Formula
A rectangle covers a slab and a footing. A circle covers a column or a sonotube: enter the diameter in feet and the height as the depth in inches. Steps are the solid stacked volume, not a hollow form.
slab = length × width × depth
column = π × (diameter ÷ 2)² × height
steps = width × tread × riser × n(n+1)/2
bags = ceil(cubic feet × (1 + waste ÷ 100) ÷ bag yield)
tons = cubic feet × 150 / 2,000, after waste
150 lb/ft³ is a typical normal-weight concrete. ACI and PCA discussions often sit between 145 and 150. This page uses 150, so a cubic yard is 2.025 tons. Bag yields are Quikrete Concrete Mix 1101: 0.30, 0.45, and 0.60 cubic feet. The stair calculator sizes the rise and run. It does not pour the concrete.
Worked examples
10 × 10 ft × 4 in, 80 lb bags, no waste. 33.33 ft³. 56 bags. 1.23 yd³. 33.33 × 150 / 2,000 = 2.50 tons. A yard at 150 lb/ft³ is 2.025 tons, and 1.235 × 2.025 is the same 2.50 tons.
Three solid steps, 4 ft wide, 12 in tread, 7 in riser, 10% waste. 4 × 1 × 7/12 × 6 = 14 ft³. With waste, 15.4 ft³, which is 26 of the 80 lb bags.
A 12 inch column, 4 ft tall. Diameter 1 ft, depth 48 in. π × 0.5² × 4 = 3.14 ft³ before waste.
Reference
Quikrete Concrete Mix data sheet 1101 yields. One cubic yard is 27 cubic feet.
| Bag | Yield | Bags per yard |
|---|---|---|
| 40 lb bag | 0.30 ft³ | 90 |
| 60 lb bag | 0.45 ft³ | 60 |
| 80 lb bag | 0.60 ft³ | 45 |
Questions
How many 80 pound bags for a 10 by 10 slab at 4 inches?
56 bags with no waste. The slab is 33.33 cubic feet. An 80 lb bag yields 0.60 cubic feet, and 33.33 / 0.60 rounds up to 56. That is 1.23 cubic yards and 2.50 tons at 150 lb/ft³.
What do the bag sizes yield?
From the Quikrete Concrete Mix data sheet 1101: 40 lb yields 0.30 cu ft (90 bags per yard), 60 lb yields 0.45 (60 per yard), 80 lb yields 0.60 (45 per yard). Sakrete publishes the same three yields. A 50 lb and a 90 lb bag exist on some sheets and are not in this list.
How are steps measured?
As a solid stair: width × tread depth × riser × n(n+1)/2. Three steps, 4 ft wide, 12 in tread, 7 in riser, is 14 cubic feet. Open risers and hollow block are not this shape.
Sources
- Quikrete Concrete Mix data sheet 1101: 40 lb → 0.30 cu ft, 60 lb → 0.45 cu ft, 80 lb → 0.60 cu ft. https://www.quikrete.com/construction-products-guide/-/assets/files/data_sheet-concrete%20mix%201101.pdf
- Normal-weight density used here: 150 lb/ft³. Tons = cubic feet × 150 / 2,000.
- Solid steps: width × tread × riser × n(n+1)/2. The sum of the first n riser rectangles.