Compound Interest Calculator
See how your investment grows over time with compound interest — instantly, with optional monthly contributions.
Growth over time
This calculator projects a result assuming a constant interest rate for the entire period — real investments fluctuate and returns are never guaranteed. Use it to compare scenarios, not as financial advice or a promise of actual returns.
How it works
Enter your numbers
Principal, annual rate, years and how often interest compounds.
Add contributions if you like
Turn on the optional monthly contribution to see how regular deposits change the outcome.
See your projected growth
Future value, total interest earned, and a year-by-year chart update instantly.
What people use a compound interest calculator for
Seeing the numbers ahead of time makes long-term decisions easier.
Savings accounts
Investment planning
Retirement goals
College savings
Comparing accounts
A few things worth knowing about compound interest
Compound interest is often summarized as "interest on interest" — instead of only earning a return on the money you originally put in, each round of interest gets added to the balance and starts earning its own return too. Over short periods the effect is small, but stretched across ten, twenty or thirty years, that snowball is usually what separates a modest ending balance from a dramatically larger one, even when the starting amount and the rate stay exactly the same.
Compounding frequency — how often interest is calculated and added, whether annually, quarterly, monthly or daily — changes the result too, but usually by less than people expect. Going from annual to daily compounding on the same nominal rate nudges the outcome up a little; it doesn't replace the much bigger effect of a higher rate or, especially, more time. Time in the market tends to matter more than fine-tuning the compounding schedule.
Adding a regular contribution changes the shape of the growth curve more than almost anything else on this page, since new money keeps arriving and compounding on its own timeline in addition to the original principal. And because every projection here assumes one constant, unbroken interest rate for the entire period, it's worth treating the future value as an illustration of how the math works rather than a guarantee — real accounts and markets don't move in a straight line.
Frequently asked questions
How is compound interest calculated?
Compound interest is calculated by applying the interest rate to the current balance at each compounding period, so interest itself starts earning interest. This calculator uses the standard formula: future value = principal × (1 + rate/n)^(n × years), where n is the number of compounding periods per year.
What does compounding frequency change?
More frequent compounding (daily vs. annually, for example) applies interest in smaller steps more often, which produces a slightly higher future value for the same nominal annual rate — though the difference is usually modest compared to changing the rate or the time horizon.
How are monthly contributions included in the calculation?
Monthly contributions are treated as a separate stream added at the end of each month and grown at the equivalent monthly rate implied by your chosen compounding frequency, then combined with the growth of your initial principal for the total future value.
Is the future value guaranteed?
No. This calculator projects a result assuming a constant, unchanging interest rate for the entire period, which real investments rarely provide. It's useful for comparing scenarios and understanding how rate, time and contributions interact, not as a promise of actual returns.
Is this compound interest calculator free to use?
Yes, completely free, with no account or sign-up, and it runs entirely in your browser.