Compound Interest Calculator

Match the rate to where the money actually sits. For savings accounts and CDs, use the APY your bank quotes (the FDIC publishes national average deposit rates for comparison). For long-term diversified stock investments, a common planning assumption is 7%–10% a year before inflation, based on historical averages — with big year-to-year swings and no guarantee. When in doubt, run the calculator at a lower and a higher rate and treat the spread as your realistic range.

Each dollar compounds for however many years remain, so a dollar invested in year 1 of a 20-year plan grows through all 20 years, while a dollar added in year 15 only gets 5. That's why starting earlier — even with smaller amounts — often beats contributing more but later.

No. This shows nominal growth before taxes and inflation. In a tax-advantaged account (401(k), IRA) growth compounds untaxed until withdrawal (or tax-free in a Roth); in a taxable account, tax on interest, dividends, or realized gains will slow the compounding. And at ~3% inflation, money roughly halves in purchasing power over 24 years — so think of long-horizon results in today's-dollar terms with caution.

Less than most people expect. At 7%, monthly compounding instead of annual adds only about 0.23 percentage points of effective annual return. The rate itself and the number of years dwarf the effect of compounding frequency.

A quick mental shortcut for how long money takes to double: divide 72 by your annual return. At 7% a year, 72 ÷ 7 ≈ 10.3 years to double; at 9%, about 8 years. It's an approximation — it works best for returns in the 6%–10% range and assumes you're not adding contributions — but it's a fast sanity check on the compounding you see in the results here. It also cuts the other way for debt: a balance at 22% interest doubles in a little over three years if you don't pay it down.