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.

Bob Schmidt

Product Manager

Sakrete of North America

## 185 USER COMMENTS

Glyn, it will take 14 of the 80 lb bags of concrete mix to fill this area, or 18 of the 60 lb bags.

- Lee-Technical ServiceWednesday, December 7, 2016 at 12:40 PM

how many bags to fill a 2' X 2' X 2' hole?

- GlynTuesday, December 6, 2016 at 7:52 PM

Dale, it would take 223 of the 80 lb bags of concrete mix to fill this area. That is almost 5 yards of concrete.

- Lee-Technical ServiceTuesday, November 15, 2016 at 11:15 AM

i want to make a pad 20ft by 20ft and four inch thick how many bags of 80 pounds would i need

- daleSaturday, November 12, 2016 at 2:47 AM

Jay, you are at almost 4.5 yards of concrete. It will take 316 of the 50 lb bags. However, the only concrete that we have available in a 50 lb bag Is the Fast Setting Concrete Mix. You would not want to attempt that with fast setting material. It will be hard enough with regular setting material because of the quantity that you need. If you use the 60 lb bags of concrete mix you need 267 bags. If you use the 80 lb bags you will need 200 bags.

- Lee-Technical ServiceMonday, September 26, 2016 at 10:17 AM

Rik, we have Sakrete 5000 Plus Concrete Mix that would work fine for that project. It reaches 2500 psi in 3 days, 3500 psi in 7 days, and 5000 psi in 28 days.

- Lee-Technical ServiceMonday, September 26, 2016 at 10:06 AM

I'm pouring 24 x 10 x 6 in patio how many 50lb bags will i need? thnx

- jaySunday, September 25, 2016 at 12:31 PM

I need a 32"x66"x24" block that can withstand 3200 lbs. What type of mixture is best and how much?

- RikFriday, September 23, 2016 at 11:57 AM

Greg, it will take approximately 56 of the 80 lb bags of concrete mix to fill this area.

- Lee-Technical ServiceTuesday, September 20, 2016 at 8:56 AM

how many bags to do an area 10 feet by 10 feet to a dept of 4 inches?

- GregSunday, September 18, 2016 at 7:11 PM