How many yards of cement do you need for a 20 x 21x 3 slab?

Answer 1

This sounds like a typical small project and this is a very common question. First of all, although the material may be the same chemical composition as cement; it is called concrete when it is used for building slabs, bases and structures. The term cement is used when brick, block, and slabs of the material are joined together. Think of the term cement as glue.

This is the basic formula for measuring the volume of a slab or structure: Length X Width X Height or Thickness. Length is always going to be the largest of the dimensions; and the third dimension should always refer to the thickness. This will give you a cubic measurement. Since three feet equals a yard: each side must be measured. 3 X 3 X 3 = 27 In a cubic yard, there are 27 cubic feet. Concrete is ordered in cubic yards.

Your question is "How many Yards..." Slabs are usually measured in sq. ft.: so since you gave the figures above, a new formula has to be utilized. L X W X H or T now must be also divided by 27 to get cubic feet.

I'm going to assume that the 3 in your measurement is in feet; so, 21 X 20 X 3= 1260 cu feet. Now divide that by 27 and you get 46.67 cubic yards.

This time I'm going to assume that the 3 in your formula is in inches. Because there are 12 inches in a cubic foot you must first divide your measurement by 12. 21 X 20 X (3/12= .25) 21 X 20 X .25 = 105 cu ft /27= 3.89 cu. yds.

When ordering your concrete, make sure you tell the sales person your reason for use. You may need reinforcing bars (rebar.) Rebar is measured in 1/8 inch increments (#4 rebar is a half inch thick). If your dimensions are for 3 inch thickness; then a 1/8" welded wire mesh is probably all you'll need. If you're going to build anything heavier than an aluminum carport or awning on it ; consider making the slab at least 4 to 6" thick and using #4 or #5 rebar.

Some thoughts when ordering your cubic yards of concrete. Make sure you:

1 order 10% more than your measurements require due to waste;

2 have a ready delivery system, like a slide or wheel barrows, for getting the concrete from the truck to the intended site;

3 have enough labor on hand to get it done before it starts to set-up and becomes unworkable (usually 30 to 45 minutes);

4 have a vibrator on hand to remove the air bubbles, which cause weakness and spalling;

5 create expansion joints (notice a sidewalk;)

6 have the proper floats and finishing tools available.

Finally; your project is a lot of hard work; but, very satisfying. Have fun with it; and, it won't be a job.