Compound interest calculator.
This free compound interest calculator shows how money grows when interest earns interest. Enter a starting amount, a monthly contribution, your rate and the years to get the future value, the interest earned, and a year-by-year growth schedule. The guide below covers the formula, compounding frequency, and what APY really means.
See it grow
LiveFuture value
$125,510
in 20 years
Assumes a constant rate, compounded monthly. Estimate for planning, not financial advice. Calculations run in your browser; nothing you enter is stored.
The short answer
What is compound interest?
Compound interest is interest earned on both your original money and the interest it has already earned. Each period the balance grows, and the next period earns on that larger balance, so growth accelerates over time. Simple interest, by contrast, pays only on the original amount, forever.
How this compound interest calculator works
The calculator compounds your balance monthly: each month it applies one-twelfth of your annual rate, then adds your contribution, and the next month earns on the larger total. Monthly compounding matches how most savings accounts and investment projections work in practice.
Starting with $10,000 and adding $200 a month at 6% for 20 years grows to about $125,500. You pay in $58,000; the other $67,500 is interest the money earned for you. By the later years, the interest column in the schedule below outearns your own deposits.
Make it count
Grow it faster.
Time is the biggest lever
The longer money compounds, the more the interest earns its own interest. Starting earlier beats adding more later, usually by a wide margin.
Add regularly
Steady monthly contributions compound alongside your starting balance. In the default example they end up contributing more growth than the lump sum.
Compare accounts by APY
APY already includes compounding frequency, so it is the one number that makes two accounts directly comparable. The guide below shows why.
Year by year
Your growth, year by year.
Each row is one year: the total you have contributed, the interest earned so far, and the balance at the end. Watch the interest column overtake your contributions the longer the money stays invested.
| Year | Contributions | Interest earned | Balance |
|---|
Frequency & APY
Daily vs monthly vs annual compounding.
How often interest is added changes what the same rate is worth. Here is $10,000 at a 5% nominal rate for 10 years under each compounding frequency, and the APY each one works out to.
| Compounding | APY at 5% nominal | Value after 10 years |
|---|---|---|
| Annually | 5.00% | $16,288.95 |
| Quarterly | 5.09% | $16,436.19 |
| Monthly | 5.12% | $16,470.09 |
| Daily | 5.13% | $16,486.65 |
The jump from annual to monthly matters far more than the jump from monthly to daily. That is why comparing accounts by the nominal rate misleads, and why the SEC’s investor.gov and every bank quote APY: the yield with compounding already counted.
How to calculate compound interest
Divide the annual rate by the number of compounding periods per year: by 12 for monthly, 4 for quarterly, 365 for daily.
Multiply the periods per year by the number of years to get the total number of compounding periods.
Add 1 to the period rate, raise it to the total number of periods, and multiply by your starting amount. That is the future value.
Subtract the starting amount from the future value to see the compound interest earned on its own.
The full guide
The complete compound interest guide.
The formula behind the snowball, what APY really tells you, the doubling shortcut, and where compounding quietly works against you.
Simple vs compound interest
Simple interest is paid only on the original amount. Compound interest is paid on the original amount plus all the interest already added, so each period earns a little more than the last. The difference is small at first and enormous over time.
Take $100 at 10% a year. Simple interest pays $10 every year, flat. Compound interest pays $10 in year one, $11 in year two because you now earn on $110, then $12.10, and the gap keeps widening. After 30 years simple interest has built $400; compound interest has built about $1,745. That widening gap, interest earning its own interest, is the whole point.
The compound interest formula, worked through
The future value of a single amount is A = P(1 + r/n) raised to the power of n times t, where P is the starting amount, r is the annual rate as a decimal, n is how many times a year it compounds, and t is the years. Worked example: $10,000 at 5% compounded annually for 20 years is 10,000 times 1.05 to the 20th power, which is $26,532.98, of which $16,532.98 is interest.
Add a regular contribution and each deposit gets its own run of compounding from the day it goes in. You never need to work that by hand: the calculator above applies it month by month, and the schedule shows the running totals.
