How to Calculate Yards of Concrete

1. Measure length and width in feet. Convert inches to feet by dividing by 12 — a 10 ft 6 in side is 10.5 ft.
2. Convert thickness to feet. Thickness is quoted in inches, so divide by 12. A 4-inch slab is 0.333 ft; 6 inches is 0.5 ft.
3. Multiply for cubic feet. Length × width × thickness.
4. Divide by 27 for cubic yards. This is the number your ready-mix supplier wants.
5. Add waste. Tack on 5–10% and round up to the next quarter yard.

• Thickness: 4 ÷ 12 = 0.333 ft
• Cubic feet: 20 × 20 × 0.333 = 133.3 ft³
• Cubic yards: 133.3 ÷ 27 = 4.94 yd³
• Plus 10% waste: 5.43 yd³ → order 5.5 yards

That is a single truck with room to spare, and it sits above most short-load thresholds, so you avoid the small-order surcharge.

Once thickness is fixed, yardage is just square footage divided by a constant. One cubic yard covers:

• 162 sq ft at 2 inches
• 108 sq ft at 3 inches
• 81 sq ft at 4 inches
• 65 sq ft at 5 inches
• 54 sq ft at 6 inches

So a 4-inch slab is simply square feet ÷ 81. A 400 sq ft patio: 400 ÷ 81 = 4.94 cubic yards, the same answer as the long way round.

Footings, holes, and columns follow the same divide-by-27 rule; only the volume formula changes.

• Footing: length (ft) × width (in ÷ 12) × depth (in ÷ 12) ÷ 27. A 40 ft footing 16 in wide and 8 in deep = 40 × 1.333 × 0.667 ÷ 27 = 1.32 yd³.
• Post hole or column: π × radius (ft)² × depth (ft) ÷ 27. A 12 in hole 3 ft deep = 3.1416 × 0.5² × 3 ÷ 27 = 0.087 yd³ each.
• Wall: length (ft) × height (ft) × thickness (in ÷ 12) ÷ 27.

Presets for all of these live in the main concrete calculator, or use the yardage calculator for straight rectangular pours.

Working in metres? Volume in cubic metres = length (m) × width (m) × thickness (m), no division needed. One cubic yard is 0.7646 cubic metres, and one cubic metre is 1.308 cubic yards.