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How Much Concrete Do I Need?

posted by Bob Monday, August 15, 2011 at 9:16 AM

The number one question that we get on our hotline (1-866-Sakrete) is, “how much concrete do I need”? But let’s face it, unless you have a practical real world application for those boring math problems, you just don’t care enough to retain it. Well, now you do have a real world application so let’s go through the exercise so you will know how to do it without calling the next time. You can even impress your friends at dinner parties (if the subject about concrete should happen to come up). I would only recommend this topic for those ”friends” where you don’t mind if you were never invited back to again.

For those who are more direct, and want the short cut answer please scroll to links at the end of this blog.

The first thing you need to do is relax. It really ain’t that hard. Don’t let terms like “cubic feet” and “yield” and “density” bother you. There are basically two steps. The first step is figuring the volume of the space you want to fill. It doesn’t make any difference if you want to fill it with concrete or vanilla ice cream. Volume is volume. Because of the amount of concrete needed for most jobs it is usually easiest to figure the volume in cubic feet. If your project is large enough to calculate the volume in cubic yards instead of feet, call a ready mix concrete truck. When you hear “cubic” think ice cube. An ice cube usually has three sides (at least the ones you make in the trays in your freezer at home do). The way to find the volume of a cube is to multiple all three sides. If your cube were a perfect cube and it was 2 feet wide and 2 feet long and 2 feet deep, the volume would be 8 cubic feet. Concrete slabs are usually not perfect cubes so the math is a little different. If you were going to pour a slab that was 2 feet wide and 2 feet long it would not likely be 2 feet deep. It might be 4” deep which is a normal depth for a concrete slab. So now the equation looks like this: 2’ x 2’x 4”. The problem is you have to get everything into the same units; you can’t multiple feet by inches. This is where folks get hung up but it’s actually quite easy. Just multiple 2 x 2 x 4 ÷ 12. The answer is 1.3 cubic feet.

If you are filling a round hole in the ground to set a post or mail box, the calculation for volume is different than a cube or rectangle. The formula for the volume of a round hole is pi r2 x depth. Or 3.14 x radius x radius x depth. The radius is half the width of the hole. Unless you are doing a monster hole it might be easiest to do this all in inches. If your hole is 36” deep and 10” wide, the calculation is 3.14 x 5 x 5 x 36 ÷ 1728. This gives you 1.6 cubic feet. If you forget the 1728 it is simply 12” x 12” x 12” or the number of cubic inches in a cubic foot.

So now that we know the volume the second step is to calculate how many bags of concrete it will take to fill the hole. On the bag it will tell you how many cubic feet the bag will fill. This is the “yield”. The yield on an 80 lb bag is about 6/10 (.6) cubic feet. If you take the example above of 1.6 cubic feet, the calculation would be :

1.6 ÷ .6. = 2.7 bags


If you happen to be enthralled with math calculations I provide the following information on how to calculate the yield that I said was .6 cubic feet for an 80 lb bag. The first thing you need to know is the “density” of the concrete. If I give you a box filled with concrete that measures 12” x 12” x 12” (or 1 cubic foot), how much do you think it will weight? Go ahead and take a guess- almost everyone gets it wrong. The answer is 145 lbs. (If it was filled with beer it would weigh 64 lbs.) This means the “density” of concrete is about 145 lbs per cubic foot. Now that we have that information we can calculate the yield. Add up both the dry material in the bag (80 lbs) and the water it takes to mix it up (1 gallon which weighs 8.3 lbs) for a total weight of 88.3 lbs. Then divide the total weight by the density of the concrete.

88 ÷ 145 = .60 cubic feet.

There is one method for calculating the number of bags required which is even easier. We have already done the math for you on concrete as well as a number of other products. Visit www.Sakrete.com and select the calculator button or follow this link http://www.sakrete.com/products/calculators.cfm. Or for those interested in doing calculations on a mobile device please go to http://www.sakrete.com/mobile/calculators.cfm where our mobile site can help you determine how much you need wherever you are from the store aisle to the jobsite.

Bob Schmidt
Product Manager
Sakrete of North America

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90 USER COMMENTS

Dave, you will need 53.3 80lb bags.
- Chris Technical Services
Friday, July 11, 2014 at 9:45 AM

I need to pour a patio slab that's 16' Long by 8' wide by 3" thick. How many 80# bags will I need to complete this project?
- Dave
Thursday, July 10, 2014 at 8:38 PM

Joe, The best approach to this would be to compact the dirt, lay down plastic, and use 2”-4” of gravel then pour your concrete slab. You should not have a problem with freeze thaw issues in a crawl space at all.
- Chris Technical Services
Monday, June 23, 2014 at 10:43 AM

I am pouring a 100 ft sq x 3.5 in slab for a crawl space. Is it best put it on gravel (how thick?) over dirt, or with a plastic sheet over dirt underneath? Would a plastic sheet possibly cause some problem from freezing during winter?
- Joe Cotruvo
Sunday, June 22, 2014 at 3:20 PM

SC, if you mean you need to fill a 2'x2'x7" space you will need 4, 80lb bags. If you need to fill a 2'x2'x7' space you will need 47, 80lb bags.
- Chris Technical Services
Friday, June 20, 2014 at 1:33 PM

I need to fill a 2x2x7 foot space. How much Sakrete do I need?
- SC
Friday, June 20, 2014 at 11:24 AM

Wendy, If the material is getting hard it is sufficiently hydrated and cured.
- Chris Germain Technical Services
Thursday, April 17, 2014 at 8:05 AM

Thanks. How do I know it is sufficiently hydrated and cured? There must have been some slump since it was two guys who mixed by hand...
- Wendy
Wednesday, April 16, 2014 at 6:41 PM

Wendy, your potential problems include the slab not being strong enough and possible point load issues. You always want to make sure you have enough water for the mix to hydrate and cure correctly. Typically the mortar joints between bricks are approximately 3/8” to ½”.
- Chris Germain Technical Services
Tuesday, April 15, 2014 at 9:55 AM

My husband mixed the concrete for a 4" slab so there was almost no slump at all. I'm noticing on the edge of the finished slab that at the bottom, there are crevices where it was not possible for the concrete to lie flat because there wasn't enough liquid. What are the potential problems down the road? The slab will be covered on top ith bluestone and bricks will be laid up along the side. Also, is it a problem if brick mortar is mixed without enough water? Lastly, what is the maximum acceptable distance between bricks. It is just a 2' wall set against a concrete slab. Thanks.
- Wendy
Saturday, April 12, 2014 at 5:16 PM

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