Compound Interest Calculator
Calculate how your money grows over time with compound interest -- model recurring contributions, compounding frequency, fees, and inflation to see your real future balance.
Last updated August 24, 2026
Compound Interest Calculator
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Compound interest is what happens when the interest (or investment return) your money earns starts earning its own return. Leave $10,000 earning 7% a year and after year one you have $10,700 -- but in year two, you earn 7% on the full $10,700, not just the original $10,000. Over enough years, the growth on your growth becomes larger than the growth on your original contributions.
This calculator models that effect precisely: a starting amount, recurring contributions on whatever schedule you choose, a chosen compounding frequency, and optional fee and inflation adjustments -- so you can see not just a final number, but exactly how much of it came from what you put in versus what your money earned on its own.
How It Works
The core formula
For a lump sum with no further contributions, future value follows the standard compound-interest formula:
FV = P × (1 + r/n)n×t
where P is your starting amount, r is the nominal annual rate, n is how many times per year it compounds, and t is the number of years.
Recurring contributions: a real simulation, not a shortcut
Contribution frequency and compounding frequency are independent choices in this calculator -- contributing monthly while interest compounds quarterly, for example, is common and is not silently assumed to match. Rather than force a single formula to handle every combination, the engine simulates growth month by month: any compounding frequency you choose converts to an exact monthly-equivalent growth rate (so a switch between monthly, quarterly, semiannual, annual, or daily compounding never changes the true annual growth those frequencies imply), and your contributions post on their own schedule on top of that.
Contribution timing
End of period (an "ordinary annuity" in financial terminology) means each contribution is added after that period’s growth has already applied -- the standard assumption for most savings and retirement accounts, where a paycheck contribution lands and starts growing the following period. Start of period (an "annuity due") means each contribution is added before that period’s growth applies, so it earns a full extra period of growth compared to the same contribution made at the end. Over a long horizon, start-of-period contributions produce a meaningfully larger balance for exactly this reason -- not because of a different rate, but because of one additional period of compounding per contribution.
Fees
When you enter an annual fee or expense ratio, it is subtracted directly from your entered annual return before compounding -- the same way a mutual fund or ETF’s quoted return is already net of its expense ratio, rather than billed as a separate flat charge. A 7% expected return with a 1% fee compounds as an effective 6% return in this calculator, and the "Cost of Fees" figure in your results is the difference between your balance with and without that fee, holding everything else equal.
Inflation
If you enter an inflation rate, the calculator shows a second figure -- your final balance divided by (1 + inflation rate)years -- representing your balance in today’s purchasing power. This never replaces or is averaged into your nominal final balance; both numbers are shown side by side so you can see the real growth of your purchasing power alongside the nominal dollar total.
Understanding Your Results
Final Balance is your total nominal balance at the end of the period you entered -- the actual dollar amount, not adjusted for inflation. Total Growth and Total Contributions split that final balance into what you put in yourself versus what your money earned; Growth Share of Final expresses that split as a percentage, which tends to rise the longer your money has to compound.
Effective Annual Rate converts your nominal rate and compounding frequency into the single annual percentage yield (APY) that would produce the same growth if it compounded only once a year -- useful for comparing accounts that quote rates with different compounding frequencies. Monthly Contribution Equivalent restates your recurring contribution as a steady monthly amount regardless of the frequency you actually chose, so a quarterly or annual contribution plan is still easy to compare to a monthly budget.
If you entered an inflation rate or a fee, you will also see the inflation-adjusted value (your balance in today’s purchasing power) and the cost of fees (how much smaller your balance is because of the fee, compared to the same plan at 0% fee) -- both are secondary figures next to, never in place of, your nominal final balance.
The year-by-year table beneath the chart shows the same three figures -- contributions, growth, and balance -- for every year of your timeline, so you are never limited to reading values off the chart alone.
Compound interest vs. simple interest
Simple interest only ever applies to your original principal: $10,000 at 7% simple interest earns exactly $700 every year, forever. Compound interest applies to your growing balance, so the same $10,000 at 7% compounded annually earns $700 in year one, but $749 in year two (7% of $10,700), $801.43 in year three, and so on -- an amount that keeps increasing every year even though the rate never changes. Over short periods the difference is small; over decades, it is the entire reason long-term investing works the way it does.
Worked example: the cost of waiting
Consider two savers, each planning to contribute $300 a month at a 7% annual return, compounded monthly. The first starts at age 25 and stops contributing at 35 (10 years of contributions, then lets it sit until 65). The second starts at 35 and contributes every month until 65 (30 years of contributions). Run each scenario through this calculator and you will see the first saver, despite contributing for only a third as long, ends up with a comparable or larger balance by 65 -- because those early contributions had decades longer to compound. This is the single most common illustration of why starting earlier tends to matter more than contributing more, dollar for dollar.
Compounding frequency in practice
Switching from annual to monthly compounding at the same nominal rate increases your effective annual rate slightly (more frequent compounding means interest starts earning its own interest sooner), which is why the calculator reports an explicit Effective Annual Rate figure -- so you can compare a 7% rate compounded monthly against a 7.1% rate compounded annually on equal footing, rather than assuming the higher quoted number always wins.
What this calculator does not do
It does not model taxes on investment growth (tax treatment depends heavily on account type -- a 401(k), Roth IRA, and taxable brokerage account are all taxed completely differently -- and mixing that into a general compounding calculator would misrepresent the result for most account types). It does not guarantee any rate of return; every number here is a projection based on the constant annual rate you enter, and real investment returns vary year to year, sometimes losing value, in ways a single average rate cannot capture.
Pros and Considerations
Benefits
- •Simulates recurring contributions on their own schedule, independent of compounding frequency, instead of assuming they match
- •Models contribution timing (start vs. end of period), fees, and inflation as separate, clearly labeled figures
- •Shows a full year-by-year breakdown, not just a single final number
- •Free, no account required, and does not require contact information to see a result
Considerations
- •Assumes a constant annual rate of return for the entire period -- real investments fluctuate year to year and can lose value
- •Does not model taxes, which vary significantly by account type (401(k), Roth IRA, taxable brokerage, etc.)
- •Comparison scenarios are mathematical variations on your inputs, not predictions of future performance
Important Notes
- •This calculator assumes a constant annual rate of return for the entire time period entered. Real investment returns vary year to year, including years with losses, in ways a single average rate cannot represent.
- •Fees are modeled as a flat annual percentage drag on the return you enter. Real fund expense ratios, advisory fees, and account fees can vary and may be structured differently than a simple annual percentage.
- •This calculator does not model any taxes on investment growth or withdrawals. Tax treatment depends on your specific account type and current tax law.
- •The comparison scenarios are mathematical variations on your entered assumptions, not predictions or guarantees of future returns.
Warnings
- •This tool provides an estimate for educational and planning purposes only. It is not financial or investment advice, and it does not guarantee any rate of return.
- •Results depend entirely on the assumptions you enter, especially the annual rate of return, which you should treat as a planning assumption, not a promise.
- •Consult a qualified financial professional before making investment decisions.