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.
Round Concrete — The Formula Explained
The formula for a cylinder is π × radius² × height. For a 12-inch sonotube at 36 inches deep: π × 0.5² × 3 = 2.36 cubic feet per footing. Divide by 0.60 for 80lb bag count = 4 bags. The most common error is using diameter instead of radius — always halve the diameter first. A 12-inch tube has a 6-inch radius, or 0.5 feet in decimal.
Round slabs such as fire pit pads, circular patios, and fountain bases use the same formula. Measure across the widest point to get diameter, halve it for radius, then calculate π × radius² × thickness in feet. Add 10% for waste on any round pour because forming a circle always creates more edge waste than a square or rectangle of the same area.