📈 Track your actual DCA performance — real P&L, avg cost, weekly AI signalsTry Pro free →
The eighth wonder of the world

Compound interest
calculator

See exactly how your money grows over time. Compare compounding frequencies and watch small differences in rate or time become enormous differences in outcome.

DCA calculator Position size calculator Compound interest calculator
$
Starting amount
$
Added every month (can be 0)
% per year 8%
Years 20 yrs
% per year 0%
Adjusts final value for purchasing power
Final balance
—
Total deposited
—
Interest earned
—
Real value (inflation adj.)
—
Total balance
Total deposited
Interest earned
Chart showing compound interest growth over time.
Milestones
Start compounding your money today.

Open an account and put your money to work.

Alpaca Robinhood Coinbase Fidelity

How compound interest works

+
Interest on interest

With simple interest, you only earn returns on your original deposit. With compound interest, you earn returns on your deposit AND on all the interest you've already accumulated. Over time, this difference becomes enormous.

⏱
Time is the biggest variable

The earlier you start, the more dramatic the results. $10,000 invested at 8% for 30 years grows to ~$100,000. Wait 10 years to start and that same $10,000 only reaches ~$46,000. You lose more than half just by waiting a decade.

↻
Frequency matters

Daily compounding produces slightly more than monthly, which produces more than annual. The difference is small at low rates but meaningful at higher rates over long periods. Most investment accounts compound daily or monthly.

$
Regular contributions amplify it

Adding even a modest monthly contribution dramatically accelerates growth. The monthly addition isn't just adding linearly — each contribution immediately begins compounding, creating a snowball effect that grows faster over time.

Frequently asked questions

What is the compound interest formula?
A = P(1 + r/n)^(nt) — where A is the final amount, P is the principal, r is the annual rate (as a decimal), n is the compounding frequency per year, and t is the time in years. With regular contributions the formula extends to account for each deposit compounding from its own start date.
What's a realistic interest rate to use?
For long-term stock market investments, 7–10% is historically reasonable (S&P 500 averages ~10% nominal, ~7% after inflation). High-yield savings accounts currently offer 4–5%. Bonds typically 3–5%. Crypto is far higher but far more volatile and unreliable as a planning assumption.
How does inflation affect compound interest?
Inflation erodes purchasing power over time. A balance of $100,000 in 20 years won't buy what $100,000 buys today. The "real value" shown in this calculator adjusts for inflation so you can see what your final balance is actually worth in today's dollars. US inflation has averaged around 3% historically.
What is the Rule of 72?
The Rule of 72 is a quick mental shortcut: divide 72 by your annual interest rate to estimate how many years it takes to double your money. At 8%, your money doubles in ~9 years (72 ÷ 8). At 6%, it takes ~12 years. It's not exact but it's a fast sanity check.
Is compound interest the same as DCA?
They're related but different. Compound interest describes how returns are calculated — returns generating more returns over time. Dollar-cost averaging (DCA) is an investment strategy — investing fixed amounts at regular intervals. DCA works best when the underlying investment benefits from compounding, which is why they're often discussed together. Try our DCA calculator to model both together.

How the compound interest calculator works

Compound interest is growth on top of growth — returns earned on your principal plus all previously earned returns. Over long periods, this creates exponential rather than linear growth. The Rule of 72 gives you a quick mental estimate: divide 72 by your annual return rate to find how many years it takes to double your money. At 7% your money doubles roughly every 10 years. At 10%, roughly every 7 years.

Time in the market is the most powerful variable in the compound interest equation. The difference between starting at 25 vs 35 — just 10 years — is typically more than one full doubling cycle. Starting earlier consistently outperforms contributing larger amounts started later.

How to use this calculator

Read our full guide on how compound interest works for deeper explanation with real-world examples.