Compound Interest (US)

The Compound Interest Calculator helps you project how your savings and investments grow over time by applying the power of compounding — where interest earns interest on itself. Whether you are setting up a long-term retirement fund, evaluating a fixed deposit, or simply curious about how a lump sum grows at a given annual rate, this tool gives you a clear, itemized projection. Enter your starting principal, set the annual interest rate and compounding frequency, add optional recurring monthly contributions, and choose your investment horizon in years. The calculator instantly displays your projected final balance alongside a breakdown showing exactly how much came from your own contributions versus interest earned. This distinction matters because it reveals the true impact of compounding: even modest monthly additions can generate tens of thousands in interest over a couple of decades. The underlying math uses the standard compound interest formula A = P(1 + r/n)^(nt) for the lump sum, combined with the future value of an annuity formula for regular contributions. You can adjust any parameter — rate, term, contribution amount, or compounding frequency — to run unlimited what-if scenarios and compare outcomes side by side. The tool is especially useful for students learning about exponential growth, young professionals planning early investments, and anyone evaluating savings accounts, certificates of deposit, or index fund projections. Every calculation runs entirely inside your web browser using JavaScript, so your financial figures are never transmitted to or stored on any server. There are no sign-ups, no ads interrupting your workflow, and no daily usage caps. Use it as often as you need to plan confidently and see exactly how time in the market can work in your favor.

1. Enter your starting principal (the lump sum you already have invested).
2. Set the annual interest rate and choose the compounding frequency (monthly is the default).
3. Add a recurring monthly contribution and the number of years you plan to invest.
4. Read the projected balance and the split between your own contributions and interest earned.

Compound interest is the reason time in the market beats timing the market: interest earns interest, so early money grows disproportionately. This calculator makes that curve visible so you can see exactly how much a modest monthly contribution adds over 10, 20, or 30 years.

• Monthly compounding with support for regular recurring contributions.
• Separates total interest earned from the money you actually put in.
• Adjustable rate, term, and contribution amount for quick what-if scenarios.
• Shows how small contribution increases compound over decades.
• Runs entirely in your browser — no figures are sent to a server.

The principal grows as A = P(1 + r/n)^(nt), where P is principal, r is the annual rate, n is compounds per year, and t is years. Regular contributions use the future value of a series: FV = PMT × [((1 + r/n)^(nt) − 1) / (r/n)]. The total balance is the sum of both. Each extra contribution has less time to compound than earlier ones, so contributing sooner (not just more) is what accelerates growth.