Compound Interest Calculator
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See exactly how your money grows with compound interest — choose any compounding frequency and see the power of time.
📈 Compound Interest Calculator
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The Power of Compound Interest
Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. Albert Einstein allegedly called it the "eighth wonder of the world" — whether or not he said it, the math is remarkable: $10,000 invested at 8% for 40 years grows to over $217,000 with no additional contributions.
📐 Compound Interest Formula
A = $10,000 × (1 + 0.08/12)^(12×20)
A = $10,000 × (1.006667)^240
A = $10,000 × 4.9268 = $49,268
How to Use the Compound Interest Calculator
Enter your starting principal
Input your initial investment or savings balance. Even a modest starting amount compounds significantly over long horizons.
Set a monthly contribution
Add a regular monthly contribution. Combining an initial amount with consistent contributions produces dramatically faster growth than either alone.
Choose compounding frequency
Daily compounding (standard for savings accounts) produces slightly more interest than annual. The difference is small but compounds over decades.
Adjust the time horizon
Compare 10, 20, and 30 year outcomes. The curve accelerates sharply in later years — this is why long time horizons are compound interest's most powerful ally.
Frequently Asked Questions
What is Compound Interest and How Does it Work?
Compound interest is interest calculated on both the original principal and the accumulated interest from previous periods. Albert Einstein is often (though likely apocryphally) credited with calling it the "eighth wonder of the world" — its power lies in exponential growth: interest earns interest, which earns more interest, growing wealth faster over time.
Formula: A = P(1 + r/n)^(nt)
Where A = final amount, P = principal, r = annual rate, n = compounding frequency, t = time in years.
Compounding Frequency — Does It Matter?
| Compounding | Times/Year | $10,000 at 5% after 10 years |
|---|---|---|
| Annually | 1 | $16,289 |
| Quarterly | 4 | $16,436 |
| Monthly | 12 | $16,470 |
| Daily | 365 | $16,487 |
Daily compounding earns about $200 more than annual compounding over 10 years on $10,000 — meaningful on larger sums and longer timeframes. Most savings accounts and investment accounts compound daily or monthly.
The Rule of 72 — Quick Mental Math for Compound Growth
The Rule of 72 is a shortcut for estimating how long it takes to double your money: divide 72 by the annual interest rate. At 6% interest: 72 ÷ 6 = 12 years to double. At 8%: 9 years. At 4%: 18 years. It's remarkably accurate for rates between 4–20% and useful for quick mental comparisons.
Compound Interest vs Simple Interest
Simple interest is calculated only on the principal: I = P × r × t. Compound interest calculates on principal plus previous interest. Over short periods, the difference is small. Over decades, it's enormous: $10,000 at 7% simple interest for 30 years grows to $31,000. At 7% compound interest, it grows to $76,123 — more than 2.4× more.
How Regular Contributions Supercharge Compound Growth
Adding regular contributions (monthly or annual) dramatically accelerates compound growth. Investing $500/month at 7% annual return for 30 years results in $567,000 — from just $180,000 in total contributions. The extra $387,000 is entirely from compound interest. Starting 10 years earlier could nearly double the final amount. Time in the market is the most powerful variable.
Sources & Methodology
Calculations are based on the most current publicly available data from authoritative government and industry sources: