Compound Interest Calculator: Compare 3 Scenarios
How do different growth rates, investment durations, starting values and annual contributions effect your long term investment performance? Now you can find out! With our Interactive Compound Interest Calculator you can compare up to 3 investments, each with different initial values, annual contributions, compound growth rates and time horizons. Check it out:
To use simply enter up to 3 different starting values, annual contributions, growth rates and durations:
For example, already pre-filled out in the calculator is 3 different investments. Each starts with $5,000 initially, but they have different annual contributions ($5,000, $4,800 and $4,500), growth rates (6%, 5% and 4%) and durations (20, 22 and 25 years). Notice that they all come out to roughly similar totals – even though investment 3 has a much lower growth rate, and has lower annual contributions – showing the importance that your investment horizon has on your performance!
Having trouble? See more detailed instructions and definitions below.
Some terms that may need defining:
Initial Investment – The amount of money to start your investment.
Annual Additions – This is the amount of additional money you contribute to the investment each year.
Growth Rate – This is the rate that your investments grow each year.
Duration – This is how long you let your investment ride.
Like this? Check out our other investing calculators:
Expense Ratio Calculator – How much is that fund’s expense ratio hurting your investment performance? Now you can easily find out. Compare 2 investments with different expense ratios and see the difference!
How to use this calculator
Each of the three columns is an independent scenario. Fill in a starting amount, a recurring contribution, an assumed annual growth rate, and a number of years, and the results update as you type.
The controls above the columns apply to all three at once, you are changing one variable and holding the rest still. You can set how often interest compounds, how often contributions are made, whether contributions land at the start or the end of each period, whether contributions rise each year, and whether results are shown in nominal dollars or adjusted for inflation.
Two features to know: The scenario names are editable. Click the heading and type over it, so a comparison reads “Start now” against “Start in ten years” rather than “Investment 1” against “Investment 2.” And, the presets across the top load complete three-way comparisons (cost of waiting, lump sum against monthly investing, and three contribution levels) so you can see the shape of a question before building your own version of it.
When you have something worth keeping, the share button copies a link that encodes every input, and the download button saves the chart as an image.
4. What compound interest is, and the formula behind it
Compound interest is interest calculated on the money you have already earned, not only on the money you put in. Simple interest on $10,000 at 7% pays $700 a year forever. Compound interest pays $700 in the first year, then $749 in the second, because the second year’s interest is calculated on $10,700.
The standard formula for a lump sum is:
A = P(1 + r/n)^(nt)
Where A is the ending balance, P is the starting principal, r is the annual interest rate as a decimal, n is the number of times interest compounds per year, and t is the number of years.
Recurring contributions are handled with the future value of an annuity, added to the lump-sum result:
FV = C × [((1 + i)^N − 1) / i]
Where C is the contribution per period, i is the rate for that period, and N is the total number of contribution periods. If contributions are made at the start of each period rather than the end, the whole term is multiplied by (1 + i).
This calculator runs the arithmetic period by period rather than through the closed-form equation, which produces the same answer and also generates the year-by-year table. Where the compounding schedule and the contribution schedule differ — daily compounding with monthly contributions, for instance — the annual rate is converted to an equivalent rate for the contribution period before the two are combined. Calculators that skip that conversion overstate results, sometimes by a wide margin over long horizons.
5. Comparing three scenarios, and why it changes the answer
A single-scenario calculator tells you what a plan produces. Three scenarios tell you what the difference between plans is worth.
Take the simplest version: At an assumed 7% annual return compounded monthly, $100 a month for thirty years grows to about $121,997. Two hundred fifty dollars a month grows to about $304,993. Five hundred dollars a month grows to about $609,985. Contributions scale linearly, so the ending balances do too — which is a useful thing to see plainly, because it is not what most people expect. The magic in compounding is not that saving more produces disproportionately more. It is what happens when you change the other variables.
Hold the contribution steady at $500 a month and change only the assumed rate, and thirty years produces about $502,258 at 6%, $609,985 at 7%, and $745,180 at 8%. A single percentage point, in either direction, is worth roughly $100,000 to $135,000 on the same deposits. That is the arithmetic behind almost every argument about fees, asset allocation, and the cost of sitting in cash — and it is far easier to believe when three lines are drawn on the same chart.
6. The cost of waiting to invest
The most expensive variable in the formula is the exponent, and the exponent is time.
At an assumed 7% annual return compounded monthly, $500 a month invested for forty years grows to about $1,312,407. The same $500 a month invested for thirty years grows to about $609,985. The ten years cost about $702,421 — more than the entire thirty-year result.
What makes that figure hard to internalize is that the ten missing years are the first ten, and the first ten look like the least productive ones. In the early years the balance is mostly your own deposits; growth is a rounding error. The compounding that matters happens in the final decade, and it happens on a balance that only exists because the early deposits were made. Skipping the boring years removes the fuel from the interesting ones.
The same comparison framed by age: someone contributing $300 a month from 25 to 65 accumulates about $787,444 on $144,000 of contributions. Someone starting at 35 accumulates about $365,991 on $108,000 of contributions. The second person put in 75% as much money and finished with 46% as much.
Load the Start now vs. 10 years later preset to run this with your own numbers.
Historical episodes make the same point in a less tidy way. Investors who stepped out of the market during the panics catalogued across this site — the silver collapse of 1980, the Penn Central failure of 1970, the corner of 1901 — did not merely miss a rebound. They removed years from the exponent, and those years compound in the arithmetic whether or not the decision felt reasonable at the time.
7. Lump sum vs. dollar-cost averaging
The question comes up whenever someone receives a bonus, an inheritance, or a rollover: invest it now, or spread it over the next year.
The arithmetic of compounding favors investing sooner, because money in the market earns for longer. Consider $36,000 invested at once against $300 a month for ten years, which contributes the same $36,000. At an assumed 7% annual return, the lump sum grows to about $70,817 and the monthly schedule to about $51,925. The gap of roughly $18,900 is entirely a function of the average dwell time of each dollar.
That is not the whole picture, and the calculator cannot show the rest of it. The lump sum has a worse distribution of outcomes as well as a better average: it carries the full exposure from day one, which is precisely why it earns more on average and precisely why it hurts more when the timing is poor. Studies of historical periods generally find that investing immediately beats averaging in most rolling windows, but “most” is not “all,” and the investor who cannot tolerate the bad windows is the one who abandons the plan.
A calculator answers the arithmetic question. The behavioral question — which approach you will actually stick with — is the one that decides the outcome.
Load the Lump sum vs. monthly preset to compare the two directly.
8. How fees quietly erode returns
Fees are the clearest illustration of what a single percentage point does over a long horizon, because a fee is nothing more than a permanent reduction in the compounding rate.
Take $100,000 growing for thirty years. At 7% it becomes $761,226. At 6% — the same portfolio, minus a 1% annual fee — it becomes $574,349. The difference is $186,876, or roughly 24.5% of the fee-free result.
The number that surprises people is not the dollar figure but the ratio. A 1% annual fee does not consume 1% of the outcome, or even 30% of 1% for thirty years. It consumes about a quarter of the ending balance, because each year’s fee also removes all the growth that fee would have produced in every subsequent year. The fee compounds exactly the way the returns do, in the opposite direction.
To model this, enter your assumed gross return in one column and the same return minus the expense ratio in another. The third column can hold a middle case. The gap between the lines is the fee, expressed in the only unit that matters.
Nothing here is a comment on whether any particular fee is worth paying. Some are. The point is that the cost is knowable in advance, and it is larger than the percentage suggests.
9. Inflation, and what “today’s dollars” means
A balance thirty years from now is denominated in thirty-years-from-now dollars, which buy less than today’s.
The inflation toggle divides every result by cumulative inflation at the rate you set, converting the projection into purchasing power. It is a sobering adjustment. That $609,985 from thirty years of $500 monthly contributions becomes about $251,306 in today’s dollars at 3% annual inflation — still a substantial sum, and less than half the headline number.