APY vs the nominal rate
The nominal rate, sometimes labelled APR on savings products, ignores compounding. The APY, annual percentage yield, is what you actually earn in a year once compounding is included: APY = (1 + r/n) to the power of n, minus 1. A 5% nominal rate compounded monthly is a 5.12% APY; compounded daily it is 5.13%.
Banks advertise APY precisely so accounts with different compounding schedules can be compared on one number. When you shop savings accounts or CDs, compare APY to APY and the frequency takes care of itself. A savings calculator projects a goal from there.
The Rule of 72
The Rule of 72 estimates how long money takes to double: divide 72 by the annual rate as a whole number. At 6% that gives 12 years, and the exact answer is 11.9, so the shortcut is impressively close. At 8% it gives 9 years, almost exactly right.
It also works in reverse: to double your money in 10 years you need about 7.2% a year. Use it to sanity-check projections in your head, then let the calculator do the precise version.
Why starting early wins
Because growth compounds, the early years do a surprising amount of the work. Here is $100 a month at a 7% annual return, compounded monthly:
| Investing $100/month | You paid in | Balance at 7% |
|---|---|---|
| 10 years | $12,000 | $17,308 |
| 20 years | $24,000 | $52,093 |
| 30 years | $36,000 | $121,997 |
| 40 years | $48,000 | $262,481 |
Doubling the time does far more than doubling the money paid in: the last decade in that table adds more growth than the first three combined. This is why retirement and 401(k) projections reward early contributions so heavily. Time, not timing, builds the balance.
Where compound interest works against you
The same snowball rolls both ways. Credit cards typically compound interest daily, so a carried balance grows the way a savings account does, only faster, because card rates are far higher than deposit rates. Unpaid interest joins the balance and starts charging interest of its own.
Inflation compounds too: prices rising 3% a year halve money’s buying power in about 24 years, by the same Rule of 72. The inflation calculator shows that erosion, which is the quiet argument for keeping long-term money invested rather than idle.
The formula
No black box.
Here is the math.
Your balance is the starting amount compounded by the rate, plus the future value of every contribution compounding from the day it goes in.
See the investment calculator ›# Future value
A = P(1 + r/n)^(n × t)
+ future value of contributions
# APY from a nominal rate
APY = (1 + r/n)ⁿ − 1
# Rule of 72
years to double ≈ 72 / rateQuestions
Compound interest questions.
What is compound interest?
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Compound interest is interest earned on both your original amount and the interest that has already been added. Because each period earns on a larger balance, your money grows faster the longer it stays invested.
How is compound interest calculated?
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Each period the balance grows by the period rate, then the next period earns on the larger balance. The formula is A = P(1 + r/n) to the power of n times t, where n is compounding periods per year and t is years, with contributions compounding from the day they go in.
What is the difference between APY and interest rate?
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The nominal interest rate ignores compounding; APY includes it. A 5% rate compounded monthly is a 5.12% APY, so APY is the true yearly yield. Banks quote APY so accounts with different compounding frequencies can be compared on a single number.
How often do savings accounts compound?
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Most US savings accounts compound daily or monthly and credit the interest monthly. The difference between the two is small: at 5% nominal, daily compounding yields 5.13% APY versus 5.12% for monthly. This calculator compounds monthly, which closely matches everyday accounts.
What is the Rule of 72?
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A shortcut for doubling time: divide 72 by your annual rate as a whole number. At 6% that is about 12 years (the exact answer is 11.9), at 8% about 9 years. It also reverses: doubling in 10 years needs roughly a 7.2% annual return.
How much is $10,000 worth after 20 years of compound interest?
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At 5% compounded annually, $10,000 grows to $26,532.98 in 20 years, earning $16,532.98 of interest. At 7% it reaches about $38,697. Adding even a small monthly contribution raises the result dramatically; try it in the calculator above.
Is this compound interest calculator free and private?
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Yes. It is completely free with no sign-up, and every calculation runs locally in your browser, so nothing you enter is stored or sent anywhere.
About the developer
Jean Borg
Jean builds and maintains every calculator on freecalculators.pro from Malta, with a focus on tools that are fast, free and show their working. The compound interest calculator uses the same standard future-value maths as the SEC’s investor.gov calculator and is provided for planning and education, not as personalised financial advice. Page last updated June 10, 2026.