The formula: feet, then divide by 27
Concrete is ordered by the cubic yard. A cubic yard is 3 ft × 3 ft × 3 ft, which is 27 cubic feet. The safe habit is to put every size in feet first, multiply, and divide by 27 last.
- Slab cubic feet = length (ft) × width (ft) × thickness (in.) ÷ 12.
- Footing cubic feet = number × length (ft) × width (in.) ÷ 12 × depth (in.) ÷ 12. For a continuous footing, the length is its run around the building. For square pads, it is one pad's side.
- Cubic yards = cubic feet ÷ 27.
- With waste = cubic yards × (1 + waste percent ÷ 100).
The waste allowance covers spillage, an uneven subgrade and the concrete left in the truck and pump. Pick your own figure, or use the one a question gives. The calculator rounds the total up to whole yards. Your supplier may sell in smaller steps, so ask.
Worked example 1: a slab with a perimeter footing
A slab is 50 ft by 30 ft and 4 in. thick. A footing runs 160 ft around it and is 12 in. wide and 10 in. deep. Waste is 10%. These are invented study numbers.
- Slab = 50 × 30 × 4 ÷ 12 = 1,500 × 0.3333 = 500 ft³, which is 500 ÷ 27 = 18.52 yd³.
- Footing = 160 × (12 ÷ 12) × (10 ÷ 12) = 160 × 1 × 0.8333 = 133.33 ft³, which is 4.94 yd³.
- Total = 500 + 133.33 = 633.33 ft³, and 633.33 ÷ 27 = 23.46 yd³.
- With 10% waste: 23.46 × 1.10 = 25.80 yd³, so order 26 yd³.
Check: 18.52 + 4.94 = 23.46, and 27 × 23.46 = 633.4, back to the cubic feet.
Worked example 2: six pad footings
Six pads are each 3 ft square and 12 in. deep, with 5% waste and no slab (enter 0 for the slab).
- Each pad = 3 × (36 ÷ 12) × (12 ÷ 12) = 3 × 3 × 1 = 9 ft³. Six pads = 54 ft³.
- 54 ÷ 27 = 2 yd³ exactly. With 5% waste, 2 × 1.05 = 2.1 yd³, which rounds up to 3 whole yards.
The second example shows why the rounding step is yours to judge: 2.1 yd³ is the quantity, and whether you order 2.25, 2.5 or 3 depends on what your supplier sells. These are original arithmetic examples, not exam questions.
Concrete yardage questions
Why divide by 27 and not by 9?
Nine converts square feet to square yards. Concrete is a volume, and a cubic yard holds 3 × 3 × 3 = 27 cubic feet.
Can I enter the slab thickness in feet?
The calculator takes inches and divides by 12 for you. A 4 in. slab is 0.3333 ft, and a 6 in. slab is 0.5 ft.
Where is this on the NASCLA exam?
Concrete is one of the twelve subject areas on the NASCLA outline, with 6 of the 115 scored questions. The Bid Bond course works this takeoff in its Concrete lesson.
See the NASCLA exam outline for all twelve subject areas and their question counts.