http://www.learningconstruction.org In order to estimate the concrete that you need for certain area, you need to know that concrete is ordered in cubic yards. Or CuYards. The conversion between Cuft and Cuyds is defined by the constant (27). But I'm not going to make it more complicated for you. I'm here to make it easy, so let's start:

1) You need to know how many cubic feet you have in the area by doing the following formula: width x length x dept = cubic feet.
2) Then, divide the cubic feet by the constant 27 and that will result in the cubic yards that you will require to fill a determined area.

The following are some some quick tips to determine concrete when you already know the depth:

If you are pouring a 4" thick slab:

Multiply width by length and divide the resulting number by the constant (80).
Ej.,

(20'x40') = 800sqft / 80 = 10 Cu Yards of concrete!
If you are pouring a 6" thick slab:

Multiply width by length and divide the resulting number by the constant (50).

Ej.,

(20'x40') = 800sqft / 50 = 16 Cu Yards of concrete!
General Formula:

Multiply width by length by depth and divide it by the constant (27).

Ej.,
(20'x40'x1'6")=1200Cuft. /27= 44.44 Cu yards or 44 ½ yards.

Tip: Try to order 1/4 yard more for every 50 ft of hose you if your using a concrete pump.