CalculateY
Quick Answer
A 10 x 10 foot slab at 4 inches thick requires approximately 1.23 cubic yards of concrete, or about 56 bags of 80 lb premix.
Common Examples
| Input | Result |
|---|---|
| Slab: 10 ft x 10 ft x 4 in | 33.33 cu ft, 1.23 cu yd, 75 bags (60 lb), 56 bags (80 lb) |
| Slab: 20 ft x 20 ft x 6 in | 200 cu ft, 7.41 cu yd, 445 bags (60 lb), 334 bags (80 lb) |
| Column: 12 in diameter x 48 in tall | 3.14 cu ft, 0.12 cu yd, 7 bags (60 lb), 6 bags (80 lb) |
| Stairs: 5 steps, 7.5 in rise, 10 in run, 36 in wide | 6.51 cu ft, 0.24 cu yd, 15 bags (60 lb), 11 bags (80 lb) |
How It Works
The Formulas
Rectangular Slab:
Volume (cu ft) = Length (ft) x Width (ft) x Depth (in) / 12
This is the most common concrete calculation. The depth is divided by 12 to convert inches to feet so all dimensions share the same unit.
Cylindrical Column:
Volume (cu ft) = pi x (Diameter in inches / 24)^2 x (Height in inches / 12)
The diameter is divided by 24 (converting inches to feet and halving to get the radius). This formula covers round post holes, sonotube forms, and cylindrical footings.
Stairs:
Stair volume is calculated by summing the volume of each progressive step. Each step has a volume equal to the cumulative rise up to that step times the run times the width. The total is the sum of all individual step volumes.
Unit Conversions
- Cubic Yards = Cubic Feet / 27 (ready-mix concrete is sold by the cubic yard)
- 60 lb bags each yield approximately 0.45 cubic feet of mixed concrete
- 80 lb bags each yield approximately 0.60 cubic feet of mixed concrete
Ordering Tips
For small projects (under 1 cubic yard), premixed bags from a home improvement store are practical. For larger pours, ordering ready-mix concrete by the truck is more economical. Most ready-mix companies have a minimum order of 1 cubic yard. Adding 5% to 10% extra to the calculated volume accounts for spillage, uneven subgrade, and slight form variations.
Worked Example
For a patio slab measuring 10 feet by 10 feet at 4 inches thick: Volume = 10 x 10 x (4 / 12) = 10 x 10 x 0.333 = 33.33 cubic feet. In cubic yards: 33.33 / 27 = 1.23 cubic yards. In 80 lb bags: 33.33 / 0.60 = 55.56, rounded up to 56 bags. In 60 lb bags: 33.33 / 0.45 = 74.07, rounded up to 75 bags. Adding 10% for waste brings the estimate to approximately 1.36 cubic yards or 61 bags of 80 lb premix.