I'm pouring a 12 by 20 foot shed slab, 4 inches thick.

Order 3.0 cubic yards.

Math:

1. Volume: 12 ft × 20 ft × 0.333 ft (4 in.) = 80 ft³
2. Yards: 80 ft³ ÷ 27 ft³/yd³ = 2.96 yd³
3. Waste/Over-excavation: Add 10% (standard for grade variance, spillage, pump loss) → 2.96 × 1.10 = 3.26 yd³

Order 3.0 yards (most plants sell in ¼ yd increments). If your subgrade is perfectly flat and compacted, 2.75 yd might squeak by, but 3.0 is the safe pro order. Short loads incur fees ($100–$200) and cold joints.

Specify to dispatch:

• Mix: 3,500–4,000 psi, ¾" aggregate, 4–5" slump (no water added on site).
• Air: 5–6% if freeze/thaw exposure (IRC R402.2).
• Reinforcement: #3 rebar 18" O.C. or 6x6-W2.9xW2.9 WWF, chairs to mid-depth (ACI 332).
• Vapor barrier: 10-mil poly under slab (IRC R506.2.3).

Pro tip: Have screed boards, bull float, and help ready before truck arrives.

To calculate the concrete needed for your 12 ft × 20 ft shed slab, 4 inches thick:

1. Convert thickness to feet: 4 inches = 4/12 = 0.333 ft

1. Calculate volume in cubic feet: Volume = Length × Width × Thickness = 12 ft × 20 ft × 0.333 ft = 80 cubic feet

1. Convert cubic feet to cubic yards: 1 cubic yard = 27 cubic feet So, 80 ÷ 27 ≈ 2.96 cubic yards

1. Account for waste and overage: Always order 5–10% extra for spillage, uneven subgrade, or slight over-pour. 2.96 × 1.10 ≈ 3.26 cubic yards

Order 3.5 cubic yards to be safe — this covers typical waste and ensures you won’t run short mid-pour. Concrete is usually sold in 0.5-yard increments, so 3.5 yd³ is standard and practical.

> ✅ Tip: Ensure your subgrade is compacted gravel (4–6”) and use a vapor barrier if required by local code (check IRC R506.2.3). For a shed, reinforcement (wire mesh or rebar) is often optional but recommended for crack control — especially if soil is expansive or loads are heavy.

Order: 3.5 cubic yards of concrete. (If unsure about soil conditions or local amendments, consult your building inspector or a licensed contractor.)