Slab volume = net slab area × specified thickness. Convert thickness to feet before multiplying, divide cubic feet by 27 for cubic yards, and add any ordering allowance only after the exact geometric volume is known.
Concrete slab volume = net slab area × specified thickness. Convert inches to feet, divide cubic feet by 27 for cubic yards, then add any documented ordering allowance after the exact geometry is known.
Published: September 29, 2026
Last reviewed: September 29, 2026
Published by BuildYard. The guide reflects BuildYard's calculation methodology and editorial standards and does not replace project-specific professional advice.
Calculate the slab footprint from drawings or field measurements and split irregular slabs into simple shapes.
Deduct only openings that truly displace concrete; small penetrations may be handled differently by project practice.
Volume = area × thickness in consistent units.
A 4 in slab uses 4 ÷ 12 = 0.333 ft for the volume calculation.
Cubic yards = cubic feet ÷ 27.
Keep geometric volume separate from any allowance for subgrade variation, spillage, form deflection, or finishing needs.
Ready-mixed concrete is supplied by fresh concrete volume, while bagged concrete uses product-specific package yield.
Confirm minimum-load, delivery, placement, and project specification requirements before ordering.
Estimate concrete for driveways, patios, and sidewalks from segmented area and specified slab thickness, with joints, base, slope, and ordering allowance kept explicit.
Read guide →ConcreteCalculate concrete for rectangular columns, round piers, and cylindrical form tubes from cross-sectional area, height, and quantity.
Read guide →ConcreteEstimate cast-in-place concrete wall volume from gross wall area, wall thickness, and true openings while keeping footings, pilasters, and waste separate.
Read guide →Geometry example with a separate illustrative 5% ordering allowance; the allowance is not a universal requirement.
| Step | Calculation | Result |
|---|---|---|
| Thickness | 4÷12 | 0.333 ft |
| Volume | 1,500×0.333 | 500 cu ft |
| Cubic yards | 500÷27 | 18.52 yd³ |
| Illustrative 5% order | 18.52×1.05 | 19.45 yd³ |
Current manufacturer data, supplier specifications, drawings, field measurements, applicable codes, permit requirements, and qualified professional guidance can take priority over a general-purpose planning estimate.