The base unit conversion
Concrete is geometric: length × width × thickness in consistent units gives the volume. In Imperial, the convention is cubic feet (ft³) for the raw multiplication, then divide by 27 to get cubic yards (CY) for ordering. In Metric, multiply lengths in meters to get cubic meters (m³) directly.
Quick references:
- 27 ft³ = 1 CY (3 × 3 × 3)
- 1 CY ≈ 0.7646 m³
- 1 m³ ≈ 1.308 CY
- 1 ft = 0.3048 m
Geometry formulas
Slab on grade
Volume = length × width × thickness. Convert thickness from inches to feet for an Imperial unit-consistent calculation: a 4-inch slab is 4/12 = 0.333 ft. A 30 × 20 ft slab at 4 inches:30 × 20 × (4/12) = 200 ft³ ÷ 27 ≈ 7.41 CY
Strip footing
Volume = length × width × depth. A 100 ft continuous footing 2 ft wide and 1 ft deep:100 × 2 × 1 = 200 ft³ ÷ 27 ≈ 7.41 CY
Rectangular column
Per column: length × width × height. Multiply by the number of columns. Eight columns at 12 × 12 × 10 ft:12 × 12 × 10 = 1,440 in² × ft = wait, units don't match.Convert dimensions consistently. 12 inches = 1 ft, so the column cross-section is 1 × 1 ft, volume per column = 10 ft³ ÷ 27 ≈ 0.37 CY. Eight columns total ≈ 2.96 CY before waste.
Round column
Volume = π × (diameter / 2)² × height. A 12-inch diameter, 10-ft tall column:π × (0.5)² × 10 = π × 0.25 × 10 ≈ 7.854 ft³ ≈ 0.29 CYMultiply by column count for the total.
Wall
Volume = length × height × thickness. A 20 ft wall, 8 ft tall, 8 inches thick:20 × 8 × (8/12) ≈ 106.67 ft³ ÷ 27 ≈ 3.95 CYSubtract openings (doorways, window penetrations) before multiplying by waste.
Waste percentage
The geometric volume is exact. The order volume is the geometric volume plus a waste factor that covers:
- Spillage during placement
- Concrete left in the chute and the mixer drum
- Formwork voids and minor scope creep
- Trim cuts and rough edges that need extra material
Common defaults:
- 5-8% for large simple slabs (warehouse floors)
- 10% as a general default
- 12-15% for complex pours: curved walls, multiple columns, congested rebar
- 15-20% for awkward small pours where the short-load fee makes over-ordering safer than under-ordering
Ordering from ready-mix
Round the with-waste volume up to the next 1/4 cubic yard. Most suppliers price in quarter-yard increments and deliver in 9-10 CY mixer trucks. A typical residential foundation might need 18 CY — order as 18.25 or 18.50 to give the driver some margin.
Watch the short-load fee. Suppliers charge a flat fee on orders under 4-5 CY (varies by market). A 3 CY order with $100 short-load fee may make a 5 CY order cheaper per yard. If you have multiple small pours, batch them into one truck.
Specify the mix design when ordering: psi rating (3,000 / 4,000 / 5,000), aggregate size, slump, air entrainment for cold climates, accelerator or retarder for weather, integral color if you're not painting later. Your structural engineer's spec dictates these.
Verifying the order
When the trucks arrive, the delivery ticket lists the CY delivered. Reconcile against your order. If you ordered 18.5 CY and the ticket says 17 CY, the driver was short — call the dispatcher before placement. Volume discrepancies under 5% are common (the truck's drum doesn't empty perfectly); over 5% is a billing issue you should resolve before signing.
Keep the delivery tickets with the bid file. Compare actual delivered yardage to your computed yardage on each pour; over time this builds a calibration for your waste percentages by pour type. That feedback loop is what separates a one-job estimator from a career estimator.
Concrete in the bidding workflow
Calculator-grade math gets you a preliminary sizing. For a competitive bid you want each concrete element measured from the drawing, with the per-element quantities flowing into a single formula-backed Excel workbook. BidScreen XL adds PDF measurement directly to Microsoft Excel — trace a slab, footing, or column on the plan and the quantity drops into the cell where you need it. Waste percentages, unit prices, and totals follow as formulas. When the drawing changes (and it will), you re-measure once instead of rebuilding the spreadsheet.