Are you planning to construct a concrete slab in your backyard? While you may be a DIY expert, determining the exact amount of concrete needed is not as easy as you might think.
Before starting any project with concrete, you need to know how to measure the exact amount. Not just so you could buy or order it but also so you can pour it in its entirety at one go.
Concrete poured later than the first lot does not dry out to be as strong and results in a “cold joint.” Therefore, running out of concrete is a situation you must avoid!
For measuring concrete, you can choose from various online calculators, which make the job much easier. But if you want to know the process in detail, you’ve come to the right place.
So, without further ado, let’s get calculating!
Measuring Your Concrete
Cement, aggregate and water combine to form concrete. You can either buy convenient pre-mixed bags for a quick job or order concrete by the yard for larger projects. Either way, you need to know the formula behind accurately measuring the concrete required. Before getting started, you will require a measuring tape, notepad, pencil and calculator.
1. Measure The Dimensions
Whether you’re planning to construct a column, slab or footing, you need to measure its length, height and width. If the area you are trying to cover is not a four-sided shape, try to divide it into simpler shapes and measure each part accurately.
If you want to use an online calculator, you’re good to go. Simply enter the measurements and the calculator will find the exact volume you require.
2. Calculate Cubic Metres
Just how we learned in school, multiply the length, width and height to find out the required volume in cubic meters. If your measurements are in centimetres, you can convert them to meters by dividing by 100.
3. Calculate Cubic Feet
One foot equals 12 inches, so you will have to divide the measurements by 12 to get the cubic feet. Also, keep in mind that fractions must be converted to decimals while calculating cubic feet.
For example, assume that the length, width, and height of your concrete project are 12 feet, 14 feet and 6 inches, and 7 and ¾ inches, respectively. In this case, the length is mentioned in feet, so it does not need any conversion.
The width is 14 feet, 6 inches. To convert 6 inches to feet, divide it by 12. The result will be 0.50 feet. Thus, 14 feet, 6 inches equals 14.50 feet.
Similarly, the height is also in inches, so we need to convert that as well. First, we’ll convert the fraction (¾) into a decimal by dividing the numerator by the denominator. The answer will be 0.75. Then divide 7.75 by 12, and you’ll get 0.64 feet.
Finally, you can multiply the length, width, and height to get the volume in cubic feet. Here, the volume of concrete required will be 12 x 15.50 x 0.64 = 119.04 cubic feet.
4. Calculate Cubic Yard
A yard is the most common unit of measurement for concrete, so it is best to convert your calculation to yards. You can do this by multiplying the cubic feet by 0.037 or by dividing it by 27.
So, 119.04 cubic feet equals 4.40 cubic yards.
Measuring Concrete Needed For Other Shapes
Let’s be real; concrete projects are not always the same. After all, how many rectangular footings will your patio need?
Thus, in cases of special shapes like a circle or cylinder, you require a different method and formula. Read on to find out how…
1. For Circles and Cylinders
We know that the formula of finding the volume of a circle/cylinder is
Volume = π x r2 x h; wherein ‘r’ stands for radius, and ‘h’ is the height of the figure.
So, when you measure the volume of a circular/cylindrical area, multiply the cubic feet by 0.037 or divide it by 27.
Therefore, the formula for volume of a circle in cubic yards is
yds3 = (π x r2 x h) x 0.037 OR (π x r2 x h)/27
2. For Staircases
Figuring out the concrete needed for a staircase may sound intimidating, but it’s a lot easier than it looks. Simply divide the staircase into multiple rectangles/squares and calculate the volume of each cuboid/cube.
Take out the sum of the volumes and multiply the cubic feet by 0.037 or divide it by 27.
Number Of Concrete Bags Required
Although mentioning the volume in cubic yards should suffice, knowing the number of concrete bags you need will make things easier for you and the seller. Especially if you’re working on a small project, buying pre-mixed bags is a convenient option.
For reference, check the bag measurement and its yield.
- 40 pounds yield 0.011 cubic yards of concrete
- 60 pounds yield 0.017 cubic yards of concrete
- 80 pounds yield 0.022 cubic yards of concrete
Moreover, ensure that you buy at least 10% extra concrete so that you are left with enough even after accidental spills and variations in volume.
Concrete mixtures offer varying strengths for different cases, so make sure you tell the seller about your project. Accordingly, they will recommend a specific strength of concrete.
Additionally, you may get a certain percentage of air-entrained concrete, depending on the climatic conditions of your area. This enhances the strength of the concrete to withstand extreme temperature and climatic changes.
Cost Of Concrete
The total cost of concrete varies according to the different factors mentioned above. While concrete mixes are rated by the yard, pre-mixed concrete is charged per bag.
Usually, the price of ready-mix concrete starts from $100 per yard and may surpass $150 depending on the strength and quality. The seller may charge you for delivery, loading and more.
In the case of pre-mixed concrete, 40 lb bags cost around $2.50-$5 per bag. The cost of 60 lb bags ranges from $3 to $6 per bag and bags of 80 lbs start from $4.50 and may go up to $7. Clearly, pre-mix concrete bags are much cheaper, making them the go-to choice for small projects.
Furthermore, the entire process of constructing a concrete slab may cost you anywhere between $6 and $20 per square foot. This includes the cost of labour for prepping, pouring and finishing. Needless to say, a simple brush finish will not cost as much as a decorative coloured finish.
Ready Mix Concrete vs. Pre-Mix Bags
As mentioned earlier, your choice of concrete depends on the project. For instance, large projects like a driveway, foundation slabs, sidewalks and so on will require concrete by the yard for easy mixing and pouring from a truck. On the other hand, pre-mix bags are ideal for smaller projects like a patio slab, staircases, ramps, repair work and so on.
Also, it must be noted that you will require a rented mixer or a wheelbarrow if you opt for bagged concrete. Delivery is optional, but in the case of fewer bags, you might have to transport them yourself.
If you opt for ready mix concrete, the seller will likely deliver the requirement unless you order partial batches. Then you may be charged some shortage fees.
Tips And Tricks For Beginners
Simply knowing the formula to calculate the exact amount of concrete required is not enough to start your own project. Especially if you’re working with concrete for the first time, things might get tricky. So, here are a few tips to ensure that you make the most out of your beginner’s luck!
1. Mix The Concrete FIRST!
Instead of getting your tools and location ready, start with finely mixing the concrete if you’re using a pre-mix. You can use a concrete spreader or a mixing auger to ensure that no chunks of concrete stick together.
2. Hot Water Is A No-go
You wouldn’t want the concrete to harden quicker than it is supposed to, and hot water does just that! Simply use cold water and pour a bit more if you’re inexperienced. This will slow down the setting process, allowing you to work at your own pace.
3. Pour Slowly And Steadily
To avoid a sludge-like texture, make a hole in the concrete mixture and pour water bit by bit. This will ensure that the water reaches all the parts and maintains the cement-to-water ratio.
“Cold joints” in concrete result in an uneven surface, premature cracks and many more hassles. It is crucial to know how to measure the exact amount of concrete you will require. This way, you can pour the mixture all at once and ensure the durability of your construction.
Before we sign off, we recommend that you wear your mask and gloves while working with concrete. Long sleeves are also a suitable option to protect your skin from lime present in concrete.
That’s all we have got for you in this informational guide.
Until next time!