What this calculates
This calculator finds the volume of a flat rectangular slab from its length, width and thickness, then adds a waste allowance so you order enough to finish the pour.
Switch the project type to square or circular for round patios and fire-pit pads.
Formula
Volume (cu ft) = length (ft) × width (ft) × thickness (ft)
Thickness in feet = inches ÷ 12 → 4 in = 0.333 ft
Cubic yards = cubic feet ÷ 27
Worked example
10 × 10 ft patio, 4 in thick
- 1.10 × 10 × 0.333 = 33.33 cu ft
- 2.33.33 ÷ 27 = 1.23 cu yd
- 3.Add 10% waste: 1.23 × 1.10 ≈ 1.36 cu yd to order
Common mistakes
- Entering thickness in feet when you meant inches — a 4 ft slab is 12 times more concrete than a 4 in slab.
- Measuring the finished size but forgetting thickened edges or a sloped subgrade, both of which add volume.
- Skipping the waste allowance; running short mid-pour can leave a cold joint.
Frequently asked questions
How thick should a concrete slab be?
Many patios and walkways are poured around 4 inches thick, while slabs carrying vehicles are often thicker. Local code and your contractor determine the right thickness for your use — see the slab thickness guide.
How many yards of concrete for a 20 × 20 slab?
At 4 inches thick, a 20 × 20 ft slab is about 4.94 cubic yards before waste, or roughly 5.43 cubic yards with 10% added.
Should I round up my order?
Yes. Suppliers typically sell in fractions of a yard, and it is cheaper to have a little left over than to order a second short load.