PreConstruct AIFree calculator
Concrete calculator — cubic yards and cubic metres
Slabs, strip footings and walls, pads and columns, and stair flights. Type the sizes from the drawings and get the volume before and after waste, ready-mix rounded up to the half yard, with the working printed underneath.
Order
8 CY
5.95 m³ with waste
Before waste
7.41 CY
5.66 m³ · 200 ft³
The working
- 30 ft × 20 ft × 4" ÷ 12 = 200 ft³
- 200 ft³ ÷ 27 = 7.41 CY (5.66 m³)
- with a waste allowance of 5 per cent = 7.78 CY → order 8 CY, to the half yard
A starting figure, not an engineered quantity. Sizes, reinforcing and code requirements come from your drawings, your specifications and your engineer — check them before you order.
Where a calculator stops
This does one run, from numbers you type. A bid is every run on every sheet.
- — It doesn't read the drawing set or its schedules
- — It doesn't measure on the PDF or keep the lengths
- — It doesn't show what an addendum changed
- — It doesn't price, add labour or build a proposal
Slabs, footings and walls
Volume is length × width × depth. With the length and width in feet and the depth in inches: ft × ft × (in ÷ 12) = ft³, then ft³ ÷ 27 = CY.
A 30 × 20 ft slab at 4 in is 200 ft³, or 7.41 CY — 5.66 m³.
Pads and columns
Count them, then multiply one piece by the count: n × L × W × D (in) ÷ 1728 = ft³. For a column, length × width is its section and the depth is its height.
Six 36 × 36 × 12 in pads are 54 ft³, or 2.0 CY.
Stair flights
A cast-in-place flight is a sloped waist slab plus the triangle of steps on top of it: width × √(rise² + run²) × waist plus ½ × run × rise × width.
A 4 ft wide flight rising 8 ft over a 10 ft run with a 6 in waist is about 6.87 CY.
Waste and ordering
Estimators add a waste allowance for spillage, over-excavation and uneven subgrade — more on a slab poured on grade than a wall held in forms. Ready-mix is ordered in half-yard steps in Canada and the United States, so the order is rounded up.
One cubic yard is 0.7646 m³.
Questions estimators ask
- How do I convert cubic yards to cubic metres?
- Multiply cubic yards by 0.7646. One cubic metre is 1.308 cubic yards. The calculator shows both, so a metric drawing and an imperial ready-mix order can be checked side by side.
- How much waste should I add to a concrete order?
- It depends on the element and the site. Walls held in forms lose little; slabs on grade over uneven or over-excavated subgrade lose more. Use the allowance your own past jobs support, and type it into the waste box.
- How thick is a typical concrete slab?
- Many interior slabs on grade are 4 in (100 mm), and garage and exterior slabs are often 5 in or more — but the drawings and the specifications set it. In Ontario, Part 9 also sets minimum strengths for interior floors, garage floors and exterior flatwork.
- How do I calculate concrete for stairs?
- Treat the flight as a sloped slab (the waist) plus the steps. Use the stair tab: flights, width, total rise, total run and the waist thickness from the section.
Stop retyping sizes into a calculator
On PreConstruct AI you measure on the PDF, type the sizes the drawings print, and the concrete, forms, bars, board and stringers come out with the arithmetic beside every line — in imperial, metric or both. Your first bid is free, with no card.