Step 1 — Measure Your Project Area
Start by measuring the length and width of the area you plan to pour, and the thickness of the concrete. Use a tape measure and record all dimensions in feet and inches. For a simple rectangular slab, you only need three numbers: length, width, and thickness. For footings, you need the length, width, and depth of each footing section. For posts or columns, you need the diameter and depth of each hole.
Write your measurements down before using the calculator. It is easy to misremember a dimension when you move from the job site to your phone or computer. A small error in measurement, especially in thickness, compounds across the entire pour and can significantly affect how much material you need to order.
Step 2 — Calculate the Volume in Cubic Feet
Once you have your measurements in feet (convert inches by dividing by 12), multiply them together:
Volume (cubic feet) = Length (ft) × Width (ft) × Thickness (ft)
For example: a 10-foot by 10-foot slab at 4 inches thick (0.333 feet) equals 10 × 10 × 0.333 = 33.3 cubic feet. Our calculator handles the unit conversion for you automatically, so you can enter thickness in inches without manually dividing by 12.
Step 3 — Convert to Cubic Yards
Ready-mix concrete is ordered and priced by the cubic yard. One cubic yard equals 27 cubic feet, so divide your cubic footage by 27:
Cubic yards = Cubic feet ÷ 27
Continuing the example: 33.3 ÷ 27 = 1.23 cubic yards. This is the theoretical volume. You will add a waste factor in the next step before placing your order.
Step 4 — Add a 10% Waste Factor
No concrete pour goes exactly as calculated on paper. Subgrades are never perfectly flat, forms deflect slightly under pressure, and you always need a little extra to finish and screed properly. Add at least 10 percent to your calculated volume:
Order quantity = Cubic yards × 1.10
For our example: 1.23 × 1.10 = 1.35 cubic yards to order. Running out of concrete mid-pour creates a cold joint, a structural weak point that invites cracking and water infiltration. The 10 percent buffer is cheap insurance against this outcome.
Concrete Estimates by Shape
The formula changes slightly depending on the shape of your project. Here are the three most common shapes with their volume formulas and a worked example for each.
Multiply length by width by thickness. All in feet.
10 × 10 × 0.333 = 1.23 yd³
Multiply length by width by depth, then by the number of footings.
2 × 2 × 1 × 4 = 0.59 yd³
Use radius (half the diameter) squared times pi times depth.
3.14 × 0.42² × 3 × 4 = 0.25 yd³
Ready-Mix vs. Bagged Concrete
For projects under 1 cubic yard, bagged concrete from a home center is typically the most practical option. You can mix only what you need, work at your own pace, and avoid minimum-order charges and delivery fees from ready-mix suppliers. The trade-off is labor: mixing bags is physically demanding and time-consuming.
For any pour over 1 cubic yard, ready-mix concrete delivered by truck is almost always the better choice. It arrives at a consistent, tested mix design and the driver can often discharge directly into your forms, saving hours of mixing time. Ready-mix also eliminates the risk of inconsistent mix ratios that can weaken bagged concrete poured in batches. At 1.5 cubic yards and above, the labor savings alone justify the delivery cost for most homeowners.
Most ready-mix suppliers charge a short-load fee for orders under 3 to 5 cubic yards. If your project falls in the 1 to 3 cubic yard range, call local suppliers and compare the short-load fee against the cost of the extra bags and labor time before deciding.
How Much Concrete Do I Need for Common Projects
Use this reference table for quick estimates on the most common residential concrete projects. All slab estimates assume a 4-inch thickness. Bag counts are for 80-pound bags and include a 10 percent waste factor.
| Project | Cubic Yards | 80 lb Bags (with 10% waste) |
|---|---|---|
| 10 x 10 ft slab at 4 in | 1.23 yd³ | 62 bags |
| 12 x 12 ft slab at 4 in | 1.78 yd³ | 89 bags |
| 20 x 20 ft slab at 4 in | 4.94 yd³ | Ready-mix recommended |
| 20 x 30 ft driveway at 5 in | 9.26 yd³ | Ready-mix required |
| 10 in dia. fence post, 36 in deep | 0.06 yd³ | 3 bags (80 lb) |
| 8 x 8 ft garden shed pad at 4 in | 0.79 yd³ | 40 bags |
These are estimates based on standard formulas. Use the calculator above to get an exact figure for your specific dimensions. Always verify measurements on site before placing a material order.
For projects with complex shapes, footings combined with a slab, or any structural concrete work, consult your local building department or a licensed contractor before ordering materials. Local codes may require specific concrete strengths, reinforcement layouts, and minimum dimensions that will affect the total volume needed.