How to use the compound interest calculator
Enter your starting balance, a regular contribution and how often you add it, your assumed annual return and compounding frequency, and a time horizon. Optional fields let you model contribution timing, annual contribution increases, a one-time deposit, inflation, fees, and taxes, or switch on goal mode to solve for what a target requires. Everything calculates in your browser — no sign-in and no personal data collected.
How to read your results
The headline is your estimated future value. Below it, the breakdown separates the money you put in (starting balance plus contributions) from the interest and growth it earned, shows the effective annual rate (APY), the inflation-adjusted real value, total fees and estimated taxes, and how the result compares with plain simple interest. Charts and a year-by-year schedule show how the balance builds, and the Excel model lets you keep the full plan.
How this calculator works
A month-by-month schedule drives every figure, so the summary, charts, year table, scenarios, and goal solver always agree.
- Your stated rate and compounding frequency are converted into an effective monthly rate, so monthly compounding reproduces the exact effective annual rate (EAR/APY) of the frequency you chose — annual, quarterly, monthly, daily, or continuous.
- Contributions are added at the start or end of each period based on your timing choice; beginning-of-period money earns one extra period of growth.
- Annual contribution increases, a contribution stop year, and a one-time deposit are applied on the schedule as they occur.
- Inflation discounts the balance back to today's purchasing power; an annual fee % and fixed annual fee are applied proportionally each month.
- Tax can be applied to interest each year or to total growth at the end. Goal mode solves for the required contribution, return, time, or starting amount that reaches your target.
Compound interest: A = P(1 + r/n)^(n x t)
With contributions: FV = P(1 + i)^m + C x [((1 + i)^m - 1) / i]
Effective annual rate (APY): EAR = (1 + r/n)^n - 1
Continuous compounding: A = P x e^(r x t)
Simple interest: A = P(1 + r x t)
Inflation-adjusted: Real FV = Nominal FV / (1 + inflation)^t
Rule of 72: years to double ~= 72 / annual return %
- P
- Starting amount (principal)
- r
- Annual interest rate (as a decimal)
- n
- Compounding periods per year
- t
- Time in years
- i
- Periodic rate = r / n
- m
- Total number of periods = n × t
- C / PMT
- Regular contribution each period
Worked example
A $10,000 starting balance with $500 added every month at a 7% annual return, compounded monthly, over 10 years (3% inflation) produces this estimate.
Estimated future value
$106,639
Total contributed (incl. start)
$70,000
Interest / growth earned
$36,639
Effective annual rate (APY)
7.23%
Simple-interest comparison
$97,825
Real value after inflation
$79,349
What changes if the return is different
- Same plan at a 5% return≈ $94,100 future value
- Base case at a 7% return$106,639 future value
- Same plan at a 9% return≈ $121,300 future value
Same starting balance, contribution, and time horizon — only the assumed annual return changes. Returns are not guaranteed.
Mistake to avoid
Do not compare a nominal future value with today's prices without adjusting for inflation. Here the $106,639 balance is worth about $79,349 in today's money at 3% inflation — roughly a quarter less. Check the inflation-adjusted (real) value before deciding whether a projected balance will actually cover a future goal.
The Complete Guide to Compound Interest
What is compound interest?
Compound interest is interest earned on both your original principal and the interest that has already been added to the balance. Instead of earning a flat amount each period, your money earns on an ever-larger base, so growth speeds up the longer it runs. This is what people mean by interest-on-interest, and it is the single most important idea in long-term saving and investing.
The opposite is simple interest, which is calculated only on the original principal and grows in a straight line. Over a year or two the two look similar, but over decades the compounding curve pulls dramatically ahead.
How this compound interest calculator works
This tool runs a month-by-month simulation of your balance. Each month it adds any contribution, applies growth based on your rate and compounding frequency, subtracts any fees, applies tax if you chose to, and records the new balance. The summary numbers, charts, year-by-year table, scenarios, and goal solver all read from that same schedule, so they always agree to the cent.
Because the engine works in months, it can show you both a clean yearly summary and a detailed period-by-period schedule, and it can model contributions, increases, one-time deposits, fees, taxes, and inflation together — things a single textbook formula cannot capture on its own.
The compound interest formula
For a single lump sum, future value is A = P(1 + r/n)^(n x t). Here 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. The exponent n x t is the total number of compounding periods.
For example, $10,000 at 7% compounded monthly for 10 years is 10,000 x (1 + 0.07/12)^(12 x 10), which works out to about $20,097 — roughly double, with no contributions at all.
Monthly contributions explained
Most people do not invest a single lump and walk away — they add money regularly. Each contribution becomes part of the balance and earns its own interest from the moment it lands. The future value of a stream of equal contributions is FV = C x [((1 + i)^m - 1) / i], where C is the contribution, i is the rate per contribution period, and m is the number of contributions.
In the worked example, $500 a month for 10 years is $60,000 of contributions. Combined with the $10,000 start, you put in $70,000 — but the ending balance is about $106,639, meaning roughly $36,639 came from growth.
Compounding frequency explained
Compounding frequency is how often earned interest is added back to the balance: annually, semi-annually, quarterly, monthly, daily, or continuously. The more often interest compounds, the slightly higher the effective return, because interest starts earning interest sooner.
The effect is real but usually modest. The frequency comparison chart on this page holds the rate and time constant and varies only the frequency, so you can see exactly how much it changes the outcome — typically far less than a change in rate or time would.
