Real Examples: Common East Valley Pours
Backyard Patio — 10×20 ft at 4 inches
10 × 20 × 0.33 ÷ 27 =
2.44 cubic yards
Add 10% waste buffer:
order 2.7 yards
This is a classic small-truck job — under 3 yards, the kind of pour that most ready-mix suppliers charge a short-load fee for or won't take at all. King Concrete's small trucks start at 1 yard with no penalty.
Standard Driveway — 12×30 ft at 4 inches
12 × 30 × 0.33 ÷ 27 =
4.40 cubic yards
Add 10% waste buffer:
order 4.9 yards (round up to 5)
A good candidate for either a small or large truck depending on scheduling. At 5 yards, a large truck is more efficient.
Double-Wide Driveway — 20×30 ft at 4 inches
20 × 30 × 0.33 ÷ 27 =
7.41 cubic yards
Add 10% waste buffer:
order 8.2 yards (round up to 8.5)
Standard large truck territory. This is one of the most common residential pours in Mesa, a two-car driveway running from the street to a double garage.
Sidewalk Repair — 4×12 ft at 4 inches
4 × 12 × 0.33 ÷ 27 =
0.59 cubic yards
Add 10% waste buffer:
order 0.65 yards (round up to 1)
This is where the 1-yard minimum matters. You can't order half a yard from most suppliers — and you shouldn't try to mix this by hand either. One yard delivered by small truck is the right call.
Garage Floor — 22×22 ft at 5 inches
22 × 22 × 0.42 ÷ 27 =
7.49 cubic yards
Add 10% waste buffer:
order 8.3 yards (round up to 8.5)
At 5 inches, this slab can handle daily vehicle traffic and tool storage weight. A common Arizona upgrade when homeowners convert an older 4-inch slab that's cracking.
Residential Foundation Slab — 30×40 ft at 6 inches
30 × 40 × 0.50 ÷ 27 =
22.22 cubic yards
Add 5% waste buffer:
order 23.3 yards
At this scale, a smaller waste buffer is appropriate — overordering on a 23-yard pour wastes significantly more material and money than on a small patio. Multiple truck loads are scheduled sequentially to keep the pour continuous.
Why the Waste Buffer Matters
Adding 5 to 10% to your concrete order isn't pessimistic, it's how professionals have always ordered concrete. Here's what eats into your calculated volume:
Subgrade variation. Your forms may look level, but subgrades are rarely perfectly flat. Low spots in the gravel base consume more concrete than your calculation assumes.
Form flex. Wood forms bow slightly under the weight of wet concrete, especially on longer runs. A form that bows out an inch across a 20-foot span adds measurable volume.
Spillage and waste. Some concrete sticks to the chute, spills at the edges of forms, or gets lost in transit between the truck and the pour. Even experienced crews lose a small amount on every job.
Cold joints. Running short mid-pour is the worst outcome. A cold joint where a first batch starts to set before the second arrives and bonds to it creates a structural weakness in the slab that can cause cracking along that line for the life of the driveway or patio. It's not cosmetic; it's structural.
The rule: for pours under 10 yards, use 10%. For pours over 10 yards, 5% is usually sufficient. When in doubt, round up to the next half yard. The cost difference is small. The downside of running short is not.
Irregular Shapes: Breaking Them into Rectangles
Not every slab is a clean rectangle. An L-shaped driveway, a patio that wraps around a corner, or a foundation with a bump-out all require a little more math. The approach is simple: break the shape into rectangles, calculate each one separately, and add them together.
Example: L-Shaped Driveway
Imagine a driveway that's 12×20 ft running from the street, plus a 10×10 ft apron in front of a side-entry garage. Those are two rectangles:
- Section A: 12 × 20 × 0.33 ÷ 27 = 2.93 yards
- Section B: 10 × 10 × 0.33 ÷ 27 = 1.22 yards
- Total: 4.15 yards → order
4.6 yards with 10% buffer
The same method works for any shape you can sketch on paper. Draw it, divide it into rectangles, calculate each piece.
Circles and Curves
For a circular pour (a round patio, a column footing), use:
3.14 × Radius² × Thickness (ft) ÷ 27 = Cubic Yards
A 12-foot diameter circular patio (radius = 6 ft) at 4 inches:
3.14 × 36 × 0.33 ÷ 27 =
1.38 yards → order
1.5 yards
Should I Order Bags or Ready-Mix?
This is the question almost every DIY homeowner asks, and the honest answer depends entirely on one thing: how many cubic yards your project requires.
When bags make sense
For projects under about 0.5 cubic yards, a fence post base, a step repair, patching a crack, or a very small footing, bagged concrete is practical. An 80-pound bag of Quikrete yields roughly 0.6 cubic feet, so you need about 45 bags to make a cubic yard. At roughly $6.75 to $8.00 per bag, that's $303 to $360 per yard in materials alone, before your time, mixer rental, and the physical effort of mixing and pouring it.
When ready-mix makes sense
Above 1 cubic yard, ready-mix is almost always cheaper per cubic foot and dramatically faster. At East Valley pricing of $160 to $195 per yard delivered, you're paying less per yard than bagging, and you're getting a consistent, professionally batched mix with the right water-to-cement ratio, not a batch that varies based on how carefully you added water to each bag.
More importantly: bagging a yard of concrete by hand takes a full day and a rented mixer. A ready-mix truck delivers it in 20 minutes, and your crew pours it in one continuous operation — which produces a better slab.
