Free Compound Interest Calculator
Calculate compound interest growth on an initial balance plus optional monthly contributions. Free. No signup required.
60% of your final balance is money you never had to put in yourself — that's compounding doing the work. Tap any bar to see that year's balance.
- Total Contributed$58,000.00
- Interest Earned$86,572.72
ƒShow your work
A = P(1 + r/n)^(nt), contributions grow as an ordinary annuity
- 1Principal growth: $10,000.00 × (1 + 7%/12)^(12×20) = $40,387.39
- 2Contributions growth: $200.00/mo for 240 months at 0.5833%/mo = $104,185.33
- 3Total future value = $40,387.39 + $104,185.33 = $144,572.72
About the Free Compound Interest Calculator
Compounding is the whole reason this calculator exists as something separate from a simple multiplication: interest earned in year one starts earning its own interest in year two, so an account's real growth curve bends upward instead of climbing in a straight line. A back-of-envelope estimate (just rate times years times principal) badly understates long horizons — $10,000 at 7% for 20 years isn't $24,000, it's closer to $38,700, and the gap is entirely compounding.
It's used by long-term savers and investors projecting where a lump sum plus recurring monthly contributions will land years or decades out, by people comparing how much compounding frequency (daily vs. monthly vs. annual) actually matters, and by anyone who's heard the phrase "compound interest is the eighth wonder of the world" and wants to see their own numbers instead of someone else's example.
Your balance, your contribution amount, your timeline to a goal — none of it is sent anywhere or stored. The math runs entirely in your browser, so you can project real numbers for a real financial decision without creating a record of what you're planning.
How it’s calculated
The principal grows on its own compounding schedule — A = P(1 + r/n)ⁿᵗ, where n is how many times per year interest compounds. Going from annual to monthly compounding makes a real difference on paper; going from monthly to daily makes almost none, since the extra compounding periods are tiny fractions of the rate applied very slightly more often.
Monthly contributions are treated separately as an ordinary annuity, compounding monthly regardless of the compounding frequency you picked for the lump sum, since recurring deposits are inherently a monthly event. The two growth streams (initial principal and ongoing contributions) are calculated independently, then added together for the total.
Frequently asked questions
What's the actual difference between compound interest and simple interest?
Simple interest is calculated only on the original principal every period, so it grows in a straight line. Compound interest is calculated on the principal plus all interest already earned, so it grows on an upward curve — the longer the time horizon, the bigger that gap gets.
Does compounding monthly vs. daily actually matter?
Barely, in practice. The jump from annual to monthly compounding is the one that meaningfully changes your total; going from monthly to daily typically changes the result by well under 1% over most realistic time horizons.
How much difference do small monthly contributions really make?
More than intuition suggests, because every contribution gets its own compounding runway — a dollar contributed in year one compounds for the full period, while a dollar contributed in year nineteen barely compounds at all. Run the numbers with and without the monthly contribution field to see the gap for your own timeline.
What interest rate should I use to estimate stock market growth?
There's no single right answer, but a common long-run reference point for a diversified US stock portfolio before inflation is around 7-10% annually — and that figure includes years of sharp losses smoothed out over decades, not a steady year-by-year return.
Why doesn't my result match the "Rule of 72" estimate?
The Rule of 72 (divide 72 by your rate to estimate years to double) is a quick mental shortcut, not an exact formula — it's most accurate for annual compounding around 6-10% and drifts further off at very high or low rates, or with contributions added along the way.
Related calculators
Powered by GetCalculator.online