How to Calculate Concrete in Cubic Yards
Learn how to calculate concrete yards with the cubic yards of concrete formula, worked examples, waste factors, and a fast way to convert any slab or footing.
By The Concrete Calculator Team ·
How to Calculate Concrete in Cubic Yards
To calculate concrete in cubic yards, multiply length by width by thickness in feet, then divide by 27. That single step converts cubic feet into the cubic yards that ready-mix suppliers actually sell. Get the depth into feet first, add a waste factor, and you have an order number you can trust.
I’ve been ordering ready-mix for twenty years, and the number one reason a pour goes sideways is bad math on the truck order. Come up short and you’re chasing a second delivery while the first load starts to set. Order way over and you’re paying for concrete that ends up in a wheelbarrow graveyard. Getting the yardage right is the cheapest insurance on the whole job.
The cubic yards of concrete formula
Concrete is priced and delivered by volume, and in the US that unit is the cubic yard. One cubic yard is a block three feet on every side, which works out to 27 cubic feet. That 27 is the number you divide by every single time. Here is the concrete yardage formula in plain terms:
cubic yards = (length_ft x width_ft x thickness_ft) / 27Every measurement has to be in feet before you multiply. Thickness is the one that trips people up because slabs are spec’d in inches. Divide the inch depth by 12 to get feet. A 4-inch slab is 4 / 12 = 0.333 ft, a 6-inch slab is 0.5 ft, and an 8-inch footing is 0.667 ft. Skip that conversion and your whole answer is off by a factor of twelve.
A worked example
Say you’re pouring a 10 ft by 12 ft patio at 4 inches thick.
- Convert depth: 4 in / 12 = 0.333 ft
- Multiply: 10 x 12 x 0.333 = 40 cubic feet
- Divide by 27: 40 / 27 = 1.48 cubic yards
Round up and add waste, and you’re ordering right around 1.6 to 1.75 yards. That’s the whole method. It scales to any rectangle, and for anything that isn’t a rectangle you break the shape into rectangles, solve each one, and add the totals.
Quick reference by slab size
The table below shows cubic yards for common slab sizes before waste, so you can sanity-check your own math at a glance.
| Slab size | 4 in thick | 6 in thick |
|---|---|---|
| 10 x 10 ft | 1.23 yd | 1.85 yd |
| 12 x 12 ft | 1.78 yd | 2.67 yd |
| 20 x 20 ft | 4.94 yd | 7.41 yd |
| 24 x 24 ft | 7.11 yd | 10.67 yd |
If you want the numbers for your exact dimensions filled in automatically, the concrete slab calculator does the depth conversion and the divide-by-27 in one screen.
Always add a waste factor
The formula gives you a perfect-world number, and job sites are not perfect. Subgrade is never dead level, forms bow, some concrete sticks to the chute and the wheelbarrow, and a little spills. Add 5 to 10 percent to your calculated volume to cover it. On a small pour I lean toward 10 percent, because being a quarter yard short on a one-yard job is a real problem, while a little extra just gets used up on a mow strip or the base of a post.
For ready-mix, round up to the nearest quarter yard, since that’s how most plants bill. For bagged concrete on smaller jobs, the same volume tells you how many 40, 60, or 80 lb bags to buy, and an 80 lb bag yields about 0.6 cubic feet.
Footings, columns, and odd shapes
Calculate each element on its own, then sum them. A footing is just a long, skinny slab, so length x width x depth divided by 27 still works. Round columns use the area of the circle, which is 3.14 times the radius squared, times the height, divided by 27. The habit that keeps you out of trouble is to never try to average a complicated shape into one guess. Split it, solve each piece, and add.
If you want to see how the finished quantity connects to a truck order and bag counts, walk through how much concrete do I need, and if you’re also planning delivery weight or slab load, concrete weight per yard covers the roughly 4,000 lb per yard you’ll be moving.
FAQ
How do I calculate concrete yards from inches of depth?
Divide the depth in inches by 12 to get feet, then use length x width x depth all in feet, divided by 27. A 5-inch slab becomes 0.417 ft of depth before you multiply.
Why do you divide by 27 in the concrete yardage formula?
Because a cubic yard is 3 ft by 3 ft by 3 ft, which equals 27 cubic feet. Multiplying your dimensions in feet gives cubic feet, so dividing by 27 converts that into the cubic yards suppliers actually sell.
How much extra concrete should I order?
Add 5 to 10 percent for waste, spillage, and uneven subgrade, then round up to the nearest quarter yard for ready-mix. Coming up short costs far more than a little leftover.
Can I calculate concrete in cubic yards for a non-rectangular pour?
Yes. Break the area into rectangles and simple circles, calculate the cubic yards of each piece separately, and add the totals together for your order.
How many 80 lb bags equal a cubic yard?
An 80 lb bag yields about 0.6 cubic feet, and a cubic yard is 27 cubic feet, so it takes roughly 45 bags to make one yard. That’s why bagged concrete only makes sense on small pours.
Ready to skip the pencil work? Drop your dimensions into the concrete yardage calculator and get cubic yards, cubic feet, and bag counts instantly.