Concrete Slab Calculator
Enter your slab's length, width, and thickness to get the cubic yards of ready-mix to order — or how many 60 lb / 80 lb bags for a small pour.
Slab dimensions
A 4" slab is typical for patios and walkways; 5–6" for driveways.
You'll need
Order ready-mix by the cubic yard for slabs over ~1 yd³. Bags (60 lb ≈ 0.45 ft³, 80 lb ≈ 0.6 ft³) suit small pours and post holes.
How to calculate concrete for a slab
Volume in cubic feet is length × width × (thickness ÷ 12). Divide by 27 for cubic yards — how ready-mix is sold. Always add 5–10% for spillage, uneven subgrade, and over-excavation so you don't come up short mid-pour.
Scope the whole job, not just the slab
ProBuildCalc turns your iPhone's LiDAR scanner into a job-site measuring kit — walk the space and it captures the square footage, material takeoff, and a blueprint automatically. No tape measure, no graph paper.
Concrete calculator FAQ
- How do I calculate cubic yards of concrete?
- Multiply length × width × thickness in feet (convert the thickness from inches by dividing by 12) to get cubic feet, then divide by 27 to get cubic yards. A 10×10 ft slab at 4 inches is about 1.23 cubic yards.
- How many bags of concrete are in a cubic yard?
- A cubic yard is 27 cubic feet. An 80 lb bag yields about 0.6 ft³, so it takes roughly 45 bags to make a cubic yard; 60 lb bags yield about 0.45 ft³, or roughly 60 bags per yard.
- Should I order ready-mix or use bags?
- For anything over about 1 cubic yard, ordering ready-mix delivered by the truck is cheaper and faster than mixing bags. Bags make sense for small pours, footings, and setting posts.
- How thick should a concrete slab be?
- Four inches is standard for patios, sheds, and walkways. Driveways and anything carrying vehicle loads are usually 5–6 inches with rebar or wire mesh.
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See also: estimating guides · ProBuildCalc by trade