There are different formula for different shapes. The formula for each of these different geometric shapes can be found in a book on geometry; just plug the measurements into the formula and compute.
How Do I Find The Area Of Different Geometric Shapes?
The formulas for a number of different shapes can be found here.
These are among the most commonly used.
triangle = bh/2
rectangle, parallelogram = bh
trapezoid = (b1 + b2)h/2
circle = Pi*r2
These are among the most commonly used.
triangle = bh/2
rectangle, parallelogram = bh
trapezoid = (b1 + b2)h/2
circle = Pi*r2