There are three ways on how to get it:
The "base times height" method where
b is the length of the base
a is the altitude (height).

The "diagonals" method where
d1 is the length of a diagonal
d2 is the length of the other diagonal

Using trigonometry where
s is the length of any side
a is any interior angle
sin  is the sine function

First pick one side to be the base. Any one will do, they are all the same length. Then determine the altitude - the perpendicular distance from the chosen base to the opposite side. The area is the product of these two, or, as a formula:where
b is the length of the base
a is the altitude (height).
ENTER ANY TWO VALUESBaseclearHeightclearAreaclear Calculate Clear  
Use the calculator on the right to calculate the area of the trapezoid given base (side) length and altitude (perpendicular height).Enter any two values and the missing one will be calculated. For example, enter the area and base length, and the height needed to get that area is calculated.
2. The "diagonals" methodAnother simple formula for the area of a rhombus when you know the lengths of the diagonals. The area is half the product of the diagonals. As a formula:where
d1 is the length of a diagonal
d2 is the length of the other diagonal
3. Using trigonometryIf you are familiar with trigonometry, there is a handy formula when you know the length of a side and any angle:where
s is the length of any side
a is any interior angle
sin  is the sine function 
(see Trigonometry Overview)
It may seem odd at first that you can use any angle since they are not all equal. But the angles are either equal or supplementary, and supplementary angles have the same sine. 
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