Compound Interest Calculator
How your savings grow over time
Saving or investing? Enter a starting balance, monthly contribution, expected return and time horizon to see how compounding grows your money.
The power of compounding
Compound interest means you earn returns on your past returns, not just your contributions. The two biggest levers are time and rate — small differences in either produce huge differences decades out. Try adding ten years and watch the "interest earned" line.
Why starting early beats saving more
Because growth compounds on itself, a dollar invested in your twenties can outweigh several dollars invested in your forties. Someone who saves for ten years and then stops often ends up ahead of someone who starts ten years later and saves for decades — the early money simply has more time to multiply.
Compounding frequency and real returns
More frequent compounding (monthly vs. annually) helps a little, but rate and time matter far more. Remember that inflation eats into returns: a 7% nominal return is closer to 4% in real buying power, so for long-term goals it's wise to think in inflation-adjusted terms (see the inflation calculator).
Compound Interest Calculator: frequently asked questions
How does compound interest work?
Each period's earnings are added to your balance, so future earnings are calculated on a larger amount — growth accelerates over time.
What return should I assume?
Historically the broad US stock market has averaged roughly 7% per year after inflation, but returns vary and aren't guaranteed.
What's the rule of 72?
Divide 72 by your annual return to estimate the years it takes money to double. At 7% a year, money doubles in roughly 72 ÷ 7 ≈ 10 years — a quick mental shortcut for the power of compounding.