Compound interest calculator

Starting from $10,000.00 and adding $200.00 at the end of each month, a 7% annual rate compounded monthly grows to $144,572.72 in 20 years. Of that, $58,000.00 is money you put in and $86,572.72 is interest. The 7% rate is an example you can change. It is not a forecast.

Your numbers

Starting amount What you have on day one.

Monthly deposit Added at the end of each month. Use 0 for a lump sum.

Annual rate (%) Nominal rate, compounded monthly. Not APY.

Years How long the rate stays constant in this projection.

$144,572.72 after 20 years. $58,000.00 is money you put in, and $86,572.72 is interest. The equivalent APY is 7.23%.

Ending balance $144,572.72

Put in $58,000.00

Interest $86,572.72

Equivalent APY 7.23%

Year by year, for the numbers above
Year	Put in	Interest	Balance
1	$12,400.00	$801.42	$13,201.42
2	$14,800.00	$1,834.27	$16,634.27
3	$17,200.00	$3,115.28	$20,315.28
4	$19,600.00	$4,662.39	$24,262.39
5	$22,000.00	$6,494.83	$28,494.83
6	$24,400.00	$8,633.24	$33,033.24
7	$26,800.00	$11,099.74	$37,899.74
8	$29,200.00	$13,918.03	$43,118.03
9	$31,600.00	$17,113.55	$48,713.55
10	$34,000.00	$20,713.58	$54,713.58
11	$36,400.00	$24,747.34	$61,147.34
12	$38,800.00	$29,246.20	$68,046.20
13	$41,200.00	$34,243.79	$75,443.79
14	$43,600.00	$39,776.14	$83,376.14
15	$46,000.00	$45,881.93	$91,881.93
16	$48,400.00	$52,602.60	$101,002.60
17	$50,800.00	$59,982.60	$110,782.60
18	$53,200.00	$68,069.60	$121,269.60
19	$55,600.00	$76,914.70	$132,514.70
20	$58,000.00	$86,572.72	$144,572.72

Put in includes the starting amount. A row is the balance at the end of that year.

Educational calculator, not financial advice. The rate stays constant for the whole period, which real accounts do not promise.

The sentence and the figures above are filled in for the starting example, so they can be read with JavaScript off. With JavaScript on, they follow the numbers you type.

How this is calculated

Each month the balance is multiplied by one plus the monthly rate, and then the deposit is added. The monthly rate is the annual rate divided by 12. This is the ordinary-annuity convention, and it is the same month-by-month model used in Compound's lessons.

The rate you type is a nominal annual rate, not APY. APY is what that rate becomes after a year of monthly compounding. The result panel shows the APY for the rate you entered, so the two are not mixed up.

Deposits land at the end of the month and earn nothing until the following month.
The rate does not change, and no fee, tax, or inflation adjustment is applied.
Totals are rounded to the cent for display. The projection itself compounds the unrounded monthly balance.

A savings account can change its rate. A stock portfolio does not grow by the same percentage every month. Change the rate and the deposit to see how sensitive the ending balance is. The figure is the formula, not a prediction.

Questions

What rate should I enter?

Enter the annual rate before monthly compounding. If a bank quotes only APY, the result here runs a little high, because this page compounds a nominal rate. For an investment illustration, try a rate and then try a lower one. Nothing on this page predicts what you will earn.

Does the monthly deposit earn interest in the month I add it?

No. Deposits are added at the end of the month, so they earn nothing until the next month. A deposit at the beginning of the month would earn one extra month of interest. The gap is small at first and noticeable over decades.

How is this different from the rule of 72?

Divide 72 by an annual rate for a rough count of the years a lump sum takes to double. That shortcut ignores monthly deposits, and it is an approximation. This calculator includes deposits and compounds monthly. The guide linked below explains the shortcut itself.

Will I really end up with this balance?

Only if the rate you entered is the rate you actually receive, every month, for the whole period, with no fees and no taxes taken out. The calculator applies that assumption consistently. It does not know which assumption will come true.

Learn the ideas behind the formula

Compound is a free-to-start course of short daily lessons on saving, credit, investing, and taxes, plus a $10,000 paper-trading portfolio. The portfolio uses simulated money — no real trades and no brokerage account.

Start learning free