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How Much Concrete Do I Need?

posted by Bob

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. 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 from the store aisle to the jobsite.

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Mike, it will take approximately 10 of the 60 lb bags of concrete mix to fill your form tubes.
- Lee-Technical Service
Thursday, May 11, 2017 at 12:30 PM

... 4 sonotubes of 3 feet each... going with 8" diameter tubes... how many 60 lb (I can't lift the 80s... hehehe) will I need?
- Mike
Wednesday, May 10, 2017 at 2:31 PM

Terry, it takes 45 of the 80 lb bags of concrete mix to make 1 yard.
- Lee-Technical Service
Wednesday, May 10, 2017 at 10:22 AM

how many bags of 80 lbs cement in one yard
- Terry Lasseigne
Tuesday, May 9, 2017 at 5:42 PM

Question, this project would take a vey large amount of concrete. With the dimensions given, you are looking at more than 400 yards of concrete. it would not be possible to pour these slabs with our bag mix. You need to contact a ready mix concrete company for a job this size.
- Lee-Technical Service
Tuesday, April 25, 2017 at 9:55 AM

Michaelangelo, it will take 67 of the 80 lb bags of concrete to fill this area or 90 of the 60 lb bags.
- Lee-Technical Service
Tuesday, April 25, 2017 at 9:45 AM

How much concrete 4000 PSI will it take for Footing:12"x 20"x 8" All Frost-walls: 4'6" tall by 10" wide One Slabs: 60'x 260 x 6" One Addition: 60'x 100'x 6". Rebar 18" Grid-squares every 2" Styrofoam under all the slab 2"x4'x8' hope this a enough information so you can figure this out. I am kind of in a bind. My spouse is going to bid on tis work and I am not sure he is up to estimating such a big project because sometimes he forgets some things.. Just wanted to double check his figuring. thank you for your time.
- Question
Tuesday, April 25, 2017 at 8:43 AM

How much concrete will it take to fill a 25 (length) X 5'6" (width) X 3 1/2 deep hole
- Michaelangelo
Sunday, April 23, 2017 at 7:09 PM

Zane, it will take approximately 45 of the 60lb bags of concrete mix to fill that area.
- Lee-Technical Service
Friday, April 7, 2017 at 8:45 AM

How many 60# bags will it take for an 8ft. X 10ft. X 3" deep slab?
- Zane Westmoreland
Friday, April 7, 2017 at 8:12 AM