How Much Concrete Do You Need?

• Zero black-box math: every result shows the exact formula used.
• ASTM C94: 27 cubic feet per cubic yard, normal-weight concrete 145–150 lbs/cu ft.
• Waste protection: 5–15% buffer prevents short loads and cold joints. A 1/2-inch thickness error alone can cause an 11% shortfall.
• Ready-mix vs. bags: over ~1.5–2 yd³ (60–90 eighty-pound bags), a truck is usually cheaper and faster. Under ~4 yd³, expect a short-load fee.

1. Convert thickness to feet: divide inches by 12 (4" = 0.333 ft).
2. Volume in cubic feet: Length × Width × Thickness.
3. Convert to cubic yards: divide cubic feet by 27.
4. Add waste for uneven subgrade, spills and form spread.

Worked example: a 10 × 12 ft patio at 4" thick needs 10 × 12 × 0.333 ÷ 27 = 1.48 cubic yards — order about 1.63 yd³ with 10% waste. In bags, that's roughly 74 eighty-pound bags.

Bag yields (ASTM): 80 lb = 0.60 cu ft, 60 lb = 0.45 cu ft, 50 lb = 0.375 cu ft, 40 lb = 0.30 cu ft.

Safety: for load-bearing foundations, retaining walls over 4 feet, or structural columns, consult a licensed structural engineer and local building codes.