Concrete Square Footage Calculator — Slab Area, Volume, and Cubic Yards
Concrete projects require two separate calculations: the surface area (square footage) and the volume (cubic yards). Ready-mix concrete is ordered and priced by the cubic yard, not the square foot. The square footage tells you how large the pour will be; the cubic yard figure tells you how much to order. Missing either calculation leads to either a short pour (which creates cold joints in the slab) or wasted concrete that you cannot return once mixed.
This calculator starts with length and width to get surface area, then uses the depth input to calculate volume. Standard residential slab depths are 4 inches for patios, sidewalks, and garage floors, and 6 inches for driveways. Footings, stem walls, and foundations require site-specific engineering specifications.
Concrete Volume Formula
Formula: Cubic Yards = (Length ft × Width ft × Depth ft) ÷ 27
Convert depth from inches to feet first: divide by 12. Common slab depths:
- 3.5 inches (standard 2×4 form board): 3.5/12 = 0.292 ft
- 4 inches: 4/12 = 0.333 ft
- 5 inches: 0.417 ft
- 6 inches: 0.5 ft
Example: A 20'×30' patio at 4 inches depth. Volume = 20 × 30 × 0.333 = 200 cubic feet ÷ 27 = 7.41 cubic yards. Order 7.5 cubic yards — always round up to avoid a short pour. Ready-mix trucks typically deliver in 0.5-yard increments.
Concrete Waste Factor and Short Load Fees
Always order 5–10% more concrete than the calculated volume. Concrete cannot be returned, but running short creates a cold joint — a structural weakness where fresh concrete is poured against partially cured concrete. A cold joint is a permanent defect in a structural slab. The cost of 0.5 cubic yards of extra concrete (approximately $75–$120) is trivial compared to cutting out and replacing a cold-jointed section.
Concrete suppliers charge short-load fees for orders under 3–5 cubic yards because the truck is not full. A 3 cubic yard short load may cost the same per yard as 7 cubic yards plus a $150–$250 short load surcharge. For small slabs where volume is under 3 cubic yards, consider using bagged concrete mixed on-site: 60 lb bags yield about 0.45 cubic feet; 80 lb bags yield 0.6 cubic feet. You need roughly 45 × 80 lb bags per cubic yard.
Concrete Applications by Square Footage
- Sidewalk section (4'×10', 4" deep): 0.49 cubic yards — use bagged concrete
- Small patio (12'×16', 4" deep): 2.4 cubic yards — borderline for ready-mix short load
- Standard patio (20'×24', 4" deep): 5.9 cubic yards — ready-mix delivery
- 2-car garage floor (20'×20', 4" deep): 4.9 cubic yards
- Driveway (12'×40', 6" deep): 8.9 cubic yards
Related tools: cubic yards calculator · rectangle calculator · garage calculator · cost per sq ft
Concrete Reinforcement and Square Footage
Reinforced concrete slabs require rebar or wire mesh in addition to the concrete volume. Rebar (typically #3 or #4 rods at 18 inch spacing in both directions) is calculated from the slab area. For 18-inch spacing in a grid, multiply the slab length by the slab width by 2 (two directions) divided by 1.5 (spacing in feet) to get linear feet of rebar. A 20'×20' slab: (20/1.5 + 1) × 20 ft per row = 280 linear ft in one direction, doubled = 560 linear ft total. Add 10% for overlap at splices. Wire mesh (6"×6" welded wire) is sold in 5'×50' rolls covering 250 sq ft. For a 400 sq ft slab: 2 rolls (500 sq ft) with remainder for overlap. Fibre reinforcement (polypropylene fibres added to the concrete mix) is dosed by the cubic yard, not the square footage — typically 1.5 lbs per cubic yard of concrete.