Key Takeaways
- Concrete is ordered by the cubic yard, but small projects are measured in cubic feet
- The basic formula is length x width x depth, then convert to cubic yards by dividing by 27
- Always add 5 to 10 percent for waste, spillage and uneven subgrade
- A standard 80-pound bag of premix yields about 0.6 cubic feet
- Ordering a little more is cheaper than a second delivery
Cubic yard is a unit of volume equal to a cube measuring 3 feet on each side, commonly used in the US for ordering concrete and other bulk materials.
Slump is a measure of concrete consistency, indicating how much a cone of fresh concrete settles under its own weight. It affects workability and finish quality.
Figuring out how much concrete you need is one of those tasks where the math is simple but the consequences of getting it wrong are expensive. Ordering too little means a second delivery with a cold joint where the old pour meets the new. Ordering too much means paying to haul away what you did not use. Either way, you lose money and time.
Here is how to get the number right.
The basic formula
For a rectangular slab, multiply length by width by depth. That gives you the volume in cubic feet. If you are ordering a ready-mix truck, divide by 27 to convert to cubic yards, because that is the unit concrete is sold in.
A patio that is 10 feet long, 8 feet wide and 4 inches deep:
10 x 8 x (4 / 12) = 26.67 cubic feet
26.67 / 27 = 0.99 cubic yards
That rounds up to 1 cubic yard for a small pour like this. For larger projects, the math scales the same way, but the waste allowance matters more.
Why depth matters more than you think
A 4-inch slab and a 6-inch slab look similar from the side, but the 6-inch version needs 50 percent more concrete. That is not a rounding error. People who eyeball depth instead of measuring it routinely order 30 to 40 percent too little, which is exactly the range where a second delivery becomes necessary.
Measure the depth at several points. An uneven subgrade means some spots need more concrete than others, and your order has to cover the thickest point, not the average.
The waste allowance
No pour goes perfectly. Concrete sticks to the chute. Some gets left in the wheelbarrow. The subgrade is never perfectly level. A 5 to 10 percent overage on the calculated volume covers all of that without leaving you with a pile to dispose of.
For a simple rectangular slab, 5 percent is usually enough. For something with curves, steps or odd angles, go to 10 percent. For a project where formwork is complicated and the subgrade is rough, 12 to 15 percent is not excessive.
Premix bags versus ready-mix
For small jobs like setting posts or pouring a mailbox pad, premix bags from the hardware store make more sense than scheduling a truck. An 80-pound bag of premix yields roughly 0.6 cubic feet of wet concrete. A 60-pound bag yields about 0.45 cubic feet.
Count the total cubic feet you need and divide by the yield per bag. For that 10 by 8 patio at 4 inches deep, you would need about 44 eighty-pound bags. At that volume, a ready-mix truck is almost always cheaper and faster.
Ready-mix makes sense once you pass roughly 1 cubic yard, which is about 45 bags. Below that, the bag route avoids the scheduling and minimum-order hassles that come with a truck.
Irregular shapes
Not every pour is a rectangle. A curved walkway, an L-shaped patio, or a slab around an obstacle all need the same basic approach: break the shape into rectangles, calculate each one separately and add the volumes together.
A kidney-shaped pool deck or something with compound curves is harder to estimate with a tape measure alone. For those, measure the widest and longest points, treat it as a rectangle with that bounding box, and accept that you will have a modest amount left over. That leftover is almost always cheaper than coming up short.
Column and footing volumes
Round columns use a different formula: pi times the radius squared times the height. A 12-inch diameter footing that is 3 feet deep:
3.14 x 0.5 x 0.5 x 3 = 2.36 cubic feet
Most residential footings are poured in cylindrical or rectangular forms, and the calculator handles both shapes. The key is measuring the diameter or width accurately, not guessing.
Common mistakes
The most frequent mistake is ordering by the bag count instead of the volume. Bags are convenient but the unit pricing changes dramatically with project size. The second most common mistake is forgetting that 4 inches of concrete is 0.333 feet, not 0.4. That decimal difference translates directly into how much you order.
The concrete calculator handles the conversion between cubic feet, cubic yards and bag counts so you do not have to carry the math in your head.
Editorial note: This article is reviewed for accuracy and updated regularly. All calculations use standard industry formulas. For the latest pricing and delivery terms, consult your local concrete supplier directly.
Frequently asked questions
How many bags of concrete do I need for a 10x10 slab?
A 10 by 10 foot slab at 4 inches thick is about 1.23 cubic yards, or roughly 56 eighty-pound bags of premix. At 6 inches thick, you would need about 84 bags. The concrete calculator gives you the exact count for your specific dimensions.
How much does a cubic yard of concrete weigh?
A cubic yard of plain concrete weighs approximately 4,000 pounds. That number goes up with heavier aggregates and down with lighter mixes. A standard ready-mix truck carries about 10 cubic yards.
Can I order less than a cubic yard?
Most ready-mix companies have a minimum order, often between 1 and 3 cubic yards depending on the market. For anything below that minimum, premix bags are the practical option.
How far in advance should I schedule a concrete delivery?
Call at least 3 to 5 days ahead during busy season. Same-day delivery is sometimes possible but not guaranteed, especially for specific mix designs or large orders. Have your forms set and reinforcement in place before the truck arrives, because concrete waits for no one.