Compound Interest Calculator
See how your investments grow over time with compound interest and regular contributions. Built for the USA and Canada, with any compounding frequency and an optional inflation adjustment.
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How compound interest works
Compound interest is interest earned on both your original money and the interest it has already earned. Over time that snowball effect does most of the heavy lifting, which is why starting early matters so much. The standard formula for a lump sum is:
Here P is your starting amount, r is the annual rate, n is how many times a year interest compounds, and t is the number of years. This calculator extends that formula to include your regular contributions, simulating each month so the result stays accurate for any compounding frequency. For a full step-by-step walkthrough, see our guide on how to calculate compound interest, or the regulator-backed explainer from the US Securities and Exchange Commission at Investor.gov.
What a realistic return rate is
The rate you choose drives everything, so it pays to be realistic. Over the long run, a broad stock-market index has historically returned roughly 7 to 10 percent per year before inflation, while safer holdings like high-interest savings or GICs return much less. A single annualized figure is the fairest way to compare options, which our guide on how to calculate ROI explains. Because markets do not move in a straight line, investing a fixed amount on a schedule, known as dollar-cost averaging, smooths out the timing. And once you are planning to live off your savings, the 4% rule helps translate a balance into sustainable income. Explore all our free financial calculators to plan the next step.
Frequently asked questions
Compound interest is interest calculated on your original principal plus all the interest already added. Because each period earns interest on a larger balance, growth accelerates over time, unlike simple interest, which is paid only on the original amount.
Slightly. More frequent compounding earns a little more, but the difference between daily and monthly is small at normal rates. The rate, your contributions, and the number of years matter far more than the compounding frequency.
Roughly 7 percent before inflation is a common long-run assumption for a diversified stock portfolio, based on historical averages. It is not guaranteed, returns vary year to year, and safer investments earn less. Use a rate that matches how your money is actually invested.
The balance shown is before tax. You can switch on the inflation adjustment to also see the result in today’s dollars, which shows how much buying power your savings represent rather than the raw future number.
Yes. The growth math is the same for any account. Switch the country toggle to match your accounts, then enter your balance and contributions. Tax treatment differs by account type, but the projected pre-tax growth is identical.
Last updated: June 2026. Projections are estimates, not guarantees.
This tool is for general information, not financial advice. Investment returns are not guaranteed and past performance does not predict future results. Figures are before tax. Consider speaking with a qualified financial professional before making investment decisions.