simple interest?#

With simple interest, you earn interest only on the original principal, so the balance rises by the same dollar amount every year. With compound interest, interest is also earned on past interest. In the first few years the two lines look almost identical. After a couple of decades they are far apart.

$1,000 at 6% a year: simple vs compound (annual compounding)
After	Simple interest balance	Compound interest balance	Difference
5 years	$1,300.00	$1,338.23	$38.23
10 years	$1,600.00	$1,790.85	$190.85
20 years	$2,200.00	$3,207.14	$1,007.14
30 years	$2,800.00	$5,743.49	$2,943.49

Calculated with balance = 1,000 × (1 + 0.06 × years) for simple interest and 1,000 × 1.06^years for compound interest. Illustrative rate, not a forecast.

Simple vs compound growth of $1,000 at 6%

Compound interestSimple interest

Both lines use the same rate. The gap is interest earned on earlier interest.

Why does time matter more than the rate?#

Look at the compound column again. Between year 0 and year 10 the balance grew by about $791. Between year 20 and year 30 it grew by about $2,536 — at the same 6% rate. Later years add more because they start from a bigger base. That is why investor education material from regulators keeps returning to the idea of starting early: more years means more rounds of compounding^[2].

A quick way to feel this is the rule of 72: divide 72 by the yearly rate to estimate how many years it takes money to double. The Federal Reserve Bank of St. Louis gives the example that at 2%, doubling takes about 36 years^[3]. At 6%, the rule gives 12 years; the exact answer is about 11.9 years. Try it with our rule of 72 calculator.

Can compounding work against you?#

Yes. Anything that takes a percentage of your balance every year compounds too. The SEC's bulletin on fees explains that fees reduce the amount of money in your portfolio that is earning a return^[5]. Its example: $100,000 growing at 4% a year for 20 years ends at roughly $208,000 with a 0.25% yearly fee but roughly $179,000 with a 1% fee^[5].

The same logic applies to debt. A credit card balance that is not paid off is charged interest on interest. And inflation compounds against the buying power of cash. When you compare any two choices, ask what is compounding in each, and in which direction. Our fee drag calculator shows the effect of a yearly cost on any balance.

From the SEC's fee example ($100,000, 20 years, 4% growth)

0.25% yearly fee: ≈ $208,000^[5]
0.50% yearly fee: ≈ $198,000^[5]
1.00% yearly fee: ≈ $179,000^[5]

Where do beginners see compounding in real life?#

Savings accounts and certificates of deposit, where the bank adds interest to the balance on a schedule.
Bonds and bond funds, when the interest payments are reinvested instead of spent — see what a bond is.
Dividend-paying stocks and funds, when dividends are reinvested to buy more shares.
Index funds held for many years, where gains stay invested — see index funds explained.

In each case the rule is the same: earnings only compound if they stay invested. Money that is withdrawn stops compounding the moment it leaves.

What mistakes do beginners make?#

Treating an illustrative rate as a promise
Examples use a steady 6% to show the arithmetic. Real investments move up and down, and some years lose money. Use examples to understand the mechanism, not to predict your balance.
Ignoring fees because they look small
A 1% yearly cost compounds just like a 1% gain. Check the expense ratio or account fee before you look at the expected return.
Withdrawing the earnings
Spending interest or dividends as they arrive turns compound growth back into simple growth. That can be a fine choice, but it should be a deliberate one.
Waiting for a perfect moment to start
Because the later years add the most, delaying the start removes the most valuable years from the end of the timeline.

What else do beginners ask?#

Is compound interest the same as compound returns?

They work the same way, but interest is a fixed rate set by a bank or bond, while investment returns vary from year to year and can be negative. The word compound simply means earnings are added to the base.

How often is interest compounded?

It depends on the account: yearly, quarterly, monthly or daily are all common. More frequent compounding gives a slightly higher result at the same stated rate. The SEC's compound interest calculator, for example, asks you to choose a frequency from annually to daily^[4].

What is the rule of 72?

A shortcut for doubling time: divide 72 by the yearly rate in percent. At 2% it gives about 36 years^[3]. It is an estimate and works best for rates between roughly 4% and 12%.

Does compound interest beat inflation?

Only if the rate after fees is higher than inflation. If an account pays 3% and prices rise 3%, your buying power stays about the same even though the balance grows.

What is the bottom line?#

Compound interest is a simple rule repeated many times: keep the earnings, and the base grows. The rate matters, but the number of years usually matters more — and fees, debt and inflation compound by the same rule in the other direction. Use the arithmetic to make better comparisons, not to forecast a number.

How does compound interest work, and why does time matter so much?