What this calculator does
See how much your savings could grow over time. Compound interest means you earn interest on your interest, too. Each time interest is added, your balance gets a little bigger. The next round of interest is then figured on that bigger balance, so your money grows faster and faster.
Enter a starting deposit, a yearly rate, a number of years and how often interest is added. You can also add a fixed amount each month to see how regular saving helps. The calculator shows what you'll end up with, the total you put in, the interest you earned and your balance at the end of each year.
Where it's used
Why this math matters outside the classroom:
- Long-term saving. A $5,000 deposit earning 5% compounded yearly grows to about $8,144.47 in 10 years.
- Adding monthly deposits. Savers see how regular monthly contributions add to the growth of a starting balance over many years.
- Compounding frequency. Comparing yearly, monthly and daily compounding at the same stated rate shows how much difference the frequency makes.
- Algebra 2 lessons. Students model a balance as P(1 + r/n)^(nt) and compare it with simple interest on the same deposit.
- Growth over time. A year-by-year table shows that interest earned each year grows as the balance grows.
Grade level and standards: Grades 10–12; Common Core State Standards for Mathematics HSF.LE.A.2, HSA.SSE.B.4. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.
How to use it
- Enter your starting deposit, the yearly interest rate and the number of years. Part years, such as 2.5, are fine.
- Choose how often interest is added to your balance: yearly, twice a year, quarterly, monthly, daily or continuously (all the time).
- If you plan to save each month, enter that amount. Then choose whether you add it at the start or end of each month.
- If you like, enter an inflation rate to see what the final amount would be worth in today's dollars.
- Read your final balance, total deposits, interest earned, yearly yield and doubling time. Open the yearly table to watch the balance build.
What the inputs mean
- Initial deposit
- The amount you start with. It can be zero if you're only making monthly deposits.
- Annual interest rate
- The stated yearly rate, before compounding is counted. Some accounts advertise an APY (annual percentage yield) instead, which already includes compounding. If that's what you have, enter it and choose yearly compounding.
- Years
- How long the money grows. It must be more than zero, up to 100 years.
- Compounding
- How many times a year interest is added to your balance. Daily uses a 365-day year.
- Monthly contribution
- A fixed amount you add every month. Leave it blank or at zero for a one-time deposit.
- Contributions are made at the
- Money added at the start of each month earns one extra month of interest, compared with money added at the end.
- Inflation rate (optional)
- Optional. Your guess at how fast prices will rise each year, on average. The calculator divides the final amount by (1 + inflation)^years. That shows what it would buy in today's money. Leave it blank to skip.
Formula and method
A = P × (1 + r ÷ n)^(n × t) continuous: A = P × e^(r × t) monthly equivalent rate: i = (1 + r ÷ n)^(n ÷ 12) − 1 contributions (end of month): FV = C × ((1 + i)^m − 1) ÷ i contributions (start of month): FV = C × ((1 + i)^m − 1) ÷ i × (1 + i) APY = (1 + i)^12 − 1 doubling time = ln 2 ÷ ln(1 + APY) today's dollars = A ÷ (1 + inflation)^t
In the formulas, A is the final amount and P is your starting deposit. r is the yearly rate as a decimal (5% is 0.05), n is how many times a year interest is added, and t is the time in years. For continuous compounding, e is a fixed math number, about 2.718. Your starting deposit grows at exactly the compounding you chose.
For monthly deposits, C is the amount you add each month and m is the number of whole months. FV is what those deposits grow to. They grow at i, a monthly rate that gives the same yearly growth as the compounding you chose. That keeps your deposits and your starting amount growing at the same pace, even when interest is added quarterly or daily.
If the time ends partway through a month, there's no deposit for that last part-month. Your balance still keeps growing until the end.
APY is how much your money grows in one full year once compounding is counted. ln is the natural logarithm, a calculator button that works out powers backward. The doubling time is exact for one deposit with no monthly additions. The rule of 72 is a quick way to estimate it.
Worked example
You deposit $10,000 at 5% a year, compounded monthly, and add $200 at the end of every month for 10 years.
- Initial deposit: $10,000 × (1 + 0.05 ÷ 12)^120 ≈ $16,470.09.
- Contributions: 120 deposits of $200 at a monthly rate of 0.05 ÷ 12 grow to about $31,056.46.
- Future value: $16,470.09 + $31,056.46 ≈ $47,526.55.
- Total deposits: $10,000 + 120 × $200 = $34,000.
- Effective annual yield: (1 + 0.05 ÷ 12)^12 − 1 ≈ 5.116%, so a single deposit doubles in about 13.9 years.
- If you also enter 2.5% inflation, the $47,526.55 is worth about $37,127.66 in today's dollars.
Result: After 10 years the balance is about $47,526.55, and $13,526.55 of that is interest.
Understanding the result
Interest earned is your final balance minus everything you put in. The bigger it is compared with your deposits, the harder compounding is working for you.
If you enter an inflation rate, the today's-dollars figure is the better guide to what your balance will actually buy.
Adding interest more often helps a little, but the rate and the number of years matter far more. At 5% on $1,000 for one year, yearly compounding gives $1,050.00 and monthly gives about $1,051.16.
Assumptions and limitations
- The interest rate stays the same the whole time.
- You add the same amount every month, on time. By default, it goes in at the end of each month.
- Monthly deposits start growing right away at the matching monthly rate. They don't sit idle until the next time interest is added.
- Taxes on interest and account fees aren't included. Inflation only affects the separate today's-dollars figure, at the steady rate you enter.
- Real accounts may add interest on set dates, count days differently or pay a rate that changes. So their numbers can differ from this estimate.
- These results are estimates for planning, not a promise of returns. Check the real terms with your bank or account provider.
- Investments without a fixed rate, such as stock funds, can lose value. A steady growth rate keeps the math simple, but real returns go up and down.
- It doesn't handle withdrawals or monthly amounts that change over time.
Frequently asked questions
What is the difference between APR and APY?
APR is the stated yearly rate, before compounding is counted. APY is what you actually earn in a year once compounding is included. So APY is always equal to or higher than APR. At 5% compounded monthly, the APY is about 5.12%.
How much difference does compounding frequency make?
Less than most people expect. For $10,000 at 5% over 10 years, yearly compounding gives about $16,288.95 and daily gives about $16,486.65. The rate and how long you leave the money matter much more.
Does it matter whether I contribute at the start or end of the month?
A little. Money added at the start of the month earns one extra month of interest. Over many years, that makes the start-of-month balance end up somewhat higher.
What is the rule of 72?
It's a quick way to guess how long money takes to double. Divide 72 by the yearly rate in percent. At 6%, money roughly doubles in 12 years. It's an estimate that works best for rates between about 4% and 12%.
How do I see what my savings will be worth after inflation?
Enter an inflation rate. The calculator divides your final balance by how much prices rise over the same years. In the default example, $47,526.55 after 10 years is about $37,127.66 in today's money at 2.5% inflation.
How long will it take to reach a savings goal?
Use Solve for on the Years field. Enter the balance you want, and the calculator finds how long it takes with your deposit, rate and monthly amount kept the same. It works the same way for the starting deposit or the rate. To find the monthly amount you'd need instead, try the savings goal calculator.
Why is my bank statement different from this result?
Banks may add interest on set dates, round each interest payment to the cent, use a 360- or 366-day year, or change the rate over time. Use this result as an estimate, and check your account terms.
How this calculator is checked
Method: Standard compound interest formula A = P(1 + r/n)^(nt) and its continuous limit; future value of an ordinary annuity and annuity due for contributions, at the equivalent monthly rate. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.
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