Compound Interest Calculator
Calculate how an investment grows over time with compound interest and optional monthly contributions — instantly inside your browser.
What is compound interest?
Compound interest is interest calculated not just on the original principal, but also on the interest that's already accumulated — so a balance grows faster and faster over time compared to simple interest. How often that interest is added back to the balance (the compounding frequency) also affects how quickly it grows.
This calculator projects the future value of an investment given a starting amount, interest rate, time period, compounding frequency, and an optional regular monthly contribution.
GuideHow to use it
How the calculation works
The balance is projected period by period: each compounding period, the current balance earns interest at the rate divided by the compounding frequency, and any contribution for that period is added on top. Repeating this across every period in the chosen time span produces the final balance, with total interest earned being the difference between the final balance and everything you put in.
Why it helpsFeatures & benefits
Why this runs entirely in your browser
The projection runs in JavaScript locally in the tab, which means it keeps working offline once the page has loaded and nothing entered here is ever sent to a server.
Common uses
Estimating how a savings account or investment will grow over time, comparing the effect of different compounding frequencies, or planning how much a regular monthly contribution adds to long-term growth.
Frequently asked questions
A few things people usually want to know before trusting the numbers.
Yes — more frequent compounding (like monthly or daily instead of annually) results in slightly more growth for the same nominal interest rate, since interest starts earning interest sooner.
The monthly contribution is spread evenly across each compounding period and added to the balance before the next period's interest is calculated.
No. This tool projects growth assuming a constant interest rate for the whole period — real investments fluctuate, and this is meant as an estimate, not a guarantee.
Simple interest is calculated only on the original principal every period. Compound interest is calculated on the principal plus any interest already earned, which is why it grows faster over time.
No. All calculation happens locally in your browser. Nothing entered here is sent to a server, stored, or shared.