How We Calculate

Slab volume: cubic yards = (length × width × thickness ÷ 12) ÷ 27. Thickness in inches is divided by 12 to convert to feet. All three dimensions are multiplied to get cubic feet. Dividing by 27 converts to cubic yards — there are exactly 27 cubic feet in one cubic yard.

Post hole volume: cubic feet = π × (diameter ÷ 24)² × (depth ÷ 12). Diameter in inches is halved to get radius, divided by 12 to convert to feet, then squared. Result is multiplied by depth in feet and by π.

Bag yields are taken directly from manufacturer specifications: an 80 lb bag covers 0.60 cu ft (Quikrete, Sakrete); a 60 lb bag covers 0.45 cu ft; a 40 lb bag covers 0.30 cu ft. Bag counts always round up — never down — because running short mid-pour creates a cold joint that permanently weakens the slab.

Waste factor: 10% is added to all results, in line with American Concrete Institute (ACI) guidance for residential pours. This accounts for uneven subgrade, form overflow, and mixing loss.

See: Concrete Calculator, Concrete Slab Calculator, Concrete Driveway Calculator

Volume formula: cubic yards = (length × width × depth ÷ 12) ÷ 27. Identical to the concrete slab formula. Depth in inches is divided by 12, all three dimensions are multiplied for cubic feet, then divided by 27.

Weight conversion: tons = cubic yards × material density. Density values are industry averages: crushed gravel 1.40 t/yd³, pea gravel and river rock 1.35 t/yd³, crushed limestone and granite 1.50 t/yd³. Actual delivered weight varies by moisture and supplier. If your supplier gives you a per-yard weight, use that figure instead.

Waste factor: 10% is added for settling, edge losses, and compaction. Freshly delivered gravel typically compacts 15–20% over the first few months; the 10% overage is the standard starting buffer.

See: Gravel Calculator, Driveway Gravel Calculator, Pea Gravel Calculator

Unit conversions use exact multipliers: 1 square yard = 9 square feet; 1 square meter = 10.7639 square feet. Waste factors for flooring default to 10% for straight-lay installations and should be increased to 15% for diagonal or pattern installs.

See: Square Footage Calculator, Room Square Footage Calculator

Cost ranges shown in our calculators are national averages collected from Home Depot, Lowe's, and regional ready-mix suppliers. Prices are reviewed periodically and were last updated in early 2025.

Bulk material prices for gravel and ready-mix concrete depend heavily on your distance from the quarry or plant. Treat all price estimates as planning baselines, not supplier quotes. Always confirm current pricing locally before ordering.

Our calculators produce estimates, not engineered specifications. For structural concrete work — foundations, load-bearing walls, retaining structures — consult a licensed structural engineer. Local building codes, soil conditions, and climate can require different specifications than a general-purpose calculator can anticipate.

All results should be verified with your material supplier before ordering. Suppliers can account for local aggregate density, mix design variations, and delivery minimums that no online calculator can predict.