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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197 USER COMMENTS

Pat, you will need approximately 14 of the 80 lb. bags of Concrete Mix to fill this area.
- Lee-Technical Service
Wednesday, March 22, 2017 at 12:55 PM

I have a 5' x 5' x 4" deep how many bags of concrete do I need
- Pat
Tuesday, March 21, 2017 at 5:50 PM

Weqwe, concrete weights approximately 145 per cubic foot after cured. 9'x 9'x 4"= 1 yard of concrete. It takes 27 cubic feet to make 1 yard of concrete.
- Lee-Technical Service
Friday, March 3, 2017 at 12:57 PM

How much does a concrete slab that is 9' x 9' x 4" weigh? (tons)
- Weqwe
Thursday, March 2, 2017 at 12:33 PM

Aras, you will need approximately 101 of the 30Kg bags to fill that area.
- Lee-Technical Service
Tuesday, January 24, 2017 at 2:29 PM

Carol, you will need 267 of the 80 lb bags to fill an area that size.
- Lee-Technical Service
Tuesday, January 24, 2017 at 2:25 PM

How many bags of 30Kg cement i need to concrete 120 Sqft or 10ft X 12ft X 5inch.thanks
- Aras
Monday, January 23, 2017 at 7:51 PM

im doing a 20' by 24' how many 80 pounds bags do i need at 4 inches thick
- carol
Friday, January 20, 2017 at 10:04 PM

Rana, it will take approximately 7.1 bags of the 80 lb concrete mix to fill these areas. So you will need to purchase 8 bags.
- Lee-Technical Service
Tuesday, January 3, 2017 at 1:56 PM

WANT TO KNOW HOW MUCH REquired to make square slab of 1feetx1 feet actually i have to make 2 feetx8 feet +3" slab in my kitchen Material how much i needed to make 1'x1'x3" and 2'x8'x3" 1)CEMENT IN KG (AS one BAg In PAkistan is 50 KG) 2)Stone Crush 3)Sand to make combination of aboce 3 to make one slab so that i purchase that material avoid wastage Regards/Rana Gulzar Ahmed
- RANA GULZAR AHMED QAMAR
Friday, December 23, 2016 at 8:13 AM

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