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Compound interest calculator

See how a starting amount and regular monthly deposits grow when interest is paid on interest.

$
%
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Added at the end of each month.
Only changes how amounts are displayed.

Year-by-year growth

YearDepositsInterestBalance
1$1,200.00$539.50$11,739.50
2$1,200.00$628.50$13,568.01
3$1,200.00$722.05$15,490.06
4$1,200.00$820.39$17,510.44
5$1,200.00$923.75$19,634.20
6$1,200.00$1,032.41$21,866.60
7$1,200.00$1,146.62$24,213.23
8$1,200.00$1,266.68$26,679.91
9$1,200.00$1,392.88$29,272.79
10$1,200.00$1,525.54$31,998.32
11$1,200.00$1,664.98$34,863.30
12$1,200.00$1,811.56$37,874.86
13$1,200.00$1,965.64$41,040.50
14$1,200.00$2,127.60$44,368.09
15$1,200.00$2,297.84$47,865.93
16$1,200.00$2,476.80$51,542.73
17$1,200.00$2,664.91$55,407.64
18$1,200.00$2,862.65$59,470.29
19$1,200.00$3,070.50$63,740.78
20$1,200.00$3,288.99$68,229.77

What compound interest is

With simple interest you earn interest on the starting amount only. With compound interest, each interest payment is added to the balance, and the next payment is worked out on the larger balance. The gap between the two grows every year.

A = P × (1 + r ÷ n)^(n × t) P = starting amount r = yearly rate as a decimal n = compounding periods per year t = years

Worked example

$10,000.00 at 5% for 20 years with no deposits. With simple interest you would have $20,000.00. Compounded yearly it grows to $26,532.98. Compounded monthly it reaches $27,126.40.

Add $100.00 at the end of every month and the monthly-compounded balance after 20 years is $68,229.77. You put in $34,000.00 in total and $34,229.77 is interest.

How monthly deposits are handled

The calculator moves forward one month at a time. Each month it adds that month's interest, then the deposit. When interest compounds at a different frequency from the monthly deposits, it uses the equivalent monthly rate:

monthly rate = (1 + r ÷ n)^(n ÷ 12) − 1

This gives exactly the same result as the formula above when there are no deposits.

Effective annual rate

The more often interest compounds, the more you earn at the same quoted rate. The effective annual rate shows what the quoted rate is worth over a full year:

effective annual rate = (1 + r ÷ n)^n − 1

At 5% compounded monthly, the effective rate is 5.12%.

What this calculator leaves out

Borrowing instead of saving? The loan and EMI calculator shows what compound interest costs on a loan.

Page last reviewed: 10 October 2026. Results are estimates for general information. See the terms.