Concrete Slab Calculator
This free concrete calculator turns your slab's length, width and thickness into bagged-mix or ready-mix quantities, a base gravel and wire mesh estimate, and a cost range.
What this calculator does
This planner converts a slab's length, width and thickness into a bagged-concrete estimate or a ready-mix cubic-yard order, with an optional gravel sub-base and wire mesh reinforcement suggestion. Sizing the sub-base separately? Use the gravel calculator for the compacted base layer on its own.
Pouring a basement or garage floor you plan to finish? Once the slab is in, the flooring calculator sizes LVP, laminate or carpet for it (or the tile calculator for tile, thinset and grout), and the drywall calculator covers the walls and ceiling.
How the math works
- Volume: area (length × width) × thickness, converted from inches to feet, plus your waste allowance.
- Bags: volume in cubic feet ÷ one bag's yield, rounded up. The cost estimate uses 60 lb bags (≈0.45 cu ft each); the shopping list also gives the count in 40, 50 and 80 lb bags.
- Ready-mix: volume in cubic yards, rounded up to the nearest 1/4 yard with a 1 cu yd minimum (typical supplier ordering increments).
Checking how far the bags you already bought will go? Our sister site's bags to cubic yards converter converts 40, 50, 60 and 80 lb concrete bags to cubic yards, and back.
Concrete for common slab sizes
Planning a standard size? The slab size guide has each one worked out, from small pads to 50x50 shop floors: bags, cubic yards, cost, control joints and weight.
How many bags of concrete are in a cubic yard?
One cubic yard is 27 cu ft. Bag yields are from QUIKRETE's Concrete Mix data sheet; other brands are close. Add your waste allowance on top, or let the calculator do it.
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.3 cu ft | 90 |
| 50 lb | 0.375 cu ft | 72 |
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.6 cu ft | 45 |
Worked example
A 10 ft by 10 ft slab (100 sq ft) at 4 in thick: 33.3 cu ft, plus 10% waste ≈ 36.7 cu ft. That's about 82 sixty-pound bags (or 62 eighty-pound, 98 fifty-pound or 123 forty-pound bags), or 1.5 cu yd of ready-mix once rounded up to the nearest quarter yard. With a 4 in gravel sub-base (≈1.7 tons) and 6x6 wire mesh (110 sq ft), the bagged DIY estimate runs roughly $507–$732; ready-mix delivered materials-only runs about $270–$380, including a short-load fee for an order that small.
Frequently asked questions
How much concrete do I need for a slab?
Multiply length by width by thickness (in feet) to get cubic feet, then add a waste allowance of about 10%. A 10x10 ft slab at 4 in thick needs roughly 1.5 cu yd of ready-mix, or about 82 bags.
How many 80 lb bags of concrete do I need?
An 80 lb bag of concrete mix yields about 0.60 cu ft, so it takes 45 bags to make one cubic yard. Divide your slab's volume in cubic feet (with about 10% waste) by 0.60 and round up. A 10x10 ft slab at 4 in thick needs about 62 eighty-pound bags, 82 sixty-pound bags, 98 fifty-pound bags or 123 forty-pound bags.
Should I use bagged concrete or ready-mix for my slab?
Bagged concrete is practical for small pours, generally under about 1 cubic yard (roughly 60 bags). Beyond that, ready-mix delivery is typically more practical and consistent.
Do I need wire mesh or rebar in a concrete slab?
6x6 welded wire mesh is common for crack control on residential slabs. It is not a substitute for engineered rebar on structural or load-bearing slabs — for an actual reinforcement grid, size it with the rebar slab grid calculator.
How thick should a concrete slab be?
4 in is standard for patios, walkways and sheds. Driveways and slabs supporting vehicle loads are typically 5–6 in or more — check local code requirements.
Do I need a gravel base under a concrete slab?
A compacted gravel sub-base, commonly 4 in, improves drainage and reduces cracking from ground movement — especially on clay or poorly draining soil.
Limitations
This tool estimates materials and typical costs only. It is not a structural design — wire mesh is for crack control, not a substitute for engineered rebar on load-bearing or structural slabs. Confirm thickness and reinforcement requirements for your specific use with a qualified professional.
Put this calculator on your site
Run a blog or business site for DIYers, homeowners or contractors? You can embed the Concrete Slab Calculator as a free widget — responsive, no registration, and no ads or tracking added to your page.
Cite this calculator
Using this tool in a class, a paper or an article? Copy a ready-made reference:
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