# Plane Geometry Formulas You Must Know for the ACT

Some of the questions on the ACT Math test deal with plane geometry. Plane geometry is chock-full of useful formulas. If you can’t commit all these formulas to memory and need to prioritize them, memorize everything up to the circumference of a circle. These are the must-know formulas for geometry. Then, if you have time, work your way forward from there.

**Interior angles of a polygon**(the total of all angles inside the polygon):Total measure of the interior angles = 180(Number of angles – 2)

**Area of a triangle:****Pythagorean theorem (where***a*and*b*are the legs, and*c*is the hypotenuse):*a*+^{2}*b*=^{2}*c*^{2}**Area and perimeter of a square of side***s*:*A*=*s*^{2}*P*= 4*s***Area and perimeter of a rectangle of sides***l*and*w*:*A*=*lw**P*= 2*l*+ 2*w***Area of a parallelogram:***A*=*bh***Area of a trapezoid:****Diameter, area, and circumference of a circle:****Arc length of a circle:****Volume and surface area of a cube:***V*=*s*^{3}*A*= 6*s*^{2}**Volume and surface area of a box:***V*=*lwh**A*= 2*lw*+ 2*lh*+ 2*wh***Volume and surface area of a sphere:****Volume of a right prism:***V*=*A*_{b}*h***Volume of a pyramid:****Volume of a cylinder:****Volume of a cone:**