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
© 2013 Sakrete
USER COMMENTS
Can anyone tell me what percent of porcelain cement do you need to add in order to achieve 3,000 psi in order to have a good cement content?
- Roxane
Friday, September 16, 2011 at 4:18 PM
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For a general purpose concrete mix you can mix 1 part portland cement, 2 parts concrete sand, and 3 parts gravel. When using our Sakrete Portland Cement you will add 6 gallons of clean water to the 94lb bag along with the appropriate amount of sand and gravel which will yield 4.5 cubic feet of material.
- Dean
Friday, September 16, 2011 at 5:14 PM
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I live in Chicago, IL. I hired someone to rebuild my cement porch. Can you tell me how much time I have if want to finish project for this year? It's earlier November now.
- Donna
Tuesday, November 1, 2011 at 7:48 PM
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Donna, the amount of time you have left to finish this project can depend on many things. It is best to pour concrete when you have moderate temperatures (50-90 degrees F). If the cold weather is a concern there are a few things you can do to help out. People often use hot water to accelerate the set time on the concrete as well as keep the dry material in a warm area until you use it. For more information on cold weather applications check out the "placing concrete in hot or cold weather" section in the general blog.
- Dean
Wednesday, November 2, 2011 at 2:49 PM
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This Website Helped Me With A Math Project Thanks for the info , Im Glad My Teacher Recommended This Website for infORMATION. Thanks Sakrete.com !
- Jonae
Sunday, November 6, 2011 at 10:09 PM
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How much cementitious material is in an 80-lb bag of Sakrete High Strength Concrete?
- Julie
Saturday, November 12, 2011 at 2:23 PM
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Julie, the range of cementitious material can be found on the MSDS for the High Strength Concrete Mix which is 10-30%.
- Dean
Monday, November 14, 2011 at 9:21 AM
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I'm so glad I found my solution olnine.
- Ruvell
Monday, November 21, 2011 at 1:26 AM
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you know what. my math was wrong too. 4" is .33 not .25. and you should divide by the .6 not multiply. But instead of listening to me or this blog just use the handy calculator linked up top at the button "calculators-how much do I need"
- mathman
Saturday, September 22, 2012 at 3:20 PM
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how many bags of sakrete does it take for a hole that is 24 in. round and 5 1/2 ft deep with a 4 in post in hole. i have 25 of these holes
- mike
Tuesday, December 11, 2012 at 2:06 PM
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Mike, if you are setting a 4" square post into a hole with a diameter of 24" and 5 1/2 ft deep, you will need 28 80lb bags or 44 50lb bags for each hole. For all 25 holes the total amount of concrete you will need is 700 80lb bags or 1100 50lb bags. We also have a calculator tool at the top right corner of the page that can help determine the amount of material needed.
- Dean
Tuesday, December 11, 2012 at 3:40 PM
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how much cement would I need for a 14 foot quarter round slab?
- Tamara
Sunday, January 6, 2013 at 2:51 PM
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14 foot quarter round and 4 inches deep.
- Tamara
Sunday, January 6, 2013 at 2:56 PM
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Tamara, if you have a slab that is a quarter of a slab with a diameter of 14 feet, going 4 inches deep you will need 22 80lb bags or 29 60lb bags of Sakrete Concrete Mix.
- Dean
Thursday, January 10, 2013 at 9:03 AM
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I am trying to pour a small pad that is 12 sq ft,4" deep.How many 60 lb bags of ready mix sakrete will I need.
- John Salisbury
Tuesday, January 29, 2013 at 3:30 PM
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John, for a 12 square foot slab, 4" deep you will need 9 60lb bags of Sakrete Concrete.
- Dean
Tuesday, January 29, 2013 at 4:36 PM
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I have a foundation i need to put in.. it's 28 feet long and 6 foot high.. how many blocks will i need.. yes i still have to lay a footer i know below this.. i have to figure how much i need there also which will be prob 10 inch thick.. im trying to put my self in a bid of what its going to run me price range in just materials
- tim
Tuesday, April 2, 2013 at 4:08 AM
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Tim, I can help you out with the amount of concrete you will need for the footer. If you are laying a footer for the block foundation that is 28 feet long, 10 inches deep and let's say 1 foot wide then you will need 39 80lb bags. If you are using 60lb bags you will need 52 bags. You should contact the manufacturer of the blocks to see how many blocks you will need for that foundation.
- Dean
Tuesday, April 2, 2013 at 4:55 PM
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ok so ive tryed to calculat using ur methods can quite get it i have to lay a small footing its 12" x 3" x 64ft how many 80lb bags of quick creat do i need.
- josh
Wednesday, April 3, 2013 at 5:01 PM
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Hi Josh, for an area that is 12in. x 3in.x 64 ft you will need 27 80lb bags of Sakrete Concrete Mix.
- Dean
Thursday, April 4, 2013 at 8:29 AM
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How many bag of 80 pound bags of cement will I need to a space 4" thick by 15'x18'
- Randy Carpenter
Saturday, April 27, 2013 at 7:28 AM
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Randy, for a slab that is 15' x 18' x 4" you will need 150 80lb bags of Sakrete Concrete mix.
- Dean
Wednesday, May 1, 2013 at 10:50 AM
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I am doing a footer that is 12 inches deep, 18n inches wide and 40 feet long. how many 80 pound bags of concrete will it take to fill this?
- jarad
Wednesday, May 8, 2013 at 9:09 PM
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Jarad, for a footer 12" deep, 18" wide and 40' long you will need 100 80lb bags of regular Sakrete Concrete. If you decide on using the Sakrete Maximizer for the footer that will only need 60 80lb bags due to the high yield of the Maximizer.
- Dean
Thursday, May 9, 2013 at 9:02 AM
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