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):

  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 = A_bh

• Volume of a pyramid:

  

• Volume of a cylinder:

  

• Volume of a cone: