Compound Interest Calculator
Compound interest is often called the eighth wonder of the world — and for good reason. Unlike simple interest, compound interest earns returns on your returns. Over decades, this snowball effect can turn modest, consistent contributions into life-changing wealth.
Compound Interest Calculator
Final Balance after 20 years
$142,438
Total Contributed
$53,000
Interest Earned
$89,438
Growth over time
How to Use This Calculator
- 1 Enter your initial investment (lump sum you're starting with).
- 2 Add a monthly contribution if you plan to invest regularly.
- 3 Set your expected annual return rate (historical S&P 500 average: ~10%).
- 4 Choose how many years you'll let the money grow.
- 5 Select compounding frequency — more frequent compounding means slightly more growth.
Wealth-Building Tips
- ✓ Starting at 25 instead of 35 can more than double your retirement balance — time is your biggest asset.
- ✓ Consistent monthly contributions often matter more than a large one-time investment.
- ✓ Tax-advantaged accounts (401k, IRA, Roth IRA) let your gains compound without annual tax drag.
- ✓ Inflation erodes purchasing power — the real return is your nominal return minus inflation.
Frequently Asked Questions
What is compound interest?
Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. In investing, it means your gains generate their own gains — creating exponential growth over time.
How often does compounding matter?
More frequent compounding yields slightly more — daily compounds slightly more than monthly, which compounds slightly more than annually. For most long-term investors, the difference is modest compared to the impact of return rate and time.
What is the Rule of 72?
Divide 72 by your annual return rate to estimate years to double your money. At 8% annual return, your money doubles in roughly 9 years (72 ÷ 8 = 9).
What average return rate should I use?
The S&P 500 has historically returned about 10% annually (7% inflation-adjusted). Conservative planners use 6–7%. Bond-heavy portfolios might use 3–5%. HYSA/CDs currently offer 4–5%.
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