Cubic feet to cubic yards
Multiply length × width × depth, all in feet, to get cubic feet, then divide by 27 ft³ per yd³.
The traps:
- Dividing by 9 converts square feet to square yards, not cubic feet to cubic yards.
- Dividing by 3 is the feet-per-yard figure for a single dimension.
- Inches must become feet by dividing by 12. 8 in is 0.667 ft, not 0.8 ft and not 8 ft. 6 in is 0.5 ft.
Worked volumes
- Footing 100 ft × 2 ft × 1 ft = 200 ft³; 200 ÷ 27 = 7.41 yd³.
- Wall 80 ft × 10 ft × 8 in: 8 ÷ 12 = 0.667 ft; 80 × 10 × 0.667 = 533.3 ft³; 533.3 ÷ 27 = 19.75 yd³. Treating 8 in as 0.8 ft gives 23.70 yd³, and using 8 ft gives 237.04 yd³.
- Slab 60 ft × 40 ft × 6 in: 60 × 40 × 0.5 = 1200 ft³; 1200 ÷ 27 = 44.4 yd³.
The small-volume strength-test waiver
The code text (IBC 2009 §1905.6.2) reads: "When the total volume of a given class of concrete is less than 50 cubic yards (38 m³), strength tests are not required when evidence of satisfactory strength is submitted to and approved by the building official."
Both conditions are required. The waiver is never automatic; dropping the approval condition is the trap. The threshold is 50 yd³; 25, 100 and 150 yd³ do not appear in the provision.
Measure each class on its own
The provision applies to "the total volume of a given class of concrete." With 30 yd³ of 4000 psi footings and 35 yd³ of 5000 psi slabs, each class is under 50 yd³, so tests for each may be waived with the building official's approval. Adding them to a 65 yd³ job total misreads the per-class wording, and nothing limits the waiver to one class per project.
The 44.4 yd³ slab above qualifies the same way, with the same approval.