SmartRateToolsCompound Interest Calculator
See how your money grows over time — and how much of a difference monthly contributions make. Adjust any input to update results instantly.
Set to 0 if you only want growth on your starting amount.
S&P 500 historical avg ≈ 10% (7% inflation-adjusted)
Final Balance
$345,742
After 20 years
Total Contributed
$130,000
Interest Earned
$215,742
Contributions vs. Growth
What is compound interest?
Compound interest is interest calculated on both your original deposit and the interest you've already earned. Unlike simple interest (which only calculates on the original amount), compounding means your money grows faster over time — interest earns interest.
Why does it matter?
The longer you leave money invested, the more dramatic the compounding effect. A 25-year-old investing $5,000/year at 8% will have far more by retirement than a 35-year-old doing the same — despite investing only 10 fewer years. Time is the most powerful ingredient.
Note: This calculator assumes a fixed annual return and doesn't account for taxes, inflation, or investment fees. Real returns will vary.
Related reading: What is compound interest?, APR vs APY, and Compound Interest (glossary).