Compound Interest Calculator
See how your money grows over time. Enter a starting amount, optional regular contributions, an interest rate, and a time horizon to instantly get your future value, total contributions, total interest earned, and a year-by-year growth breakdown. No sign-up — your numbers never leave your browser.
What is the Compound Interest Calculator?
The Compound Interest Calculator is a free, instant tool that shows how an investment or savings balance grows when interest is reinvested over time. Unlike simple interest, which is earned only on your original deposit, compound interest earns interest on the interest you have already accumulated — so your balance accelerates the longer it stays invested. Enter a starting amount, an optional recurring contribution, an annual rate, and a number of years, and the calculator returns your projected future value, the total you will have deposited, the total interest earned, and a complete year-by-year breakdown.
How to use it
- Enter your initial amount — the lump sum you start with (it can be 0 if you are starting from scratch).
- Add a regular contribution and choose whether you pay it monthly or yearly. Leave it at 0 for a pure lump-sum projection.
- Enter the annual interest rate as a percentage (for example, 7 for 7%).
- Pick a compounding frequency: daily, monthly, quarterly, or annually. More frequent compounding grows the balance slightly faster.
- Enter the number of years you plan to stay invested.
- Results update instantly. Open the year-by-year breakdown to see how contributions and interest stack up each year.
There is no Calculate button to wait on — everything recalculates the moment you change a value, so you can test different rates, contributions, and time horizons in seconds and close the tab when you are done.
The formulas behind it
For a one-time lump sum, this tool uses the standard compound interest formula:
A = P x (1 + r/n)^(n x t)
- A = future value of the investment
- P = starting principal
- r = annual interest rate (as a decimal)
- n = number of times interest is compounded per year
- t = number of years
When you add regular contributions, the tool layers on the future value of a series (an annuity) for those deposits:
FV = PMT x ((1 + i)^m - 1) / i
- PMT = the amount of each contribution
- i = the interest rate per contribution period
- m = the total number of contributions
Internally the calculator steps through the term one compounding period at a time, accruing interest each period and adding contributions on their own schedule. That keeps the growth accurate even when the compounding and contribution frequencies differ, and it is what lets the tool produce an exact year-by-year table.
Worked example
Suppose you start with $1,000, add $100 every month, earn a 7% annual rate compounded monthly, and stay invested for 10 years. You deposit $13,000 in total ($1,000 plus 120 monthly contributions of $100). With compounding, your balance grows to roughly $19,318 — meaning you earned about $6,318 in interest on money you simply left to compound. Push the horizon to 30 years and the same plan grows past $130,000, with the vast majority of the gain coming from interest, not deposits. That widening gap is the snowball effect of compounding.
Common use cases
- Projecting how a retirement or brokerage account could grow with steady monthly contributions.
- Comparing the long-term payoff of starting to invest now versus a few years from now.
- Estimating the future value of a savings goal — a house down payment, a child's education, an emergency fund.
- Seeing how a higher contribution or a slightly better rate changes your final balance.
- Understanding the difference between what you deposit and what compounding adds over decades.
Why use this one
Many top compound interest calculators only handle a single lump sum or hide the details behind a sign-up. This one lets you add recurring contributions, choose your compounding frequency, and view a full year-by-year growth breakdown right on the page. It is instant (results update as you type, no reload) and private: every calculation runs in your browser, so your financial numbers are never uploaded, stored, or shared. No account, no email, no app.
Results are projections for general informational purposes only and are not financial or investment advice. They assume a constant rate and exclude taxes, fees, and inflation. Real-world returns vary — always do your own research or consult a qualified professional.
Frequently asked questions
How is compound interest calculated?
For a lump sum it uses A = P x (1 + r/n)^(n x t), where P is the starting principal, r is the annual rate as a decimal, n is how many times interest compounds per year, and t is the number of years. When you add regular contributions, it also applies the future value of a series formula. Internally the tool steps through the term period by period, accruing interest and adding contributions on schedule, which is how it builds the exact year-by-year table.
Does more frequent compounding earn more?
Yes, but the difference is usually small. Daily compounding earns slightly more than monthly, which earns slightly more than annually, because interest is added back to the balance more often and starts earning its own interest sooner. The effect grows over longer horizons but is far smaller than the impact of the rate, your contributions, or the number of years.
What is the difference between simple and compound interest?
Simple interest is earned only on your original deposit, so it grows in a straight line. Compound interest is earned on your deposit plus all the interest already added, so the balance grows faster and faster over time. That accelerating curve is why starting early and staying invested matters so much.
Does this calculator account for taxes, fees, or inflation?
No. It projects gross growth based purely on your inputs and a constant rate. Taxes on gains, account or fund fees, and inflation are not included, so your real-world, after-tax, inflation-adjusted result will typically be lower. Treat the figures as an estimate for comparison, not a guarantee.
Is my financial information saved or sent anywhere?
No. The entire calculation runs in your browser. The amounts you enter are never uploaded to a server, stored, or shared — close the tab and they are gone.