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 chockfull 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 mustknow 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 = 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 = s^{3}
A = 6s^{2}

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_{b}h

Volume of a pyramid:

Volume of a cylinder:

Volume of a cone: