Calculate concrete for a patio, pad or several slabs with different thicknesses. Add each section, get one combined cubic-yard total and review the unrounded calculation before applying an optional allowance.
Add one or several slab areas with their own thicknesses.
Example: a 12 × 8 ft slab at 4 inches plus a separate 6 × 4 ft section at 6 inches.
First section: 12 × 8 × (4 ÷ 12) = 32 ft³. Second section: 6 × 4 × (6 ÷ 12) = 12 ft³. Total net volume: 44 ft³ = 1.63 yd³. With an illustrative 10% additional-material allowance, the combined volume is 48.4 ft³ = 1.79 yd³.
Calculate each section using its actual concrete-filled depth, add the unrounded volumes and apply the selected allowance once. This estimates quantity, not structural thickness or reinforcement. Convert your final volume to manufacturer-specific bags →
Use actual dimensions of each region. Make each region separate when thickness changes.
Find the usable length of each rectangular section in feet.
Measure at right angles to the length; split irregular shapes into sections.
Record the actual fill depth in inches for every section.
Use cubic feet for calculations; show cubic yards for supplier conversations.
Length (ft) × Width (ft) × Thickness (in) ÷ 12Multiply the result by the number of identical slabs. To express cubic yards, divide by 27. For multiple unique sections, calculate each separately and add the unrounded volumes.
With allowance: total net ft³ × (1 + waste % ÷ 100). Do not round each slab prematurely or add waste twice.
Dimensions refer to the concrete space, not any surrounding frame or decorative surface.
Different thicknesses, slopes, thickened edges or irregular boundaries may require a separate volume zone and additional site measurements.
The prefilled Slab Calculator demonstrates this example with a separate, editable 10% allowance.
A patio extension might be 12 ft × 8 ft × 4 in while a detached step/pad may have another thickness. Enter each section individually, then let the tool total the physical volumes before adding your global allowance.
Example: 12 × 8 × 4 in = 32 ft³; 6 × 4 × 6 in = 12 ft³; combined net volume = 44 ft³ ≈ 1.63 yd³. At 10% waste, the estimate becomes about 1.79 yd³.
Choose the right method for the geometry you actually plan to pour.
| Situation | Input approach | Important consideration |
|---|---|---|
| Rectangular patio | One length, width and thickness | Enter the actual fill thickness |
| Several separate pads | Add one section per distinct slab | Each can have different depth and quantity |
| L-shaped slab | Split into non-overlapping rectangles | Do not count shared intersection twice |
| Thickened perimeter | Base slab plus incremental edge volume | Extra depth only, not double the whole edge |
| Sloped or uneven base | Measure and sum suitable depth zones | Use documented average depth or site measurements |
| Round pad | Use round geometry on the general calculator | Inside dimensions matter |
Slab thickness is a design decision, not just a material estimate. The appropriate depth depends on use, soil/base preparation, loads, reinforcement, weather and applicable local rules.
This tool will calculate a user-entered thickness but does not certify that it is safe for patios, vehicle traffic or equipment foundations. Confirm the required design independently.
Bagged concrete may be suitable for smaller jobs, while delivered ready-mix can reduce the labor of mixing larger quantities. The right choice depends on supplier availability, crew/equipment and your pour plan.
Whole bags = ceil(adjusted ft³ / manufacturer yield per bag)For a named concrete mix, check the exact product's published yield instead of assuming every 80 lb bag produces the same volume.
Keep these decisions separate to avoid an apparently precise number that cannot be ordered in practice.
Measure forms, excavation depth and any thickened sections before calculation.
Adjust the waste percent for your actual handling and measurement conditions.
Minimum delivery, short-load charges and order increments are not added automatically.
Only need a single square pad? See the 8×8 to 30×30 ft volume chart on the general calculator. If you already know how many yards are available, convert them into square feet at your chosen depth. Comparing mixing bags versus a supplier delivery? Read the bagged vs. ready-mix guide. This page specializes in several separately measured slabs, changing thickness and combined volume.
A patio or equipment pad often has multiple sections or uneven fill depth. Keep volume, bag quantity and installed cost as separate decisions.
Helpful answers about dimensions, rounding and placing an order.
The net volume is 10 × 10 × (4/12) = 33.33 ft³, or about 1.23 yd³. With 10% allowance it becomes about 1.36 yd³.
For rectangular concrete, both use length × width × thickness. Slab or pad typically describes a project use, not a different physical volume formula.
Yes. Use Add another slab to enter a separate length, width, thickness and quantity for each independent area.
The calculator sums all net slab volumes first, then applies the single global allowance one time to the total.
No. Material volume is separate from labor, base preparation, reinforcement, delivery and contractor margin. Those require a dedicated cost estimate.
No. The examples illustrate geometry only. A qualified professional and local requirements determine appropriate structural thickness and reinforcement.
Geometry comes from rectangular-prism volume and the exact conversion 1 yd³ = 27 ft³. See the general concrete calculator for other shapes. Specific bag yields and placement instructions must be verified against current manufacturer instructions, such as QUIKRETE Concrete Mix No. 1101 documentation. This is not a structural design, building-code or professional estimate.
Independent estimating tools with transparent formulas, named product sources, seven reproducible QA baselines and engineering advisory support. Calculators provide quantity estimates, not structural designs or guaranteed supplier yields.