How many bags of concrete do I need? - DIY Chatroom

Willie T · 06-04-2009, 11:56 AM

How to Calculate Store Bought Concrete Coverage by Bag Size

All measurements will be recorded in feet and decimal equivalents of inches. 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. That would be .333 feet.

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.
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

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 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

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

3.14159265 x (.415 x .415) x 3 = 1.6 cubic feet

(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.... approximately.)

jogr · 06-04-2009, 12:26 PM

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?

Willie T · 06-04-2009, 12:39 PM

Quote:

Originally Posted by jogr

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?

1.66732 gallons per bag. Just kidding!

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 it and encourages cracks.

The best mixing setup I've found is to use a strong 1/2" drill (no smaller!!!) with a paddle the stores sell for mixing like this.

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.

Mort · 06-13-2009, 04:47 PM

Quote:

Originally Posted by jogr

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.

1forest1 · 11-05-2009, 07:41 PM

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...

Willie T · 11-09-2009, 09:46 PM

Quote:

Originally Posted by 1forest1

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 (easier to do standing by the wheelbarrow, than a division problem) any number of inches for an answer.