Concrete Slab Calculator
Enter your slab size to get the concrete you need in cubic yards, metres cubed and bags, plus an estimated cost. Waste is included.
Concrete needed
Estimated cost
How to calculate concrete for a slab
Concrete is sold by volume. For a rectangular slab, multiply length by width by thickness, with everything in feet, then convert to cubic yards by dividing by 27. Thickness is usually quoted in inches, so divide it by 12 first.
Formula: cubic yards = length (ft) × width (ft) × thickness (in ÷ 12) × (1 + waste %) ÷ 27
Worked example: 12 ft × 12 ft × 4 in slab
Volume = 12 × 12 × (4 ÷ 12) = 48 ft³. Add 10% for waste and uneven ground: 52.8 ft³. Divide by 27: 1.96 cubic yards. In bags, that is 88 bags of 80 lb mix.
Why add waste?
Sub-grades are never perfectly flat, forms bow slightly and some mix is lost in the truck or wheelbarrow. Adding 5 to 10 percent is standard. Running short mid-pour is far more expensive than having a little left over, because a cold joint weakens the slab.
Ready-mix or bags?
As a rule of thumb, bags make sense under about 1 cubic yard. Above that, mixing by hand is slow and the cost per yard of bagged concrete is usually higher than ready-mix. Ready-mix suppliers often add a short-load fee for small orders, so check your local minimum before you compare prices.
Common slab thicknesses
Patios and walkways are normally 4 inches. Driveways and garage floors are 5 to 6 inches, or more for heavy vehicles. Check your local building code for your project.
Common slab sizes
Cost tips
Prices on this page are editable estimates, not quotes. Get 2 to 3 local quotes before you buy, and ask if delivery is included. Delivery, tax and waste can change the total a lot. Order a little extra for cuts and breakage, and confirm final quantities with your supplier or a qualified professional.
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Frequently asked questions
How many bags of concrete do I need for a 10x10 slab?
A 10 ft by 10 ft slab at 4 inches thick needs about 1.36 cubic yards with 10% waste, which is roughly 62 bags of 80 lb mix.
How much does a yard of concrete cover?
One cubic yard covers about 81 square feet at 4 inches thick, or about 54 square feet at 6 inches thick.
How thick should a concrete slab be?
4 inches is typical for patios and walkways. Driveways and garage floors are usually 5 to 6 inches. Follow your local building code.
How much waste should I add?
Add 5 to 10 percent. Use 10 percent or more on uneven ground, and when you are pouring into an excavated area.
Are the prices accurate?
Prices are editable estimates. The defaults are typical US figures, and your local price can be higher or lower. Delivery and short-load fees are not included.
How do I convert cubic yards to cubic metres?
Multiply cubic yards by 0.7646. This calculator shows both.