This tool compounds a lump sum once a year. Future value is principal times (1 + rate)^years. There is no monthly contribution on this page. The rate is the number you type. It is not a market return.
$10,000 at 7% compounded once a year for 20 years grows to $38,696.84. The gain is $28,696.84. That is 3.87 times the starting amount. A monthly contribution belongs on the monthly investment page, not this one.
Fees, taxes, and a varying market return are not applied. Inflation is a separate calculator. The growth is not a forecast that a savings account or a stock index will earn 7%.