Slab Calculator
Slab Results
| Application | Min Thickness | Recommended | PSI |
|---|---|---|---|
| Sidewalk / Walkway | 3" | 4" | 3000 |
| Backyard Patio | 3.5" | 4" | 3000–4000 |
| Residential Driveway | 4" | 5" | 4000 |
| Garage Floor | 4" | 5–6" | 4000 |
| Heavy Equipment Pad | 6" | 8" | 5000 |
Circular Concrete Project Reference
| Project | Diameter | Depth | Cubic Feet | 80lb Bags |
|---|---|---|---|---|
| Fire pit pad | 5 ft | 4" | 6.54 | 11 |
| Hot tub pad | 8 ft | 6" | 25.13 | 42 |
| Patio circle | 12 ft | 4" | 37.70 | 63 |
| Pier footing (deck) | 12" | 36" | 2.36 | 4 |
| Pier footing (heavy) | 16" | 48" | 5.59 | 10 |
| Mailbox base | 18" | 24" | 3.53 | 6 |
| Tree ring (12 ft outer, 4 ft inner) | annular | 4" | 33.51 | 56 |
All values include +10% waste factor. 80 lb bag yields 0.60 cu ft. For 60 lb bags, multiply 80 lb count by 1.33.
The Circular Concrete Formula
For any circular concrete project, the volume formula is π × radius² × depth. The radius is half the diameter. All three dimensions must be in the same units — usually feet for depth conversions.
Example: A 10-foot diameter circular patio at 4 inches thick = π × 5² × (4 ÷ 12) = 3.14159 × 25 × 0.333 = 26.18 cubic feet, or 0.97 cubic yards. Add 10% waste for a final order of 1.07 cubic yards.
For pier footings or sonotubes: Use the diameter of the tube and the full depth of the hole. A 12-inch sonotube at 36 inches deep = π × 0.5² × 3 = 2.36 cubic feet (about 4 bags of 80lb concrete).
For tree rings or annular shapes: Calculate the outer circle, then subtract the inner circle. Outer area − inner area × depth = volume. Our calculator above handles this automatically.
Round vs Square: When to Use Each
Round or cylindrical concrete projects are more common than most homeowners realize. Deck footings (sonotubes), fire pit pads, circular patios, pier foundations, and some pool applications all require the circular volume formula: π × radius² × depth.
The most common mistake is using the rectangular formula on sonotube footings. A 12-inch sonotube at 36 inches deep has a volume of 2.36 cubic feet — if you accidentally calculated it as a 1×1×3 foot rectangular footing you would get 3 cubic feet, causing you to buy 27% too many bags.
For a quick field estimate: (diameter in inches)² × depth in feet × 0.0082 = cubic feet. Example: 12" tube × 12" × 3ft depth × 0.0082 = 2.36 cu ft. Divide by 0.60 = 4 bags of 80lb concrete.
Concrete Column Quick Reference
| Diameter | 1 ft tall | 2 ft tall | 3 ft tall | 4 ft tall | Use |
|---|---|---|---|---|---|
| 6" round | 0.5 bags | 1 bag | 2 bags | 2 bags | Light post, mailbox |
| 8" round | 1 bag | 2 bags | 3 bags | 4 bags | Deck post, light column |
| 10" round | 1 bag | 2 bags | 4 bags | 5 bags | Standard deck post |
| 12" round | 1 bag | 3 bags | 4 bags | 6 bags | Heavy post, pergola |
| 16" round | 2 bags | 5 bags | 7 bags | 10 bags | Large beam support |
All bag counts use 80lb bags (0.60 cu ft each), include 10% waste factor.
Column vs Footing — What is the Difference?
A vertical structural element that transfers loads from above (a beam, floor, or roof) down to the ground or a footing below it. Columns can be concrete, steel, wood, or masonry. In residential construction, concrete piers are the most common column type — they are often poured using round Sonotube forms.
The widened base at the bottom of a column or wall that spreads the load over a larger area of soil. A footing sits below the frost line and is always wider than the column above it. Think of it as the foot of the column. Most residential pier footings are round pads 18–24 inches in diameter poured 12 inches deep before the column form is set.