Concrete Slab Calculator
Enter your slab's length, width, and thickness for the concrete it needs — by the cubic yard (how ready-mix is ordered) and by the bag (for small pours).
Concrete
1.23cu yd
- Volume
- 33.3 cu ft
- Bags of mix
- 56 bags
Concrete-volume estimate only. Slab thickness, reinforcement, and the compacted base for a structural or load-bearing slab follow local code and engineering — this doesn't design the slab.
How it’s calculated
Volume = length × width × (thickness ÷ 12) cubic feet, ÷ 27 for cubic yards. Bags = volume ÷ the yield of one bag, rounded up — an 80-lb bag of concrete mix yields about 0.60 cu ft (Quikrete; 60 lb ≈ 0.45, 50 lb ≈ 0.375). The bag count climbs fast, which is why all but the smallest slabs use ready-mix.
Worked example
A 10 × 10 ft slab, 4 in thick: 10 × 10 × (4 ÷ 12) = 33.3 cu ft = 1.23 cu yd. In 80-lb bags that's ⌈33.3 ÷ 0.60⌉ = 56 bags — so a slab this size is really a ready-mix order.
FAQs
- How much extra concrete should I order?
- Add about 5–10%. The subgrade is never perfectly flat, the forms flex, and you can't pour a slab in two batches — running short mid-pour ruins it. Ready-mix is ordered to the quarter-yard, so round up.
- Bags or ready-mix?
- Bags make sense only for very small pours — a few post footings or a tiny pad. Even a 10 × 10 slab is ~56 eighty-pound bags to mix by hand. Once you're past roughly half a cubic yard, a ready-mix truck (sold by the cubic yard) is cheaper, faster, and more consistent.
- How thick should the slab be?
- 4 inches is standard for patios and walkways; 5–6 inches for a driveway or anything bearing vehicles, usually over a compacted gravel base and often with rebar or wire mesh. Thickness, reinforcement, and the base for a structural slab are a code/engineering call — this tool sizes the concrete once you've set them.
Sources
- Volume = L × W × (thickness ÷ 12) ÷ 27 (geometry). Bag yields: Quikrete Concrete Mix published values (80 lb ≈ 0.60 cu ft, 60 lb ≈ 0.45, 50 lb ≈ 0.375) — confirm against your bag.