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.

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*r

These are among the most commonly used.

triangle = bh/2

rectangle, parallelogram = bh

trapezoid = (b1 + b2)h/2

circle = Pi*r

^{2}