This is the difference between nominal return and real return, and it is the reason long-run market history is usually quoted both ways. Over the long sweep, the S&P Composite has returned roughly 10% annually in nominal terms and roughly 7% after inflation, with dividends reinvested in both figures. The three-point spread is not a detail. Compounded across a working life, it is most of the result.
When you are comparing plans, compare them in one unit or the other, consistently. Mixing a nominal projection with a real-dollar goal is one of the more common ways a sound plan is judged by an unsound standard.
10. Does compounding frequency actually matter?
At 7% for thirty years, $10,000 grows to $76,123 with annual compounding and $81,645 with daily compounding. Daily is better, and the difference over three decades is about $5,500 — real money, but roughly what a single percentage point of return produces in three years.
The clearer way to see it is the effective annual yield. Seven percent compounded annually yields 7.000%. Compounded monthly it yields 7.229%. Compounded daily it yields 7.250%. The gain from annual to monthly is most of the available benefit; everything past monthly is a rounding difference, and continuous compounding — the theoretical limit — adds almost nothing beyond daily.
The calculator displays the effective annual yield for each scenario, which is the honest basis for comparing two accounts quoted at different compounding schedules. Rate, contributions, and years move the outcome. Frequency mostly moves the decimal.
11. Contribution timing and contribution growth
Two smaller settings change results in ways worth understanding.
Timing. Contributions can be made at the start of each period or the end. Finance texts call these an annuity due and an ordinary annuity. Starting each period earns one extra period of growth on every deposit, which over thirty years of $500 monthly contributions at 7% is worth about $3,558 — a little over half a percent. Most calculators assume end-of-period without saying so. This one lets you choose, and states which convention is in force.
Growth. Contributions rarely stay flat across a career. If your $500 a month rises 3% a year alongside your income, thirty years produces about $833,580 rather than $609,985. The escalation is worth roughly $224,000, and it is the one lever in the model that most people can actually pull, since it requires only that new money follow raises rather than absorb them entirely.
Contribution cadence is here for the same reason. Investing every two weeks with each paycheck, rather than once a month, produces about $610,942 against $609,985 on the same annual total. The difference is negligible. The reason to choose the cadence that matches your paycheck is that it gets done, not that it earns more.
12. Planning with a range of returns instead of one guess
Every projection on this page rests on an assumption that will not hold: that returns arrive at the same rate every year. They do not. They arrive in clusters, with long stretches of nothing and sudden runs that make up the difference.
That matters beyond mere untidiness. A sequence of volatile returns compounds to less than its own simple average — a portfolio that gains 20% and then loses 20% is down 4%, not flat. Any single-rate projection is therefore an illustration of arithmetic, not a forecast of a portfolio.
The variance band exists to keep that in view. Enter a number of percentage points and each scenario is drawn as a shaded range rather than a line: 7% plus or minus two points shows the same plan under a 5% and a 9% world. The band is not a probability distribution and does not represent a worst case. It is a visual reminder that the line has a width.
The Rule of 72 offers the same discipline in a form you can do in your head. Divide 72 by a return rate and you get the approximate years to double: about 10.3 years at 7%, 9 years at 8%, 12 years at 6% — each within a tenth of a year of the exact answer. If a projection cannot survive that rough check, the inputs deserve another look.
13. See it in real market history
The arithmetic on this page is clean. Market history is not, and the gap between the two is where most investing outcomes are actually decided.
The long-run figures are genuinely remarkable. One hundred dollars invested in the S&P Composite in 1900, with all dividends reinvested, would be worth roughly $18.4 million today, an annualized return near 10% nominal and near 7% after inflation. A thousand dollars committed in 1929 — the worst possible entry point of the century — would still have compounded into millions on the same reinvested-dividend basis.
