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: