Free compound interest calculator. It shows what you put in, what growth added, and what share of the final total is earnings rather than contributions — plus what a fee costs you over the same period.
Runs in your browser. No signup, nothing stored.
Compound interest calculator
Shows what you put in, what growth added, and how much of the final total is earnings rather than contributions.
Assumes a constant return compounded monthly, which real markets never deliver — the destination figure over long horizons is what the assumption is for, not the smooth path. Educational tool, not financial advice, and not a prediction.
Read the growth share, not just the total
The headline number is the least interesting output. The useful one is what share of the total is growth, because it tells you where you are on the curve. Early on, almost everything is your own contributions; late on, most of the balance is earnings compounding on earnings. That shift is the entire reason compounding gets called magic, and it is just exponential arithmetic meeting patience.
The fee field is there for a reason
Fees are quoted as a percentage that sounds trivial. Enter one and the calculator prices it properly: a fee is not a slice of your contribution, it compounds against you every year for the life of the account. Over a working lifetime a one-percent difference consumes a meaningful fraction of the final total — which is why the expense ratio deserves veto power over a fund choice.
What a constant rate does and does not mean
This model assumes a fixed return compounded monthly. Real markets never deliver that; they deliver the same destination by a far uglier path, and only for people who stayed invested through the ugly parts. Treat the smooth curve as an arithmetic illustration of a long-run assumption, not a forecast — and treat anyone showing you double-digit projections as someone selling something.
The worked numbers and the honest caveats are in Compound Interest Isn’t Magic, and the fee arithmetic is in Index Funds for Beginners. Before investing, check the sequence: high-interest debt and an emergency fund come first.
Educational tool, not personalized financial advice, and not a prediction.