Concrete Slab Calculator – Patios, Floors & Driveways

For a constant-thickness rectangular slab, multiply length by width by thickness after converting all measurements to one unit. For feet and inches, divide inches by 12 before multiplication. Convert cubic feet to cubic yards by dividing by 27. This is net placed-concrete volume, not the final supplier order. Add a separately chosen allowance for the job conditions, then apply only the rounding or minimums actually required by the concrete supplier.

Worked example: For a 12 ft × 10 ft patio with a uniform 4 in thickness, convert 4 in to 1/3 ft. Net volume = 12 × 10 × 1/3 = 40 ft³ = 1.4815 yd³. With 5% allowance, required volume is 42 ft³ = 1.5556 yd³. A supplier that sells in 0.25 yd³ increments would round this to 1.75 yd³, assuming no other minimum.

Yes, for volume planning. The same geometric formula works for rectangular patios, driveways and floors when the volume is a constant-thickness slab. A large garage floor may contain thickened edges or isolated recesses. Measure these as additional non-overlapping project sections. Actual load capacity, reinforcement, joint spacing and thickness must follow a qualified design rather than being selected by a volume calculator.

Use more than one section: calculate the base slab volume, then add a separate volume for the additional thickness only. For example, if the full slab is 4 in deep and a 2 ft wide edge region is 8 in total, add a 4 in extra-depth strip, not a second 8 in strip over the original slab. Verify intersections so corners are not counted twice.

A circular solid slab is a short cylinder: π × (diameter ÷ 2)² × thickness. The round shape asks for diameter and height/depth, so use the slab thickness as its height. If concrete forms only an annular ring rather than a solid disk, choose the tube/ring shape and enter both outer and inner diameters.

Where an opening lies inside the slab footprint you measured, add a deduction section for its actual empty concrete volume. For a simple through-opening, this is opening length × opening width × slab thickness. Do not subtract the opening again if your original dimensions already excluded it. Large openings or beams may have more complex details that require separate takeoff.

The calculator defaults to an editable 5% planning allowance. That is not a universal requirement. For a well-formed, measured slab the appropriate allowance may be small; excavated, uneven or complex work may need more. If you know there will be pump priming loss, overbreak or nonuniform thickness, include the real volume or a reasoned allowance. The result keeps net volume and allowance separate so you can review both.

Not without thickness. Area measures only the footprint. A 100 ft² slab at 4 in is 33.33 ft³, while that same area at 6 in is 50 ft³. Calculate volume first, and only then convert to yd³ or m³. The field-level unit menus are useful when a slab is measured in feet but its thickness is specified in inches or millimetres.

This tool calculates the material quantities for both purchasing methods. Bag products may suit small quantities or locations that cannot accept a truck, while ready-mix may suit larger continuous pours. Compare real bag yield, local prices, delivery fees, labour, mixing capacity, placement time and supplier minimums. Bag totals and ready-mix volumes are alternatives, not quantities to add together.