The formula every pour starts with
Concrete is sold by the cubic yard (27 cubic feet). For any rectangular pour: Length (ft) × Width (ft) × Thickness (ft) ÷ 27 = cubic yards. The classic mistake is forgetting to convert thickness — a 4-inch slab is 0.333 feet, not 4. A 10×10 ft patio at 4 in thick is 10 × 10 × 0.333 = 33.3 ft³ ≈ 1.24 yd³.
Skip the math entirely with our free concrete calculator — slabs, footings, round columns and steps, in imperial or metric.
Always add a waste factor
Pros order 5–10% extra. Subgrade is never perfectly level, forms bow, some mix stays in the truck or wheelbarrow. Running short mid-pour means a cold joint — a weak seam where fresh concrete meets concrete that already started curing. The calculator adds the waste factor for you (10% by default).
Bags or ready-mix truck?
Bag yields: a 40 lb bag ≈ 0.30 ft³, a 60 lb bag ≈ 0.45 ft³, an 80 lb bag ≈ 0.60 ft³ of mixed concrete. That means one cubic yard is about 45 × 80 lb bags — roughly 3,600 lbs of material to haul and mix by hand.
- Under ~0.5 yd³ (fence posts, small pads, steps): bags are fine.
- 0.5–1 yd³: bags are possible with a mixer and help, but your back will complain.
- Over 1 yd³: order ready-mix. It's consistent, fast, and usually cheaper per yard — most trucks deliver up to 10 yd³.
Quick reference: common pours
All at 4 in thick with 10% waste:
- 10×10 ft patio ≈ 1.4 yd³ (≈ 62 × 80 lb bags)
- 12×24 ft driveway strip ≈ 3.9 yd³ — ready-mix territory
- 24×24 ft garage slab ≈ 7.8 yd³ — one truck
- 18 ft of 16×8 in footing ≈ 0.65 yd³
Pricing the whole job, not just the mud
Concrete is often only a third of a flatwork budget — forms, rebar or mesh, base gravel, labor, finishing and sealing make up the rest. Contractors on ConstructHelp price the entire pour in about a minute with the free AI Pricing Tool: tiered pricing, a full materials list, and labor — calibrated to your local market. Homeowners can post the job free and get real quotes from local concrete pros.
