How Much Concrete Do I Need?


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 our calculators page before your next project. Determine how much you need from home, the store aisle or from the jobsite.


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Comments:

  • Lee-Technical Service

    06.28.2017
    Tracy, you will need 34 of the 80 lb bags of concrete mix to fill that area at 2" thick. The concrete mix has to be poured a minimum of 2" in thickness.
  • TRacy

    06.27.2017
    How many 80lb bag do i need for a 11x11 1 1/2 thick slag
  • Tracy

    06.27.2017
    Im doing a dog pen it is 10x10 but the roof we made 11x11 so Im thinking that we sould do a 11x11 concrete floor how much concrete woud that take ? with it beingatleast 1 1/2 thick PLEASE HELP
  • Lee-Technical Service

    06.26.2017
    Nick88, it will take approximately 21 of the 80 lb bags of concrete mix to fill an area 7 ft x 7 ft x 3 inches.
  • Nick88

    06.25.2017
    How many 80 lb bags of concrete will i need to pour a 7'x7'x3" slab? Thanks
  • Lee-Technical Service

    06.22.2017
    Cory, it will take approximately 27 of the 80 lb bags of concrete mix to fill an area 8 ft x 6 ft x 4 inches.
  • Cory

    06.21.2017
    How much will it take for 8x6 and 4inchs deep?
  • Lee-Technical Service

    06.21.2017
    James, you will need to consult a structural engineer for this information. You will have to supply them with all the load information and then they will give you the minimum length, width, and depth to properly support the load of your project.
  • James

    06.21.2017
    I am putting a 10 000l water tank on a 3m high steel structure. I was wondering how to calculate how much concrete i would need for a solid concrete foot as a base, to handle this 10 tonne weight? I am working on a ratio of 1:3:3 of cement: sand:stone/aggregate respectively. I am not sure about the depth of the concrete foot, otherwise I would have tried to calculate using your calculations..Thanks.
  • Lee-Technical Service

    06.06.2017
    Jay, you will need a fire rated concrete. We do not have a fire rate concrete in our product line. If you were to use our concrete, it would break down from the heat and flames.
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