Concrete Slab Calculator
Know exactly how many bags — or how much ready-mix — before you drive to the store.
Your slab
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Generated by AllUnits.io — allunits.io/calculators/concrete-slab
How this calculator figures it
Slab volume is length × width × depth, plus a 10% allowance for uneven subgrade and spillage. Bag counts use the manufacturer yields on the Quikrete Concrete Mix technical data sheet (TDS #1101): an 80 lb bag yields approximately 0.60 cu ft, so 45 bags equal one cubic yard. Ready-mix is quoted in quarter-yard increments.
How it works
Concrete is sold by volume, so the whole job is one multiplication — with thickness converted to the same unit as length and width first:
Add the waste factor, then either divide by a bag's yield and round up to whole bags, or round up to the quarter cubic yard for a ready-mix order. Worked example — a 10 × 10 ft patio at 4" thick: 10 × 10 × 4 ÷ 12 = 33.3 cu ft (1.23 yd³). With 10% waste that's 36.7 cu ft (1.36 yd³), which is 62 bags at 80 lb, or 1.5 yd³ of ready-mix ordered.
Concrete slab FAQs
How thick should a patio slab be?
4 inches is standard for patios and walkways. Driveways are commonly 5 to 6 inches. That is general practice — confirm locally.
How many 80 lb bags make a yard?
45. A cubic yard is 27 cubic feet and an 80 lb bag yields 0.60 cubic feet: 27 ÷ 0.60 = 45.
When should I order ready-mix instead of bags?
Around 1 cubic yard — about 45 80 lb bags. Above that, ready-mix is usually cheaper and far less work. Note that short-load fees for orders below roughly 3 cubic yards vary by plant.
Why add 10% waste?
Uneven subgrade, spillage, and over-excavation all eat concrete. Adding 5–10% is standard estimating practice.
What strength is bagged concrete mix?
Quikrete Concrete Mix is rated 4,000 psi at 28 days per its data sheet.
Can I pour a concrete slab in sections?
Yes. Divide the slab with form boards and pour one section at a time, which keeps each mixing session manageable when working from bags. Plan the joints between sections as control joints — they double as crack control.
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Related calculators & sources
Setting posts instead? Use the Fence Post Concrete Calculator. Round footings and form tubes: Sonotube Calculator. Walls on the slab: Concrete Block Calculator. Bag yields come from Quikrete product data; convert measurements directly with the volume and length converters.
Estimates for planning and purchasing. Not engineering or code-compliance advice — confirm structural work with your local building department.