Concrete Volume Calculator

Concrete volume calculation determines how much ready-mix or bagged concrete is needed for a pour. Ordering too little causes cold joints (structural weakness); ordering too much wastes money. Always include a 5–10% overage buffer.

Volume formulas by shape:

Rectangular slab: Volume (ft³) = Length × Width × Thickness Convert to cubic yards: Volume (yd³) = Volume (ft³) / 27

Circular slab (patio, column footing): Volume = π × Radius² × Thickness

Cylindrical post or tube form: Volume = π × (Diameter/2)² × Height

Trapezoidal footing (wider at base): Volume = Thickness × (Top Width + Bottom Width) / 2 × Length

Unit conversions:

• 1 cubic yard = 27 cubic feet = 0.7646 cubic meters
• 1 cubic meter = 1.308 cubic yards

Bagged concrete yield:

Ready-mix ordering: Sold by the cubic yard. Minimum orders typically 1 yard. Delivery trucks carry 8–10 yards.

Worked example (the 10% setting, which is what the waste dropdown defaults to): A 12 ft × 20 ft concrete patio, 4 inches (0.333 ft) thick: Volume = 12 × 20 × 0.333 = 79.92 ft³ In cubic yards = 79.92 / 27 = 2.96 yd³ With 10% overage = 2.96 × 1.10 = 3.26 yd³ → order 3.5 yards

Using 80 lb bags: 3.26 × 45 = 147 bags. At roughly $8 a bag that is $1,176 of mix, against ready-mix at about $200/yd³ × 3.5 = $700 delivered. Prices are 2026 US ballparks and move with cement and fuel, so treat them as a shape rather than a quote.

Ready-mix is cheaper at scale. Bags make sense for volumes under 1 cubic yard or remote locations without truck access.