Compound Interest Calculator
Watch a small seed become a forest.
Set your principal, monthly contribution, return rate, and time horizon — and see the year-by-year curve where compounding takes over and does most of the work. 100% free.
Your inputs
All values in today's dollars.
Growth over time
Contributions are the floor; interest is the curve above it.
Year-by-year breakdown
| Year | Contributed | Interest | Balance |
|---|---|---|---|
| 0 | $10,000 | $0 | $10,000 |
| 3 | $28,000 | $4,294 | $32,294 |
| 6 | $46,000 | $13,782 | $59,782 |
| 9 | $64,000 | $29,671 | $93,671 |
| 12 | $82,000 | $53,455 | $135,455 |
| 15 | $100,000 | $86,971 | $186,971 |
| 18 | $118,000 | $132,486 | $250,486 |
| 21 | $136,000 | $192,796 | $328,796 |
| 24 | $154,000 | $271,345 | $425,345 |
| 27 | $172,000 | $372,384 | $544,384 |
| 30 | $190,000 | $501,150 | $691,150 |
Future value
$691,150
after 30 years at 7% (7.23% APY)
Total contributions
$190,000
Total interest
$501,150
Interest share
73%
Money doubles in
10.0 yrs
Compounding does 73% of the work.
Of your $691,150 final balance, $501,150 is pure growth — your money making money.
Stay invested. The last decade of any 30-year run produces more growth than the first two combined.
If you stopped contributing today
$81,165
Your starting $10,000 alone grows to this in 30 years. Contributions add another $609,985.
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Frequently asked questions
What is compound interest?
Compound interest is interest earned on both your original principal and the interest already accumulated. Each period, your base grows, so the next round of interest is calculated on a larger number — that's why long horizons turn small contributions into large balances.
How often should interest compound?
More frequent compounding produces slightly more growth, but the difference between monthly and daily compounding is tiny at typical rates. Most investment accounts effectively compound continuously through reinvested dividends and price growth.
What's a realistic return to assume?
The long-run average for a broad U.S. stock index is roughly 7% real (after inflation) or ~10% nominal. Use 6–7% for retirement projections to stay realistic; higher numbers make charts look great but set you up for disappointment.
Does the rule of 72 actually work?
Yes — divide 72 by your annual interest rate and you get a close estimate of how many years it takes your money to double. At 7%, that's about 10.3 years. The calculator shows the precise doubling time based on your inputs.
Is the Compound Interest Calculator free?
Yes — the calculator, growth chart, year-by-year breakdown, and coaching are completely free. Premium unlocks saving unlimited scenarios, side-by-side comparisons, a printable PDF report, and email progress reminders.
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