APR vs APY / EAR
A nominal rate (sometimes shown as APR for borrowing) does not account for compounding within the year. The effective annual rate (EAR), shown as APY for savings, does. The conversion is EAR = (1 + r/n)^n - 1.
A 7% nominal rate compounded monthly is about a 7.23% APY. When you compare two accounts, compare their APYs, not their nominal rates — APY is the apples-to-apples figure. This calculator lets you enter either and shows the resulting APY.
Simple interest vs compound interest
Simple interest pays the same amount every period because it is always based on the original principal. Compound interest grows because it is based on the principal plus all prior interest. The longer the horizon, the wider the gap.
On this page the simple-vs-compound chart plots both paths from the same contributions. In the worked example, simple interest reaches about $97,825 while compound reaches $106,639 — the roughly $8,800 difference is value created purely by compounding.
Why starting early matters
Time is the most powerful lever in compounding because growth builds on itself. Two people who contribute the same amount can end with very different balances purely because one started earlier and gave their money more years to compound.
A classic illustration: someone who invests for ten years and then stops can sometimes finish ahead of someone who starts ten years later and contributes for far longer, simply because the early money had more time to grow. Starting now, even with a small amount, is often the highest-impact decision.
Why the later years often add more growth
In the early years of a plan, your contributions make up most of the balance and growth is a small slice. As the balance gets larger, the interest it throws off each year grows too, until eventually a single year of growth can exceed a whole year of contributions.
That is why the balance-growth chart curves upward rather than rising in a straight line, and why patience is rewarded: the biggest gains usually come at the end of a long horizon, not the beginning.
Rule of 72 explained
The Rule of 72 is a shortcut for estimating how long money takes to double: divide 72 by the annual return percentage. At 6% money doubles in about 12 years, at 8% in about 9 years, and at 9% in about 8 years.
It is an approximation that works best for rates between roughly 5% and 12%, and it ignores contributions. Use it for quick intuition; use the full schedule on this page for an accurate doubling point.
Inflation-adjusted future value
A balance in the future is not worth as much as the same number today, because prices rise. To compare fairly, you discount the future value back to today's money: Real FV = Nominal FV / (1 + inflation)^t.
In the worked example, the $106,639 nominal balance is worth about $79,349 in today's purchasing power at 3% inflation. Looking at the real value keeps a long-term plan honest about what it will actually buy.
How fees reduce compounding
Fees do not just cost you the fee — they cost you all the future growth that money would have earned. A 1% annual fee may sound small, but over decades it can quietly remove a large share of the final balance because it compounds against you every year.
Enter an annual fee percentage or a fixed annual fee to see the impact. Comparing a low-fee and high-fee scenario is one of the most useful things this calculator can show.
How taxes can change real results
Depending on the account and country, growth may be taxed each year as it is earned, taxed only when you withdraw, or sheltered in a tax-advantaged account. Each treatment produces a different after-tax result.
The optional tax setting lets you apply a rate to interest annually or to total growth at the end so you can see an after-tax estimate. It is a simplified model — for your specific tax situation, account type, and jurisdiction, confirm with a qualified professional.
Savings account vs investment return assumptions
Different goals justify different return assumptions. A savings account or certificate of deposit might pay a low, relatively stable rate, while a diversified long-term investment portfolio has historically averaged more but with year-to-year ups and downs, including losses.
Choose a rate that matches what you are actually modelling, and remember that a higher assumed return also carries higher risk. This tool does not recommend any rate — it simply projects the one you enter.
Worked example
Start with $10,000, add $500 every month, assume a 7% annual return compounded monthly, and run it for 10 years. You contribute $60,000 over the decade, so with your starting balance you have put in $70,000.
The estimated future value is about $106,639. That means roughly $36,639 — more than a third of the ending balance — came from growth rather than from your own deposits. The effective annual rate is about 7.23%, and at 3% inflation the balance is worth about $79,349 in today's money.
Common mistakes
The most common mistakes are ignoring inflation, forgetting fees, assuming an unrealistically high return, and underestimating how much time matters. Another is comparing accounts by nominal rate instead of APY, which hides the effect of compounding frequency.
A good habit is to model a base case and a more conservative case side by side using the scenario tools on this page, so your plan does not depend on everything going right.
Limitations of this calculator
This calculator assumes your rate, contributions, fees, inflation, and tax rate stay constant unless you change them. Real returns vary, sometimes sharply, and a single average rate cannot capture the sequence of good and bad years that a real portfolio experiences.
The tax model is a simplified estimate, not a tax return, and it does not account for account-specific rules or contribution limits. Treat every figure as a planning estimate rather than a prediction.
When to use a professional adviser
A calculator is excellent for understanding the mechanics and running what-ifs, but it is not a substitute for personalised advice. If you are making a major decision — choosing accounts, planning for retirement, handling a windfall, or weighing tax strategies — a qualified financial or tax professional can account for your full situation.
Use this tool to arrive informed: bring your assumptions, scenarios, and questions, and let a professional help you pressure-test the plan.
How to use the Excel download
The Download Excel model button builds a workbook with six sheets: your inputs, a results summary, a yearly schedule, a full period-by-period schedule, a scenario comparison, and plain-English formula notes. It opens in Excel, Google Sheets, and Apple Numbers.
Use it to keep a record of a plan, to tweak numbers offline, or to share assumptions with a partner or adviser. The CSV download gives you just the schedule, and Copy results gives you a quick text summary to paste anywhere.