How Much Concrete Do I Need?


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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.


How To Calculate How Much Concrete I Need

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.




Comments:

  • David

    06.18.2018
    How many bags to equal a yard of ready mix.
  • Phillip

    06.15.2018
    How many bags of ready mix do I need for a 6'x 4" x 5" form
  • Kurian Medayil

    06.14.2018
    How many bags of cement and sand do i need to make a 4 inches thick yard?
  • Lee-Technical Service

    06.13.2018
    Robert, can you give us more information to go on? We will need length x width x depth of the curb and we can get you the calculations that you need.
  • Lee-Technical Service

    06.13.2018
    Tim G, it will take approximately 37 of the 80 lb bags of concrete mix to pour those stairs. If I followed your calculations correctly. Would that be 3 stairs with a small 24" x 54"x 2.5" landing area?
  • Robert

    06.12.2018
    How many l.f of 18 inch single curb do you get per yard
  • Tim G.

    06.09.2018
    I'm pouring a set of stairs the steps will have a height of 25" a width of 54" a base of 60" The stair tread will be12" and each rise will be 71/2" how many 80lb bag of concrete mix will be required to pour these steps
  • Lee-Technical Service

    06.07.2018
    Carlos, it will take approximately 32 of the 80 lb bags or 42 of the 60 lb bags of concrete mix to fill that hole.
  • Carlos

    06.07.2018
    I have a hole with a diameter of 2 ft. and is 6 ft. deep. How many bags of concrete mix do I need to fill it?
  • Lee-Technical Service

    06.07.2018
    Jon, it will take approximately 41 of the 80 lb bags of concrete mix to fill that area.
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