What those numbers omit is the experience of holding through them: the 89% decline from the 1929 peak, the quarter-century round trip back to even, the 1970s stretch when nominal gains were erased by inflation, the two drawdowns of the 2000s. The calculator draws a smooth line through all of it. Nobody lived a smooth line.
That is the argument this site exists to make, and it is why the tools here sit alongside the history rather than apart from it.
14. Frequently asked questions
How much does waiting ten years to invest actually cost? More than the contributions you skip. At an assumed 7% return compounded monthly, $500 a month for forty years produces about $1,312,407 and the same amount for thirty years produces about $609,985 — a gap of roughly $702,421 on $60,000 of missed contributions. The missing years are the early ones, which supply the balance that the later years compound.
Is it better to invest a lump sum or spread it out monthly? On average, investing sooner produces more, because each dollar earns for longer. Invested at once, $36,000 at 7% for ten years reaches about $70,817; contributed as $300 a month it reaches about $51,925. The averaged approach carries less exposure to a single entry point, which is why some investors prefer it despite the lower expected result.
Does daily compounding really beat monthly? Barely. Seven percent compounded monthly yields an effective 7.229% annually; compounded daily it yields 7.250%. The move from annual to monthly captures most of the available benefit, and everything past that is a rounding difference.
How much do investment fees cost over thirty years? A 1% annual fee applied to a portfolio growing at 7% reduces a $100,000 starting balance from about $761,226 to about $574,349 over thirty years — a cost of roughly $186,876, or 24.5% of the fee-free result. The fee compounds the same way the returns do.
What return rate should I use? That is a judgment, not a fact, and this calculator does not supply one. Long-run U.S. equity returns have averaged near 10% nominal and near 7% real with dividends reinvested, but any thirty-year stretch can differ substantially from the average. Running three rates side by side is more useful than trying to select the correct one.
What is the difference between nominal and real return? Nominal return is the raw percentage; real return subtracts inflation and describes purchasing power. Enable the inflation toggle to convert results into today’s dollars.
What is the difference between beginning- and end-of-period contributions? A beginning-of-period contribution earns one additional period of growth. Over thirty years of monthly contributions at 7%, the difference is roughly half a percent of the ending balance. Most calculators assume end-of-period silently; this one lets you set it.
How long will it take to double my money? Divide 72 by your assumed annual return for a close approximation: about 10.3 years at 7%, 9 years at 8%. The Rule of 72 is accurate within a fraction of a year across ordinary rates.
Can I compare three different investments at once? Yes — that is what this calculator is built for. Each of the three columns takes its own starting amount, contribution, rate, and duration, and each can be renamed so the comparison reads in plain language.
How do I save or share my results? The share button copies a link that encodes every input, so anyone opening it sees exactly your scenarios. The year-by-year table downloads as a CSV, and the chart downloads as an image.
15. Sources and methodology
Calculations run period by period at the compounding frequency you select, converting the annual rate to an equivalent periodic rate where the contribution and compounding schedules differ. Contributions default to end-of-period; the effective annual yield shown for each scenario reflects the compounding frequency in use. Inflation adjustment divides balances by cumulative inflation at the rate entered. Results exclude taxes, transaction costs, and fees unless you build them into the rate.
Last updated: September 9th, 2026.
Disclaimer
Hypothetical illustration, not a guarantee of future results. This calculator is provided for educational and illustrative purposes only and does not constitute investment, tax, or financial advice. It assumes a constant annual rate of return that does not vary year to year; actual investment returns fluctuate and may be negative in any given period. Results do not reflect taxes, fees, or commissions, and do not reflect inflation unless that option is enabled, all of which would reduce actual returns. Projections are estimates based on the figures you enter and are not a promise or guarantee that any investment will achieve the results shown. Past performance does not guarantee future results. All investing involves risk, including the possible loss of principal. Consult a qualified financial professional regarding your individual situation.
The results shown by this calculator are for education and entertainment only. It makes many assumptions (such as an investment always goes up the same amount every year!) and therefore should not be used solely to plan for retirement or make any life changing decisions!
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