Compound Interest Calculator
See how your savings or investments grow over time with the power of compounding — including year-by-year projections.
Year-by-Year Growth
| Year | Balance | Contributions | Interest Earned |
|---|---|---|---|
Final Balance
The Rule of 72 — How Long to Double Your Money
The Rule of 72 is a simple mental math shortcut for estimating how long it takes to double an investment. Divide 72 by the annual interest rate, and you get the approximate number of years. At 6% annual return, it takes roughly 12 years to double (72 ÷ 6 = 12). At 9%, it's only 8 years.
This rule reveals why even small differences in interest rates compound dramatically over time. The difference between a 6% and a 9% return might seem small year-to-year, but over 30 years, the 9% portfolio can be more than double the size of the 6% portfolio.
The flip side is also true: debt compounds against you. Credit card debt at 24% APR doubles in just 3 years by this rule. High-interest debt should always be addressed before prioritizing investments, since the guaranteed return of paying off 24% debt beats nearly any investment.
How Monthly Contributions Supercharge Compound Interest
The biggest lever most investors underestimate is consistent monthly contributions. While a large lump sum grows well, adding even a modest amount each month dramatically accelerates wealth accumulation because each new contribution immediately starts earning compound returns.
Consider the difference: $10,000 invested at 7% for 30 years grows to about $76,000. But if you add just $200/month, the final balance jumps to roughly $320,000 — more than 4 times higher, from contributions that total only $72,000. The interest earned on those contributions makes up the difference.
Starting early matters enormously. A person who invests $200/month for 30 years will typically end up with more than someone who invests $400/month for only 15 years, even though the second person contributed the same total amount. Time in the market beats timing the market.