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Concrete Calculator
Enter the finished slab dimensions. The result includes a 10% allowance for uneven ground and waste.Enter your measurements
Length × width × depth, converted to cubic yards, plus 10% allowance.
How this estimate works
Length × width × depth, converted to cubic yards, plus 10% allowance.
Measure twice and use the same unit throughout. Product coverage, material density, installation patterns, and local requirements can change the quantity you need.
Quick answer
For a rectangular slab, multiply length by width by depth, convert cubic feet to cubic yards, then add the selected ordering allowance. Measure depth in several places if the base is uneven.
Worked example
A 12 × 10 ft slab at 4 inches deep has 40 ft³ of concrete, or 1.48 yd³. The calculator's 10% allowance produces an order estimate of 1.63 yd³.
Assumptions
- The slab is rectangular with a uniform finished depth.
- One cubic yard equals 27 cubic feet.
- The default result includes 10% for base variation, spillage, and waste.
How is slab volume calculated?
Convert depth from inches to feet, multiply it by slab length and width, then divide cubic feet by 27. The displayed quantity includes the ordering allowance; it does not calculate reinforcement, subbase, forms, or structural capacity.
Where can the estimate be wrong?
An unlevel excavation, thickened edges, footings, ramps, steps, or material left in the chute can increase volume. Measure distinct shapes separately and ask the ready-mix supplier how it handles order increments and short loads.
What should be confirmed before ordering?
Confirm slab thickness, mix specification, reinforcement, joints, drainage, permits, and site access with the plans or a qualified local professional. A volume result is not a structural design.
How many bags is that?
An 80 lb bag of QUIKRETE Concrete Mix is published at about 0.60 cubic feet mixed. A 12 × 10 ft slab 4 inches thick is 40 ft³; applying the calculator's 10% default and rounding up is 74 bags of that mix. Confirm the actual bag yield on the product data sheet.
Last reviewed September 22, 2026. Source: QUIKRETE Concrete Mix No. 1101 data sheet. Product instructions take priority over a general estimate.
Common questions
Short answers for the measurement details people most often need before using this calculator.
How much concrete do I need for a slab?
Enter slab length and width in feet and depth in inches. The result converts the volume to cubic yards and includes a 10% allowance.
Does this concrete calculator estimate cost or weight?
It estimates order volume. Multiply the cubic-yard result by your supplier's current price or material-specific weight because local mixes and delivery charges vary.