Plane Geometry Formulas You Must Know for the ACT

By Mark Zegarelli

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):

    a2 + b2 = c2

  • Area and perimeter of a square of side s:

    A = s2

    P = 4s

  • Area and perimeter of a rectangle of sides l and w:

    A = lw

    P = 2l + 2w

  • 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 = s3

    A = 6s2

  • Volume and surface area of a box:

    V = lwh

    A = 2lw + 2lh + 2wh

  • Volume and surface area of a sphere:


  • Volume of a right prism:

    V = Abh

  • Volume of a pyramid:


  • Volume of a cylinder:


  • Volume of a cone: