Last summer, I decided to try my hand at a DIY patio project. I measured out the 10×12 foot area I wanted to cover, did some quick math, and ordered what I thought would be enough bags of concrete mix. After all, the instructions said each bag would cover 1 square foot at 1 inch thick. Seemed simple enough. But after mixing and pouring bag after bag, I realized to my dismay that I was going to run out of concrete way before I finished the job. What went wrong?
It turns out that when it comes to concrete, estimating how much you need isn’t quite as straightforward as I thought. While a “yard” of concrete is a standard unit, the actual coverage area in square feet can vary quite a bit depending on the thickness of the concrete slab. To make sure you get the right amount of concrete for patio slabs, shed foundations, sidewalks, and other projects, you need to understand the relationship between slab thickness and coverage area per yard of concrete.
What is a Yard of Concrete?
A yard of concrete is a standard volume measurement in construction, representing 27 cubic feet of concrete, or approximately 0.76 cubic meters. Some other common ways you may see a yard of concrete referred to are a “load” of concrete or CY (cubic yards) on construction drawings and blueprints.
Why Square Footage Coverage Matters
When working with concrete, accurately calculating the square footage your project requires is extremely important for several reasons:
- Underestimating yardage needs can lead to running out of concrete mid-job, leaving you with an unfinished mess.
- Ordering too little concrete can also prevent proper curing, weakening the slab.
- Alternatively, overestimating concrete needs leads to wasted material and unnecessary costs.
- Getting the right amount is especially crucial for structural projects like concrete shed slabs, foundations, and patios.
Slab Thickness Dramatically Impacts Square Footage
One of the key factors that determines how many square feet a yard of concrete will cover is the thickness of the concrete slab. Typical slab thicknesses can range from:
- 2 inches for sidewalks
- 4 inches for patios and garage floors
- 6 inches for driveways and parking lots
- 8-12 inches for heavy equipment foundations
As you can see, the thicker the slab, the less area a yard of concrete will cover. This is because concrete spreads out more when poured thinner. A yard of 2-inch sidewalk concrete covers more ground than a yard of 8-inch concrete for a foundation.
The Formula for Calculating Concrete Yardage
Now that we understand how slab thickness impacts coverage, we can use that information to calculate how much concrete is needed for a project. The basic formula is:
Length x Width x Thickness / Coverage factor = Yards of concrete needed
Where the coverage factor is the square feet per yard based on thickness from the table above. Let’s walk through a couple examples:
Example 1: A 10′ x 12′ patio that is 4″ thick
10 x 12 x 0.33 (4″ converted to feet) / 81 sq ft per yard = 0.495 yards
Round up to 0.5 yards of concrete needed
Example 2: A 6″ thick shed foundation slab measuring 8′ x 12′
8 x 12 x 0.5 (6″ converted to feet) / 54 sq ft per yard = 0.923 yards
Round up to 1 yard of concrete needed
Handy Concrete Calculator
Doing all the math by hand for different slab sizes and thicknesses can get tedious. To make it easy, we’ve created a concrete calculator where you can just plug in your measurements and thickness, and it will provide the estimated yards required.
For example, a 10×20 slab that’s 5 inches thick needs approximately 1.5 yards of concrete. A 15×15 area at 4 inch thickness calls for around 0.7 yards. Give it a try for your project here .
Common Concrete Project Examples
Here are some examples of concrete yardage estimates for common residential projects:
Patio Slabs
A 12×12 slab at 4 inches thick: ~0.6 yards
A 20×20 slab at 4 inches: ~2.5 yards
Shed Foundations
8×8 slab at 6 inches thick: ~0.5 yards
10×12 slab at 6 inches: ~1 yard
Sidewalks
3 ft wide x 20 ft long at 3 inches thick: ~0.25 yards
4 ft wide by 50 ft long at 4 inches thick: ~1.5 yards
When estimating for projects like these, remember to account for:
- Gravel base under the slab – usually 4-6 inches
- Reinforcing rebar, mesh, or fiber
- Wood or foam forms around edges
And don’t forget wastage! Add 5-10% extra for overpour, washouts, etc.
Correcting Common Concrete Estimating Mistakes
When trying to estimate how much concrete is required for a project, some common mistakes include:
- Assuming you need less thickness than required for the slab’s purpose
- Forgetting to account for additional elements like gravel, rebar, forms
- Not building in enough margin for error – at least 5% waste
- Simply dividing total area by 81 or another rule of thumb coverage
The bottom line is you can’t reliably calculate yardage by area alone. You have to consider the slab thickness for what you are building along with other factors. While rules of thumb can provide a rough estimate, doing the full math will give you the most accurate concrete quantity estimate.
Figuring out how many square feet a yard of concrete covers is essential knowledge for pouring good concrete slabs. The thickness of the slab is the primary factor affecting concrete coverage per yard. Thinner slabs spread concrete over more square footage. Once you understand that relationship, you can accurately calculate how many yards you need for any project using our simple formula.
You can feel confident that you’ll have the right amount of concrete when you use the right estimate. No more frustration from either running short or having excess concrete left over. Get your estimate right with our tips, table, calculator, and examples – then relax knowing your next concrete pour will go smoothly from start to finish.
