Compound Interest Calculator
See how an initial deposit and regular contributions may grow over time with compound interest.
Compound Interest Calculator
Compound interest is what turns a modest, steady habit into serious long-term growth — this calculator projects how an initial deposit plus regular contributions could grow over time, making the effect of starting earlier (or saving a bit more each month) far easier to see than doing the math by hand.
What is compound interest, and how is it actually calculated?
Compound interest is interest calculated on both your original amount and any interest it has already earned, so growth accelerates over time instead of staying flat — each period's interest is calculated on a growing balance, not just the starting amount. The formula is A = P(1 + r/n)^(nt), where P is your initial amount, r is the annual interest rate, n is how many times per year it compounds, and t is the number of years; this calculator also lets you add ongoing monthly contributions on top of that initial amount. It shows gross growth only — taxes on investment income vary too much by country and account type to build in automatically.
What's the difference between simple and compound interest, and does compounding frequency matter?
Simple interest is calculated only on the original principal, so it grows at a constant rate; compound interest is calculated on the principal plus whatever interest has already accumulated, so growth speeds up the longer money stays invested. Compounding frequency matters too, though less dramatically — interest that compounds daily earns slightly more over time than interest compounding annually, since each period's interest starts earning its own interest a little sooner.
What is the Rule of 72, and how long does it actually take for money to double?
The Rule of 72 is a quick mental shortcut for estimating how long an investment takes to double: divide 72 by the annual interest rate. At 6% annual growth, for example, that's 72 ÷ 6 = 12 years. The real answer also depends on your starting balance, any ongoing contributions and the compounding frequency, which is exactly what entering your own numbers above calculates precisely rather than approximately.
Does starting a few years earlier really make that much difference?
Often, yes — substantially. Because each additional year lets interest compound on top of previously earned interest, a few extra years at the start of a savings timeline can outweigh several years of larger contributions started later; the extra compounding time is frequently worth more than the extra money would have been.
Does compound interest work the same way for debt?
Yes — and that's the less pleasant side of it. Credit cards, loans and other debt can accrue interest on top of unpaid interest the same way savings compound, which is why an unpaid balance can grow faster than expected if it's left unaddressed.
Where does the idea of compound interest actually come from?
You may have heard that Einstein called compound interest "the eighth wonder of the world" — it's a great line, but there's no evidence he ever said it: none of his collected papers or the Einstein Archives mention compound interest at all, and the quote's earliest known appearance is in a 1983 advertisement, nearly three decades after his death. The real mathematical history is arguably more interesting: in 1683, Swiss mathematician Jacob Bernoulli was studying what happens to compound interest as the compounding frequency increases toward continuous — compounding not yearly or monthly, but constantly — and in working through that problem, he stumbled onto the mathematical constant e (approximately 2.71828), one of the most important numbers in all of mathematics, discovered by asking exactly the kind of "how much does more frequent compounding actually matter" question this calculator answers today.
Is Compound Interest Calculator free to use?
Yes. Compound Interest Calculator is completely free, with no sign-up and no usage limits.
Do I need to install any software?
No. Compound Interest Calculator runs directly in your web browser — nothing to download or install.
Is my data kept private?
Everything happens on your device. We never receive or store the file or text you enter into Compound Interest Calculator.