How to Calculate Store Bought Concrete Coverage by Bag Size

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How to Calculate Store Bought Concrete Coverage by Bag Size

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

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.)
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Thanks Willie! How much water is typically used per 80 lb bag of concrete? Any tips on the best way to mix up a bag or two at a time?
Thanks Willie! How much water is typically used per 80 lb bag of concrete? Any tips on the best way to mix up a bag or two at a time?
1.36732 gallons per bag. Just kidding! (Although that is probably pretty durn close.)

You simply add water, progressively, till the concrete is about the consistency of cake icing. Don't go too far. You don't want it soupy... that weakens the concrete and encourages cracks.

The best mixing setup I've found (short of renting a motorized mixer) is to use a strong 1/2" drill (no smaller!!!) with a paddle the stores sell for mixing like this... about \$15.

Screw (from the inside) the bottom of a 5 gallon plastic bucket to the center of a 2'x2' piece of plywood (3/4" is best). Stand on the left and right edges of the plywood with the bucket kind of between your legs, and mix slowly.

Start with about a third of the bucket full of dry concrete mix and enough water to keep it a little sloppy. This keeps from overworking the drill. Feed more concrete and water as it all gets mixed well.

Keep going, balancing the concrete-to-water ratio, till you have a fairly full bucket that is soft like cake icing. Keep lifting the mixer paddles up and down to be sure to get all the concrete mixed in well.

Run your paddles around the deep rim of the bucket a lot. Dry concrete likes to pocket down there. Get it out of there with the motion of the paddles, and get it mixed in.
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Thanks Willie! How much water is typically used per 80 lb bag of concrete? Any tips on the best way to mix up a bag or two at a time?
They recommend starting with a gallon, and add a little more until it is workable. I've found that a gallon gets you to about a 1" slump, which is pretty much unworkable in most situations. Another quart makes it about a 4" slump, which is good most of the time.
Alot of great formula reminders here Willie.

I'll just add another trick that most of us might find easier to remember than the 0.083 inch-to-foot ratio.

12 inches in a foot. So 4 inches is 4/12's of a foot.... There's your formula! 4 divided by 12 = .333 of a foot

Much easier to figure out, while standing beside the wheelbarrow

I much prefer using decimals. We are supposed to use metric up here in Canada, but in reality, it becomes an ugly mish-mosh of both. Dimensions will be scribbled in ft/inch (from a tape measure), but the products are supplied in Kg or Liters. Sigh...
Alot of great formula reminders here Willie.

I'll just add another trick that most of us might find easier to remember than the 0.083 inch-to-foot ratio.

12 inches in a foot. So 4 inches is 4/12's of a foot.... There's your formula! 4 divided by 12 = .333 of a foot

Much easier to figure out, while standing beside the wheelbarrow
I much prefer using decimals. We are supposed to use metric up here in Canada, but in reality, it becomes an ugly mish-mosh of both. Dimensions will be scribbled in ft/inch (from a tape measure), but the products are supplied in Kg or Liters. Sigh...
Yes, that was covered in my third paragraph. I gave the .083 so that anyone could use it to simply multiply any number of inches for an answer. (easier to do standing by the wheelbarrow, than a division problem)
Yes, that was covered in my third paragraph. I gave the .083 so that anyone could use it to simply multiply any number of inches for an answer. (easier to do standing by the wheelbarrow, than a division problem)
I politely disagree. Its easier to take the two numbers and divide them on the spot (beside the wheelbarrow), than commit to memory a 3-digit decimal number that gets you to the same place.... Why is it easier to multiply 6x0.083 than to divide 6"/12"? They both get you 0.5.
I politely disagree. Its easier to take the two numbers and divide them on the spot (beside the wheelbarrow), than commit to memory a 3-digit decimal number that gets you to the same place.... Why is it easier to multiply 6x0.083 than to divide 6"/12"? They both get you 0.5.
If you find division easier than multiplication, go for it.

Now everyone reading this has a choice.
With all the smart phones out there now there a few good apps for doing the conversions and multiplying all on the phone. Some are free and others cost just a little bit but well worth the price.
I use the wheelbarrow and hoe method. This allows mixing whole bags and when I'm done mixing it's al ready to transport and pour. I have a calibrated bucket so all I need to do is dump bag, pour water, stir, dump, repeat. The bucket has an old patch mortar stuck to the side that looks like England. I fill it to Liverpool.
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