Calculate concrete volume from slab dimensions
Concrete is commonly ordered by the cubic yard, while slab drawings typically specify length and width in feet and thickness in inches. Volume connects those measurements. Multiply the slab footprint by its thickness after converting inches to feet, then divide cubic feet by 27 to obtain cubic yards. This gives a starting quantity before any loss, excavation irregularity or extra volume at thickened edges.
The quantity calculation assumes a rectangular slab with uniform thickness. Garage slabs, foundation elements, turned-down edges, grade beams and sloped ground may need separate volume calculations. If the slab includes trenches or pads of different depth, calculate those sections individually and add the volumes before accounting for waste.
Concrete slab formulas
Volume (cu ft) = slab area × thickness (in) ÷ 12
Volume (cu yd) = volume (cu ft) ÷ 27
Order volume = volume (cu yd) × (1 + waste % ÷ 100)
For bagged concrete, the displayed bag estimate divides ordered cubic feet by an illustrative yield of 0.6 cubic feet per 80-pound bag. The actual yield depends on the product and mixture, so the label must control the number purchased. For larger pours, ready-mix delivery may be the more practical approach, but delivery charges, short-load fees and ordering increments differ by supplier.
Worked example: 20-foot by 12-foot slab
A 20-by-12-foot slab has a footprint of 240 square feet. At 4 inches thick, its concrete volume is 20 × 12 × (4 ÷ 12) = 80 cubic feet. Dividing by 27 gives approximately 2.96 cubic yards before waste. Adding a 10% allowance increases the planning quantity to approximately 3.26 cubic yards.
The same order volume equals about 88 cubic feet. Dividing by an illustrative 0.6-cubic-foot bag yield produces 147 bags after rounding up. A supplier may impose minimum delivery quantities or different batching increments, so the calculated value is not a guaranteed order size.
Cost components that change concrete slab pricing
Concrete volume is only one part of slab installation. Excavation, compacted subbase, reinforcement, vapor barriers, forms, pumping, finishing, curing and site access can change the project budget independently of cubic yards. The calculator prices material volume and square-foot installation labor separately, with an additional field for work included in a contractor's quote but not represented by those two rates.
Thickness and reinforcement should follow the applicable structural design and local requirements. A planning calculation cannot establish suitable slab thickness, load-bearing capacity or soil preparation for a building. Dimensions supplied by an engineer or qualified contractor are more appropriate when the slab carries significant loads.
Concrete slab questions
Why is concrete measured in cubic yards?
Concrete occupies three-dimensional space. Square feet describe the slab footprint, but cubic yards include thickness as well as length and width. Ready-mix concrete suppliers commonly quote by the cubic yard, making the volume conversion important for purchase planning.
Does this calculate footings or thickened edges?
No. The single-thickness model represents only a rectangular slab. Calculate the added trench or footing volume separately and combine it with the slab volume, avoiding overlap where those sections occupy the same space.
Should I always order exactly the calculated cubic yards?
No. Check supplier rounding, delivery minimums, excavation tolerances and the actual geometry before placing an order. The waste setting allows a quantity margin, but it cannot replace measurements of uneven ground or an approved construction plan.