Compound Interest Calculator

Watch your money grow — with or without monthly contributions.

Long-run US stock market average is roughly 7% after inflation adjustments vary — treat any rate as an assumption, not a promise.

Why time matters more than amount

Compounding rewards earliness disproportionately. Contributing $300 a month starting at age 25 versus age 35 can produce a retirement balance that is roughly double — not because you saved twice as much, but because the earliest dollars had ten extra years to compound.

The rule of 72

A quick mental shortcut: divide 72 by your annual return rate to estimate how many years it takes money to double. At 7%, that's about 10.3 years; at 10%, about 7.2 years.

Compounding is the only "force" in finance that works for you while you sleep — but only if you give it decades, not months.

Assumptions to keep in mind

  • Real-world returns are not smooth; annual results vary widely.
  • Taxes and fees are not modeled here — they reduce effective returns.
  • Inflation reduces the purchasing power of future dollars.