Concrete for a 20 × 20 slab
Short answer
A 20 × 20 ft slab at 4" thick needs about 4.94 cubic yards of concrete — roughly 245 80-lb or 326 60-lb pre-mix bags (including 10% waste).
Concrete
4.94 yd³
80-lb bags
245
60-lb bags
326
40-lb bags
489
20 × 20 slab at 4" thick
A 20 × 20 ft slab is 400 square feet. At the standard 4-inch thickness that works out to 133.3 cubic feet, or 4.94 cubic yards including a 10% waste allowance. For a driveway or anything bearing vehicle loads, step up to 5–6 inches.
Different size or thickness?
Open the concrete calculatorOther slab sizes
Frequently asked questions
How much concrete for a 20 × 20 slab?
A 20 × 20 ft slab at 4" thick needs about 4.94 cubic yards (133.3 cubic feet), including a 10% waste allowance.
How many bags of concrete for a 20 × 20 slab?
About 245 80-lb bags, 326 60-lb bags, or 489 40-lb bags. Above roughly 1 cubic yard it is usually cheaper to order ready-mix than to buy bags.
Is a 4-inch slab thick enough?
Four inches is standard for patios, walkways, and shed floors. Go to 5–6 inches for driveways or anything carrying vehicle loads. Change the thickness in the calculator to see the new amount.