Ordering concrete is one of the few decisions on a job that is genuinely hard to walk back. Short a load and you have a cold joint; long by two yards and you are paying to have it hauled away. This calculator works from slab dimensions to cubic yards, truck loads and installed cost, with a waste allowance built in.
How the volume is worked out
Volume is length multiplied by width multiplied by thickness, converted to cubic yards. Because the thickness is entered in inches and the other two in feet, the sum divides the thickness by twelve, then divides the whole by twenty-seven, since a cubic yard is twenty-seven cubic feet.
The waste allowance is then added on top. Eight per cent is a reasonable default for a flat slab on a prepared subgrade. Over-excavation, a soft base, spillage at the chute and an uneven subgrade all push it higher; a tightly formed slab on compacted gravel with a vapour barrier can run closer to five.
Truck loads
The truck figure assumes nine cubic yards a load, which is a common working capacity for a standard mixer even where the drum is rated higher. It rounds up, because half a load still arrives on a whole truck. On a pour of any size, talk to the plant about the last load: many will send a short load rather than a full one, usually with a short-load charge attached.
What the cost covers
Two costs are modelled separately because they scale differently. Delivered concrete scales with volume, so it is priced per cubic yard including the waste allowance. Placing and finishing scales with surface area, not depth, so it is priced per square foot — a six-inch slab costs meaningfully more in concrete than a four-inch slab, but very little more to finish.
Pump hire, a power trowel, fibre or a specified finish sit in the extras field as a flat figure. Reinforcement, formwork, base preparation and excavation are not included.
Assumptions and limits
This is a rectangular slab of uniform thickness. Thickened edges, footings, haunches, steps and slabs on a fall all add volume that is not captured here; on those, split the pour into rectangles and add the results. The figure is an estimate for ordering and budgeting, not a substitute for a takeoff or a structural design.
Common questions
How much waste should I allow?
Five to ten per cent on a straightforward slab. Go higher on a rough or hand-dug base, on small pours where a single short load is a large percentage, and on anything pumped a long distance.
Why does the cost per square foot change when I change thickness?
Because concrete is sold by volume and finishing is priced by area. Doubling thickness roughly doubles the material but leaves the finishing cost alone, so the per-square-foot total rises less than you might expect.
Does this work in metric?
Enter feet and inches, and read the cubic metre figure in the results. A direct metric entry is on the list.
Is anything sent to a server?
No. The calculation runs in your browser. Nothing is transmitted or stored.