Compound Interest Explained, With Real Numbers

Simple interest vs. compound interest

Simple interest is paid only on your original amount. Put $1,000 in an account paying 5% simple interest and you earn $50 every year, forever. After 10 years you have $1,500, after 20 you have $2,000, and after 30 you have $2,500.

Compound interest is paid on your original amount *and* on all the interest you've already earned. In year one, $1,000 at 5% earns $50, the same as before. In year two you earn 5% on $1,050, which is $52.50. In year three you earn 5% on $1,102.50. Each year's growth is a little bigger than the last because the base keeps getting larger.

At the start the difference is tiny. After 30 years, the same $1,000 at 5% compounded grows to about $4,322, compared with $2,500 under simple interest. The longer the time, the wider the gap, and the curve bends upward more and more steeply. That bend is the whole story.

The formula, in plain English

If you invest a single amount and leave it alone, the math is: Future value = Starting amount × (1 + rate) ^ years. The rate is written as a decimal, so 7% is 0.07.

For $1,000 at 7% for 10 years: 1,000 × (1.07)^10 = about $1,967. You don't need to do this by hand. Any online compound interest calculator or a spreadsheet will do it in seconds. What's important is understanding the shape: multiply by 1.07 once for every year, so the result grows by a constant percentage rather than a constant dollar amount.

If you also add money regularly, the math gets a bit longer, but the principle is the same. Every deposit starts compounding from the day it goes in.

What $1,000 grows to at different rates

Here's what a single $1,000 invested today becomes if left untouched, at three illustrative yearly rates.

Two things stand out. First, time does most of the work: at 7%, the money roughly doubles in the first 10 years, then doubles again in the next 10, and again after that. Second, small differences in rate become huge differences over long periods. At 40 years, the difference between 7% and 9% is more than $16,000 on a single $1,000.

The power of adding a little every month

Most people don't invest one lump sum. They add something every month. Here's what regular monthly investing looks like at a 7% yearly return, with money compounding monthly.

Look at the last row. At $200 a month for 40 years, you contribute $96,000 in total, but the account could be worth around $525,000. More than four-fifths of that final amount is growth, not money you put in. That's compounding at work.

These numbers assume a steady 7% every year. Real markets rise and fall unevenly, and you can lose money, especially over short periods. The point is the shape of the curve, not a forecast.

Starting early beats starting big

Because compounding rewards time, the age you start often matters more than how much you invest. Take two people who both save $200 a month at 7%.

Person A saved only $24,000 more than Person B, yet ends up with more than double the money. Ten extra years of compounding at the start were worth about $281,000.

Here's an even more striking version. Suppose someone invests $200 a month for only 10 years, from age 22 to 32, then stops and never adds another dollar. Their $24,000 of contributions, left to grow until 65, could reach about $346,000 at 7%. A person who waits until 32 and then invests the same $200 every month for 33 years until 65 puts in $79,200 and ends with about $309,000. The early starter wins with less than a third of the contributions.

None of this means it's too late if you're older. It means that whenever you start, the best time to begin is now, and the second-best way to improve your result is to add more.

The Rule of 72

You don't always need a calculator. The Rule of 72 is a quick mental shortcut for how long money takes to double: divide 72 by the yearly rate of return.

• At 6%, money doubles in about 12 years (72 ÷ 6).
• At 8%, about 9 years (72 ÷ 8).
• At 10%, about 7.2 years (72 ÷ 10).
• At 7%, about 10.3 years (72 ÷ 7).

It's an approximation, but it's close enough to sanity-check a claim. If someone promises to double your money in two years, the Rule of 72 says that requires about 36% a year, which is a big red flag for risk or a scam.

How fees quietly work against you

Compounding works on costs too. A fee charged every year reduces the base that grows, so its damage also compounds. Imagine investing $10,000 and adding $200 a month for 30 years, with a 7% yearly return before fees.

A fee that looks like a rounding error, under one percent, costs more than $58,000 over three decades. This is why low-cost index funds are so popular, and why it pays to check an investment's expense ratio before you buy. Our guide to index funds and ETFs shows what typical costs look like.

When compounding works against you: debt

The same math that builds savings can punish borrowers. A credit card charging 22% a year compounds on what you owe. If you carry $1,000 for a year at that rate, you pay about $220 in interest. If you only make minimum payments, the balance can linger for years, with the interest piling up on top of earlier interest.

That's why paying off high-interest debt is often called a guaranteed return. Wiping out a 22% balance is like earning 22% with no risk, a return that almost no investment can promise. Lower-rate debt, such as a mortgage, is a different calculation, and many people choose to invest while paying it down.

Inflation and the honest caveats

Three caveats keep compounding in perspective.

• Inflation shrinks purchasing power. $525,000 in 40 years will buy less than $525,000 buys today. A rate like 7% is often used as a planning figure because it roughly accounts for inflation, but it's a rough guide, not a rule.
• Returns aren't steady. Stocks don't deliver 7% every year. They might gain 25% one year and lose 15% the next. Over long periods the average can look smooth, but the ride is not.
• Taxes and fees take a share. Depending on the account type, some of your growth may be taxed. Retirement accounts can help, as our guide to retirement accounts explains.

Still, the core lesson holds up. Time in the market and consistent contributions do much of the heavy lifting, and they're things you control.

How to put this to work

1. Start now, even small. Time is the ingredient you can't buy back later.
2. Automate monthly contributions so growth builds without you having to remember.
3. Keep costs low by favoring low-fee, diversified funds.
4. Avoid high-interest debt, or pay it off before investing, so compounding works for you instead of against you.
5. Increase your contribution when your income rises, even by a little. Raising it by $50 a month matters over decades.
6. Stay invested through the bumps. Selling in a panic interrupts the very process that does the work.

Frequently asked questions