How the slab calculation works
A slab is a rectangular box, so its volume is length × width × thickness. The calculator converts all three measurements to feet first. Thickness is usually given in inches, which is where most hand calculations go wrong: a 4 inch slab is 4 ÷ 12 = 0.333 ft thick, not 4 ft.
Area (sq ft) = length (ft) × width (ft) Volume (cu ft) = area × thickness (in) ÷ 12 Cubic yards = cubic feet ÷ 27 Total to order = cubic yards × (1 + waste %) Cost = total cubic yards × price per cubic yard
Example: 20 × 12 ft patio, 4 inches thick
The area is 20 × 12 = 240 sq ft. Multiplied by 0.333 ft, that is 80 cu ft. Divide by 27 and you get 2.96 cu yd. Adding 10% brings it to 3.26 cu yd, which you would round up to 3.5 yd if your supplier sells in quarter or half yards. If your local quote were $150 per yard, for example, the concrete alone would be about $489 before delivery charges. That $150 is just an example, not a price we have checked.
Understanding the measurements
Length and width
Measure the inside edges of your forms. If the slab is L-shaped, split it into two rectangles, run each one, and add the results.
Thickness
4 inches is a common thickness for patios and walkways (Sakrete’sflatwork instructions, for example, call for at least 4 in of concrete). Driveways, garage floors and slabs that carry heavier loads are often thicker. The right thickness for your project depends on its use, your soil and local code, so check with your building department or contractor. The calculator only does the math.
Waste
Subgrade is rarely perfectly flat, forms bow slightly, and some concrete stays in the truck chute or wheelbarrow. A margin of 5–10% is common for slabs; NRMCA suggests ordering 4–10% over the calculated volume (CIP 31). If your base is well compacted and screeded flat, the low end is usually enough.
Common mistakes
- Entering thickness in feet. Keep the unit on “in” unless you mean feet.
- Ignoring thickened edges. Some slabs have deeper edges or footings around the perimeter. Calculate those separately and add them.
- Comparing quotes on price per yard alone. Delivery fees, small-load charges and pump costs can change the total a lot for small orders.
Limitations
This calculator estimates concrete volume and material cost for flat, rectangular slabs. It does not size reinforcement, base material or footings, and it is not structural advice.
Frequently asked questions
How many yards of concrete do I need for a 10 × 10 slab?
At 4 inches thick: 10 × 10 × 0.333 = 33.3 cu ft, or 1.23 cu yd before waste (about 1.36 cu yd with 10%). At 6 inches thick it is 1.85 cu yd before waste.
Why is my cost estimate lower than a contractor's quote?
This tool prices the concrete only. A contractor's quote usually includes excavation, base, forms, reinforcement, finishing, labor and disposal.
Planning your budget? Read How to Estimate Concrete Slab Costs.
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