Compound interest calculator
Compound interest is interest earned on your interest. $10,000 at 5% a year, compounded monthly, grows to $16,470.09 in 10 years. Add $100 at the end of every month and it reaches $31,998.32. The formula is A = P(1 + r/k)^(kt), plus the growth of each monthly contribution.
- Estimate
- How we calculate
In 10 years you will have
$94,111.23
- Total contributions
- $70,000.00
- Interest earned
- + $24,111.23
Interest is already 26% of the total. The longer you leave it, the bigger that share gets.
How your money grows
Values in USD · axis in years10 yearsBalance $94,111
- Interest earned$24,111
- Total contributions$70,000
Chart of the balance over time. Over 10 years, you contribute $70,000 and interest adds $24,111. The year-by-year table below has every value.
Tap, drag or hover over the chart to see each year.
Year-by-year growth
| Year | Contributed | Interest | Balance |
|---|---|---|---|
| 1 | 16,000 | 651 | 16,651 |
| 2 | 22,000 | 1,642 | 23,642 |
| 3 | 28,000 | 2,991 | 30,991 |
| 4 | 34,000 | 4,716 | 38,716 |
| 5 | 40,000 | 6,837 | 46,837 |
| 6 | 46,000 | 9,372 | 55,372 |
| 7 | 52,000 | 12,345 | 64,345 |
| 8 | 58,000 | 15,776 | 73,776 |
| 9 | 64,000 | 19,690 | 83,690 |
| 10 | 70,000 | 24,111 | 94,111 |
Estimate before taxes, fees and inflation. Actual returns can differ.
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How the calculation works
Interest is earned on the whole balance, including interest from earlier periods, so the curve gets steeper over time. The annual rate is a nominal rate compounded at the frequency you choose. It is turned into the equivalent monthly rate, and each monthly contribution is added at the end of the month.
A = P·(1 + r/k)^(k·t) + PMT·[(1 + i)^n − 1] / i
P = initial investment · r = annual rate · k = compounding periods per year · t = years · PMT = monthly contribution · i = (1 + r/k)^(k/12) − 1, the monthly rate · n = months
Step by step
- Enter what you start with. Type the initial investment and, if you will keep saving, the amount you add every month.
- Set the rate and how often it compounds. Use the annual interest rate and the compounding frequency: annually, semiannually, quarterly, monthly or daily.
- Choose the time. Type how many years the money stays invested, up to 100.
- Read the result. See the final balance, how much of it is interest, the growth chart and the year-by-year table.
Frequently asked questions
How do you calculate compound interest with monthly contributions?
Grow the starting balance with A = P(1 + r/k)^(kt), then add the future value of the contributions, PMT × [(1 + i)^n − 1] / i, where i is the monthly rate and n the number of months. With $10,000 at 5% compounded monthly and $100 a month for 10 years, the deposit grows to $16,470.09 and the contributions to $15,528.23, for a total of $31,998.32.
Does the compounding frequency make a big difference?
Less than most people expect. $10,000 at 5% for 10 years ends at $16,288.95 compounded annually, $16,436.19 quarterly, $16,470.09 monthly and $16,486.65 daily. Going from monthly to daily compounding adds only $16.56. The rate and the time matter far more.
What is the difference between APR and APY?
APR is the stated annual rate; APY (annual percentage yield) includes the effect of compounding. A 5% rate compounded monthly is an APY of 5.12%, because (1 + 0.05/12)^12 − 1 = 0.0512. Savings accounts and CDs usually quote the APY: to enter an APY here, choose annual compounding.
How long does it take to double your money with compound interest?
A quick estimate is the rule of 72: divide 72 by the annual rate. At 6% that gives 12 years. The exact time at 6% compounded monthly is 139 months, about 11.6 years, the first month in which $10,000 reaches $20,000.
Are monthly contributions added at the start or the end of the month?
At the end of each month, the usual convention for savings plans. Money added at the start of the month earns one extra month of interest, so the total is a little higher: in the example of $10,000 plus $100 a month at 5% for 10 years, $32,063.02 instead of $31,998.32.
Does the result include taxes, fees or inflation?
No. It is the balance before any tax on the interest, account fees or inflation. For a rough figure in today's money, run it again with the rate minus the yearly inflation you expect.