What is compound interest?

Compound interest is interest earned on both your original principal and the interest already accumulated. Unlike simple interest, which is calculated only on the principal, compound interest grows exponentially — which is why starting early matters so much.

What annual return rate should I use?

7% is a commonly cited long-term average for US stock market returns after inflation. Use 5–6% for a conservative estimate, 3–4% for bonds, and 10% for the historical nominal S&P 500 average before inflation.

How does monthly contribution frequency affect results?

Regular monthly contributions outperform investing the same total amount once a year because each contribution starts compounding immediately. Even small regular amounts add up significantly over decades.

How long does it take to double my money?

Use the Rule of 72: divide 72 by your annual return rate. At 7% returns, your money doubles roughly every 10 years (72 ÷ 7 ≈ 10.3 years).

Compound Interest Calculator

See how a starting balance and monthly contributions grow over time. Live growth curve with contributions and compounded growth shaded separately.

Final balance

$691,150

After 30 years at 7% average annual return, compounded monthly.

Total contributed

$190,000

Growth from compounding

$501,150

Growth-to-contributions ratio

2.64×

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Loading 3D scene…

Blue = money you contributed. Green = interest earned from compounding. Both jars get the same blue — only the left gets green on top. Drag the Time horizon slider to watch the gap grow. Drag to orbit.

How compound interest works

Compound interest means your earnings generate their own earnings. In period one, you earn interest on your principal. In period two, you earn interest on the principal and the interest from period one. The balance grows exponentially, not linearly — which is why the curve gets steeper over time.

This is the single most important concept in personal finance. Albert Einstein reportedly called it the eighth wonder of the world, and for good reason: given enough time, even modest monthly amounts grow into life-changing sums. $200/month at 7% for 40 years becomes over $500,000 — but only about $100,000 of that is your actual contributions. The rest is compound growth.

Why it matters to your money

Every dollar you delay investing costs you far more than it looks like. A 25-year-old who invests $200/month from age 25 to 35 (just ten years) and then stops will have more money at 65 than someone who invests $200/month from age 35 to 65 (thirty years) — even though the second person contributed three times as much. The early investor's money had more time to compound.

Read the full explainer on compound interest if you want to dig deeper into the math, common misconceptions, and how to think about it without a calculator.

Rules of thumb

Rule of 72: Divide 72 by your annual return rate to estimate how long it takes to double. At 7%, money doubles roughly every 10 years.
Start now, not perfectly: A $100/month investment at 7% starting at age 25 becomes $277,000 by 65. Waiting until 35 drops it to $129,000 — even with the same monthly amount for the remaining years.
Time beats rate: Doubling your time horizon has a bigger impact than doubling your expected return. The early years contribute the most to the final balance because they have the longest compounding window.

How return rate and time interact: $10,000 lump sum

The table below shows how a single $10,000 investment grows at three common return rates — a conservative 5% (bonds and balanced funds), a moderate 7% (common long-term real stock-market estimate), and an aggressive 10% (historical nominal S&P 500 average) — across 10, 20, and 30-year horizons. The numbers make the exponential curve concrete.

Years	5% / yr	7% / yr	10% / yr
10 years	$16,289	$19,672	$25,937
20 years	$26,533	$38,697	$67,275
30 years	$43,219	$76,123	$174,494

Notice how the 30-year column dwarfs the 10-year column even at the same return rate. Going from 10% for 10 years ($25,937) to 10% for 30 years ($174,494) is nearly a 7× difference — from tripling to tripling three times over. The same $10,000 at 5% for 30 years ($43,219) nearly matches the 10% return at just 20 years ($67,275), illustrating that time is more powerful than chasing a higher return.

How to put compound interest to work

Understanding compound interest is one thing; engineering your finances to maximize it is another. The following strategies are the highest-leverage actions you can take to harness the exponential curve.

Automate contributions and never touch them. The biggest compounding killer is withdrawing early. Set up automatic transfers to a brokerage or retirement account on payday so the money never sits in a checking account long enough to spend. Automating also removes the temptation to time the market.
Maximize tax-advantaged accounts first. A 401(k) or IRA compounds on pre-tax or tax-free growth, which is mathematically equivalent to boosting your effective return by your marginal tax rate. A 7% return inside a Roth IRA is worth more than a 7% return in a taxable brokerage account where dividends and gains are taxed along the way.
Reinvest dividends automatically. If you hold dividend-paying funds or stocks, enable dividend reinvestment (DRIP). Every dividend reinvested immediately starts compounding. Over 20–30 years, reinvested dividends can account for a third or more of your total return.
Minimize fees — they compound too. An expense ratio of 0.05% (a low-cost index fund) versus 1.0% (an actively managed fund) seems trivial, but on a $100,000 portfolio over 30 years at 7% growth, the 1% fee costs you roughly $180,000 in lost compounding. Always check the expense ratio before investing.
Start with whatever amount you have, today. The most important variable in the compound interest equation is time — and time only moves in one direction. Waiting until you can invest "a meaningful amount" sacrifices irreplaceable compounding years. $50/month at age 22 compounds to more money by age 65 than $500/month starting at 42.

Frequently asked questions

What is compound interest?: Compound interest is interest earned on both your original principal and the interest already accumulated. Unlike simple interest, which is calculated only on the principal, compound interest grows exponentially — which is why starting early matters so much.
What annual return rate should I use?: 7% is a commonly cited long-term average for US stock market returns after inflation. Use 5–6% for a conservative estimate, 3–4% for bonds, and 10% for the historical nominal S&P 500 average before inflation.
How does monthly contribution frequency affect results?: Regular monthly contributions outperform investing the same total amount once a year because each contribution starts compounding immediately. Even small regular amounts add up significantly over decades.
How long does it take to double my money?: Use the Rule of 72: divide 72 by your annual return rate. At 7% returns, your money doubles roughly every 10 years (72 ÷ 7 ≈ 10.3 years).

Learn more

The backdoor Roth IRA, step by step — High earners can't contribute to a Roth IRA directly — but they can contribute to a traditional IRA and convert it. Here's exactly how the backdoor Roth works, the pro-rata trap that blows it up, and when to bother.
The Complete Guide to Compound Interest — A deep dive into how compound interest works, why starting early matters so much, and how to harness it for retirement, savings, and debt payoff.
Dollar-cost averaging explained — Dollar-cost averaging means investing a fixed amount on a regular schedule, regardless of price. It doesn't maximize returns — but it removes the one mistake that kills most investors: trying to time the market.
The FIRE movement explained — FIRE — Financial Independence, Retire Early — is a framework for accumulating enough assets that work becomes optional. Here's what the math actually says, the different flavors of FIRE, and the things the movement gets right and wrong.