Ready-mix concrete
Concrete Slab Calculator USA 2026
Work out cubic yards of ready-mix concrete for any slab, plus bag counts, waste allowance and 2026 USA delivery costs. Free instant calculator.
Reviewed by BuildMate Editorial Team Construction Researchers Updated March 2026 2 min read Fact-checked
Concrete Calculator — instant results
Results update as you type
Volume133.3 ft³
Cubic yards (with 10% waste)5.43 yd³
60 lb bags needed326
80 lb bags needed245
Show calculation steps
- Volume = 20 × 20 × (4 ÷ 12) = 133.33 ft³
- Cubic yards = 133.33 ÷ 27 = 4.94 yd³
- With 10% waste = 4.94 × 1.10 = 5.43 yd³
- 80 lb bags = ⌈5.43 × 45⌉ = 245
Get Materials or Professional Project Help
Turn your ready-mix concrete estimate into a shopping list, or compare vetted local pros — both free.
Buy the materials
Affiliate links — BuildMate may earn a commission at no extra cost to you.
Hire a pro
Matched with up to 3 licensed local contractors. No obligation, no cost.
How the ready-mix concrete math works
Volume = length (ft) × width (ft) × thickness (in ÷ 12), converted to cubic yards by dividing by 27, then a 5–10% waste allowance for spillage, uneven subgrade and over-excavation.
Volume (ft³) = Length × Width × Thickness ÷ 12 → Yards = ft³ ÷ 27
Frequently asked questions
How much waste should I add?
10% is standard for slabs; 15% for uneven ground or complex forms.
How many 80 lb bags per cubic yard?
About 45 bags of 80 lb pre-mix per cubic yard.
What thickness for a driveway?
6 inches for cars, 8 inches for heavy vehicles.
Estimates based on 2026 RSMeans USA data and national material averages. Always confirm quantities with your supplier or contractor before ordering.