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All the measurements you take will be recorded in feet and decimal equivalents of inches to make the math easier to do. Here’s how that is done.

You all know that half a foot (6”) is 0.5 (five tenths of a foot). That’s easy. You learned that in school.

So 12’-6” would be written as 12.5 feet. But what about 4”?

4” is a third of a foot. (12" divided by 4" equals 3 parts of a foot) That would be .333 feet. (You get that decimal number by dividing 4 by 12.)

And how do we find these magic numbers if they happen not to be 6” or 4”?

Simple. The decimal equivalent of one (1) inch is .083. (1 divided by 12, if you want to see the math process. But the .083 is given to you here as a freebie, so you don't have to mess with that part of the math.)

All you do is multiply your inches by

**.083**.

For example: 5” would be done like this: .083 x 5 = .415.

And............... 7” would be done like this: .083 x 7 = .581.

Now since these numbers run into the thousandths in accuracy, and not all calculators are “spot-on”, your last number may be a little different (by 1 or 2) than mine. But that is not a big deal. It won’t affect your volume enough to matter here.

**Step 1**

Measure the

__length__and

__width__of your slab in feet. Write those two numbers down. Remember to use those decimal equivalents for the inch parts.

**Step 2**

How deep is your slab going to be? For most applications a depth of 4 inches is standard. For heavier items, machinery, vehicles, etc., you may want as much as 6 inches or more.

**Step 3**

Measure your depth. Write it down, expressed in its decimal equivalent.

**Step 4**

Multiply Length x Width together.

**Step 5**

Now multiply that mathematical “product” (the answer) by the depth you wrote down.

**Step 6**

You now have the “cubic feet” of concrete required.

**Step 7**

All you need to do now is determine how many bags of Ready-Mix Concrete it will take to fill your slab.

An 80 pound bag will fill 2/3 of one cubic foot of slab area. That is about 0.67 cubic feet.

Divide the final answer you got up in Step 5 by 0.67.

This will be the number of 80 pound bags needed.

A 60 pound bag will fill a little less of your slab area than an 80# bag. It works out to 0.45 cubic feet. It will, of course, take more 60# bags to fill a given area than 80# bags.

Divide the final answer you got up in Step 5 by 0.45.

This will be the number of 60 pound bags needed.

**Step 8**

Should you decide mixing and placing this much concrete is too much work for you, simply order the concrete from a concrete company.

But to do this, you need to do just a little more math. The reason is that commercial concrete suppliers in the U.S. sell concrete by the

__cubic__**. Not by the cubic foot. You can convert the cubic feet you already calculated to cubic yards by using division:**

__yard__Your cubic feet / 27 = cubic yards. (one cubic yard = 3’ x 3’ x 3’ = 27 cubic feet)

The formula, in total, reads like this:

L x W x D (all in “feet”) divided by 27 = cubic yards.

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As a side note.......... Some of you are trying to figure how to calculate the concrete needed for a Sonotube or a post hole.

Same thing.... ya gotta do a little math. Here's the formula:

Cylinder Volume = (pi) x radius2 x height (In case you've forgotten, PI is 3.14159265 )

Let's take a 10" tube (or hole) three feet deep.

3.14159265 x (.415 x .415) x 3 = 1.6 cubic feet (Remember how we determined that 5" [the radius of 10"] is .415? It was in the examples we did earlier.)

(Of course, for a fence post, you'll only need about half that amount because the post, itself, will be taking up the other half of the volume of the hole.... approximately.... depending on the size and type of post you're using.)