Your slab
Concrete you need
–
- Bags–
- Other bag sizes–
- Ready-mix order (rounded up to ¼ )–
- Gravel base (+10% for compaction)–
Estimate only. Bag yields are from the maker's data sheet for standard concrete mix (80 lb ≈ 0.60 cu ft); other mixes yield differently, so check your bag.
Safety: wet concrete can burn skin and cement dust harms lungs. Wear gloves, eye protection and a dust mask, and read the safety data sheet for the mix you buy (for example on Quikrete's or Sakrete's product pages). Call 811 before you dig.
Bags or ready-mix: what it costs
Default prices are typical US prices, checked October 2026, and are not quotes. Ready-mix plants charge extra for small loads; ask what their minimum and short-load fee are. Bags also mean mixing by hand or renting a mixer.
Order it
We earn a commission if you buy through these links, at no extra cost to you. As an Amazon Associate we earn from qualifying purchases.
How the concrete calculator works
Slab area in square feet × thickness in inches ÷ 12 gives cubic feet, plus your extra for spills and uneven ground. Divide by 27 for cubic yards, which is how ready-mix is ordered. For bags, divide the cubic feet by the bag's yield and round up.
Bag yields come from a maker's data sheet for standard concrete mix: 80 lb ≈ 0.60 cu ft, 60 lb ≈ 0.45 cu ft and 40 lb ≈ 0.30 cu ft (data sheet). So a cubic yard is about 45 bags of 80 lb, 60 of 60 lb or 90 of 40 lb.
Example: a 10 × 10 ft slab, 4 inches thick, is 33.3 cu ft, or 1.23 cubic yards. That's 55.6, so 56 bags of 80 lb before any extra, or 62 bags with 10% extra.
This page is for slabs and pads. For post holes and deck footings, a footing calculator fits better.
How many 80 lb bags of concrete make a yard?
About 45. An 80 lb bag of standard mix yields about 0.6 cu ft, and a cubic yard is 27 cu ft. That's 60 bags of 60 lb or 90 bags of 40 lb.
Is it cheaper to use bags or order ready-mix concrete?
For small pours, bags usually win because ready-mix trucks charge delivery and short-load fees. Somewhere around 1 to 2 cubic yards, ready-mix often becomes cheaper, and it's far less work than mixing 45 or more bags by hand.
How thick should a concrete slab be?
4 inches is standard for patios, walkways and shed floors. Driveways and slabs that carry vehicles are usually 5 to 6 inches. Local codes and the load may need more, plus reinforcement.