Learn how to calculate how much concrete is needed for projects around the home and how many bags of concrete mix to buy.
Calculate the concrete volume in cubic feet and convert that volume into bags depending on the size bag available or the weight you are able to carry.
Using basic calculations, figure out the volume of large and small projects or irregularly shaped areas.
Are you wondering, “How many bags of concrete do I need for this project?” Learn how to calculate the concrete volume and convert that into the number of bags of concrete required for your project.
We’ll cover rectangular areas, circular footings, fence posts, and complex areas that include curves.
Before we continue our article, we must note that you must take safety measures to protect yourself when working with concrete. First, you must wear protective clothing. Second, you must use shoes that are special for working on concrete. Third, you should be careful and notice Jobsite conditions.
Many outdoor and a few indoor projects around the home require concrete. When small quantities are needed, it is better to purchase bags of concrete mix rather than have it premixed and delivered.
So just how many bags of concrete do I need for a project? It’s all about volume and calculating it correctly.
Regular concrete mix sold at lumberyards and home centers comes in different sized bags ranging from 40 to 80 pounds.
An 80-pound bag yields approximately 0.60 cubic feet of concrete. 60-pound bags yield 0.45 cubic feet and 40-pound bags just 0.30 cubic feet.
The actual yield is approximate because the amount of water added to the mix may vary. 60 to 80 pounds is a lot of weight to lift, carry and handle. If you don’t feel up to that, you can buy smaller bags instead.
How much concrete do I need? Let’s look at some different home projects and learn how to calculate the volume required for each one.
Concrete should always be at least two inches thick and often a thicker amount is necessary.
Always round up the number of bags calculated and always buy five percent more concrete (by volume) than you think you’ll need to account for spillage and irregular fill levels.
The pad we’re doing is 8-feet by 9-feet and 3 1/2 inches thick.
First, we convert all our units to be the same, and we need cubic feet as a result. 3.5 inches ÷ 12 inches per foot = 0.292 feet, therefore, our pad is 8 feet x 9 feet x 0.292 feet.
Calculating volume for a rectangular or square pad is easy. Length x Width x Depth = Volume required. 8 x 9 x 0.292 = 21.024 cubic feet.
So how many bags of concrete do I need? Number of 80 pound bags = 21.024 ÷ 0.60 = 35.04. 60 pound bags = 21.024 ÷ 0.45 = 46.72 and 30 pound bags = 21.024 ÷ 0.3 = 70.08 bags.
For deck footings, we calculate the volume of a cylinder. For this example, we’ll assume the footings have to be 3-feet deep and have a 10-inch diameter.
The volume of a cylinder is the radius x radius x pi x height. The radius is 1/2 the diameter. Our radius is 10 inches ÷ 2 = 5 inches.
Then we convert inches to feet. 5 inches ÷ 12 inches per foot = 0.42 feet. Pi = 3.14. Putting it all together we get 0.42 x 0.42 x 3.14 x 3 = 1.66 cubic feet. 1.66 cubic feet ÷ 0.6 cubic feet per 80 pound bag = 2.77 eighty pound bags per footing.
Fence Post Holes
Fence post calculations are similar to deck footing calculations, except, we have to exclude the volume of the fence post itself.
For our example, we’ll use an 8-inch diameter post hole and a 4-by-4 fence post. The holes will be 3-feet deep.
Our hole radius = 0.33 feet (8 ÷ 2 = 4. 4 ÷ 12 = 0.33). 0.33 x 0.33 x 3.14 x 3 = 1.03 cubic feet of hole volume.
Now we calculate the volume of the fence post. A 4-by-4 post is actually 3.5 inches by 3.5 inches. Converted to feet this is 0.292-by-0.292 feet. We calculate the post volume the same as for a pad. 0.292 x 0.292 x 3 = .26 feet for the post.
How much concrete do I need? Concrete volume = hole volume – post volume = 1.03 – 0.26 = 0.77 cubic feet per post hole. How many bags of concrete do I need? 0.77 ÷ 0.6 = 1.3 80 pound bags of concrete per post hole.
Complex areas can be broken down into small sections. The volume of each section is calculated and totaled.
Break down a curved walkway into rectangular and triangular areas. Calculate the volume of each area and add it up. The volume of a right triangle = length x width x 1/2 x depth.
Credits, References, and Resources
- Photo by Bushie at Flickr.com
- Using a concrete vibrator
- EmerGenie can help in emergency building situations.
- The Start-to-Finish Guide to Using Concrete via Popular Mechanics
- Quikrete Cement and Concrete Products via quikrete.com SPEC_DATA-ConcreteMix.pdf
- How to Calculate Concrete Video by Bob Vila
- Concrete Calculator Formula by Improvenet.com
- How Many Bags of Concrete Mix Do I Need from Lowe’s
- Concrete Calculator: Calculate Concrete Slab and Footing Pours via ConcreteNetwork.com
- Right Away Concrete | Concrete Calculator via Right Away Redy Mix