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I would like to make a small concrete slab of the following dimensions.

Length: 1.2m
Width: 0.86m
Depth: 0.09m

The cubic volume as i understand it is all those numbers multiplied.
result: 0.0929m2

If a bag of pre-mix concrete says
0.2m2 x 50mm

How many bags would I need?

Update: Brick BBQ construction. I am doing this for "homework" :) enter image description here

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Your volume should be in cubic meters. Trivial, but true. – Tim Quinn Nov 30 '12 at 0:03
up vote 8 down vote accepted

I would say 10 or 11 bags (depending on how sloppy you are).

0.2 * .05 = .01

.0929 / .01 = 9.29 bags

Since you never get perfect yield from a bag, I would personally go with 11 bags.

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It might be that it actually says a square .2 meters on a side. This seems a more likely way of specifying the useful volume of the bag. Just in case that makes your numbers off by 1/2. The number in that case would be 19 or 20 bags. – Tim Quinn Nov 30 '12 at 0:12
Looking again, this looks like a homework problem, so it doesn't have to reflect common sense at all. – Tim Quinn Nov 30 '12 at 0:18
"Homework"? I guess it is. See my in progress home work LOL. Have updated my post. The calculation was quite accurate. After resizing the slab, we calculated 12 bags. I bought 14 and returned 1 back to the shop. – Valamas Nov 30 '12 at 2:03
Thanks for following up after the project! – Shimon Rura Nov 30 '12 at 3:11
Nice work, Valamas. – Tim Quinn Nov 30 '12 at 8:52

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