CompoundInterestCalc
Free compound interest calculator

See how small, steady investing turns into real wealth.

Plug in a starting amount, a monthly contribution and an expected return. Watch compounding do the heavy lifting — year by year, dollar by dollar.

Built for ETF & index investors Inflation & tax aware No sign-up, instant results
Your plan
$
$
%
25 years
▶ Adjust for inflation & tax
%
%
Future balance
$0
Total invested
$0
Interest earned
$0
Growth over time
Balance Invested
▸ Show year-by-year breakdown
Period Invested Interest Balance

Estimates are for illustration and education only — not investment advice. Real returns vary and are not guaranteed.

CompoundInterestCalc.cc is built for investors of every kind — first-time savers opening a brokerage account, long-term index and ETF investors, people working toward financial independence or early retirement, and dividend investors reinvesting income for the long haul. Wherever you fit, understanding how your numbers actually compound comes first — our articles explain the concepts in plain English, and our Markets page gives you live context on stocks, gold, crypto and ETFs.

Beyond this calculator, we offer dozens of free tools covering mortgages, retirement, debt payoff, dividends and more — each built to help you test a plan before you act on it, not to sell you anything.

Browse all calculators →
Live markets

See where the market stands

Compounding works whether prices are up or down, but it helps to know the backdrop. Here's a live look at the S&P 500 — for the Nasdaq 100, gold, Bitcoin, heatmaps and ten major US companies, visit the Markets page.

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S&P 500 by TradingView
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The big idea

What is compound interest?

Compound interest is interest earning interest. You earn a return not only on the money you put in, but also on every dollar of growth you've already made — so your balance snowballs, slowly at first and then surprisingly fast.

1 · You invest

You put in a starting amount and, ideally, keep adding a little every month. This is your contribution.

2 · It earns a return

Each period your balance grows by the interest rate. That growth gets added back to your balance.

3 · Growth compounds

Next period you earn returns on the larger balance — including past gains. The effect accelerates over years.

The math, demystified

The compound interest formula

A = P(1 + r/n)(nt)
A
Final amountWhat your investment is worth at the end.
P
PrincipalYour starting amount.
r
Annual rateExpected yearly return, as a decimal (7% = 0.07).
n
Compounds / yearHow often interest is added (12 = monthly).
t
YearsHow long the money stays invested.
+
ContributionsRegular deposits each compound on their own from the day they're added.
Why it matters

Compound vs. simple interest

Simple interest only ever pays you on your original deposit. Compound interest pays you on the growing total. Here's $10,000 left to grow for 30 years at 7%.

Simple interest

$0
Interest only on the original $10,000
  • Total interest$0
  • Growth patternStraight line

Compound interest

$0
Interest on principal + accumulated growth
  • Total interest$0
  • Extra vs. simple$0
Time is the secret ingredient

The power of starting early

Two people both invest $300/month at 7%. Amara starts at 25; Ben waits until 35. Amara invests for just ten extra years — but ends up with dramatically more, because her early dollars compound the longest.

Starting a decade earlier leaves Amara $0 ahead at 65 — proof that when you start can matter more than how much you add.

Amarastarts at 25
$0
Benstarts at 35
$0
Amara invested $0 Ben invested $0
Real-world scenarios

ETF & index investing examples

A few common long-term plans, calculated for you. Tap any card to load it into the calculator and make it your own.

Good to know

Frequently asked questions

What is compound interest, in plain English?
It's the interest you earn on both your original money and on the interest it has already earned. Because your gains start generating their own gains, a balance grows faster and faster the longer it's left alone — like a snowball rolling downhill.
How does this calculator work?
It simulates your investment month by month. Each month it adds your contribution and applies growth based on your interest rate and compounding frequency. It then totals your contributions versus the interest earned, and can adjust the result for inflation and tax on gains.
What return rate should I use for an ETF or index fund?
Historically a broad index like the S&P 500 has returned roughly 7% per year after inflation over long horizons, or closer to 10% before inflation. Returns swing a lot year to year and past performance doesn't guarantee future results — so it's smart to test a conservative and an optimistic rate.
Does compounding frequency really matter?
A little. More frequent compounding (monthly vs. annually) produces slightly higher returns because interest starts earning interest sooner. The difference is usually small compared to the impact of your contribution amount and time horizon.
Should I account for inflation?
If you want to know what your future balance is actually worth in today's purchasing power, yes. Enabling the inflation field discounts your final balance so you can compare it to prices you understand right now. A common long-run assumption is around 2–3%.
Is this financial advice?
No. This is a free educational tool to help you understand how compounding works. It doesn't account for fees, taxes in your specific jurisdiction, or market risk. Always do your own research or speak to a qualified advisor before investing.
From the blog

Articles & guides

Plain-English reads on compounding, investing and building wealth slowly.

View all articles →
Investing basics · 7 min read

The 8th wonder of the world: why compound interest beats timing the market

The biggest fortunes are usually built by people who simply stayed invested — and let time do the heavy lifting. Here's the simple math of why.

Read article →