About CompoundCalc
CompoundCalc exists because most compound interest calculators give you a number and no way to check it. This one shows the formula, the assumptions and the year-by-year path behind every result, so you can tell whether the answer deserves to be believed.
Why the site was built
There is a specific error that runs through an astonishing share of published compound interest content: treating a stream of monthly deposits as if every deposit had been sitting in the account since day one. It inflates results by tens of thousands of dollars on a thirty-year projection, and it appears in calculators, in spreadsheets and in articles from sites far larger than this one.
CompoundCalc was built around the opposite instinct — step through every compounding period, show the intermediate balances, and let the reader see where the growth actually comes from. Once the engine did that correctly, the guides followed, because the interesting part of compound interest is not the final number but the shape of the curve that produces it.
What is here
Alongside those sit scenario pages — $10,000 at 7% for 30 years, $100 a month for 10 years, how long to reach $500,000 — because that is how people actually phrase the question. Each one is a real calculation you can adjust, not a static article.
How the numbers are checked
Every figure printed on this site is reproduced independently of the JavaScript that generated it, and compared, before it is published. Lump-sum growth is checked against the closed-form compound interest formula, contribution growth against the ordinary-annuity formula, and both against a period-by-period simulation. Where a mathematical identity exists — beginning-of-period contributions must equal end-of-period contributions multiplied by exactly 1 + r/n — it is used as a trap for logic errors.
The full process, including how corrections are handled, is on the editorial policy page. It is not decoration: it has caught published-number errors that were wrong in the site's favour.
What this site is not
- Not a financial adviser. Nothing here is personal financial, tax or legal advice, and CompoundCalc is not regulated by any financial authority.
- Not a forecaster. The tools model constant-rate scenarios. Real markets do not deliver constant rates, and the disclaimer sets out exactly where the model stops being useful.
- Not affiliate-funded. There are no referral links to brokers, banks or apps, so there is no product the site benefits from recommending.
- Not a data collector. Nothing you type into a calculator leaves your browser. There is no server to send it to.
How the site is funded
Display advertising, and nothing else. Ads are labelled, kept out of the calculator's way, and excluded entirely from the methodology and legal pages. Advertisers have no influence over the content — see the editorial policy for the standing rules.
How to get the most out of it
Contact and policies
Corrections, questions and requests for a calculator that does not exist yet all go to contact@compoundcalc.com. Corrections are the most welcome of the three.