Concrete Slab Calculator
Size a rectangular pour: slab length × width × thickness becomes cubic feet, cubic yards, and cubic meters, a waste allowance is added, and the result is rounded to the quarter-yard figure you would actually give a ready-mix dispatcher.
Slab dimensions
Example: a 10 × 10 ft slab at 4 in thick with 10% waste → order 1.5 cu yd.
This tool sizes the pour and nothing else — slab thickness for a load, reinforcement, and permits are decisions for a licensed engineer or your building department.
From dimensions to a truck order
The volume chain is length ft × width ft × (thickness in ÷ 12) cubic feet, then
÷ 27 for cubic yards. The worked slab: 100 sq ft × ⅓ ft =
33.33 cu ft = 1.23 cu yd (0.94 m³). Adding 10% brings it to 1.36 cu yd, and quarter-yard ordering
rounds that up to 1.5 cu yd — leaving 0.14 cu yd
of margin in the truck. These figures come from the engine at build time, not from copy-editing.
The mistake that orders 12× too much
Thickness goes in as inches; length and width as feet. Entering a 4-inch slab as 4 feet thick multiplies the order by twelve — the most expensive typo in concrete estimating. If the cubic-yard figure looks wildly high, check the thickness field first.
Frequently asked questions
Why does the calculator ask for thickness in inches?
Because that is how slabs are specified in the field — a patio is "4 inches", a driveway "5 or 6 inches". The engine converts to feet internally (4 in = 1/3 ft). Typing 4 in the thickness box when you meant 4 FEET is the classic error, and it inflates the order twelve-fold.
What does ready-mix ordering in quarter yards mean?
Most ready-mix suppliers take orders in 0.25 cu yd steps, so the tool rounds your waste-adjusted volume up to the next quarter yard. It is a widespread convention, not a rule — confirm the increment and any short-load fee when you call the plant.
Why add waste to a pour that is exactly the slab volume?
Real subgrade is never perfectly flat, forms bow slightly, and some concrete stays in the chute and wheelbarrow. Coming up half a yard short mid-pour creates a cold joint, so the conventional allowance is about 10% — adjustable in the form.
Should I use bags or a ready-mix truck?
Volume decides. Below roughly a third of a yard, bags are practical; a full slab usually justifies a truck. To compare, take the cubic-feet figure from this tool into the concrete bag calculator and look at the bag count it produces.
Dimensions never leave your browser — the arithmetic runs locally. The full estimation pipeline is documented on the methodology page.