Concrete for a 16 × 20 slab
Short answer
A 16 × 20 ft slab at 4" thick needs about 3.95 cubic yards of concrete — roughly 196 80-lb or 261 60-lb pre-mix bags (including 10% waste).
Concrete
3.95 yd³
80-lb bags
196
60-lb bags
261
40-lb bags
392
16 × 20 slab at 4" thick
A 16 × 20 ft slab is 320 square feet. At the standard 4-inch thickness that works out to 106.7 cubic feet, or 3.95 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?
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Frequently asked questions
How much concrete for a 16 × 20 slab?
A 16 × 20 ft slab at 4" thick needs about 3.95 cubic yards (106.7 cubic feet), including a 10% waste allowance.
How many bags of concrete for a 16 × 20 slab?
About 196 80-lb bags, 261 60-lb bags, or 392 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.