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Circular Slab Calculator
Concrete for round pads, tank bases and fire pits. Enter diameter and thickness: get cubic yards and bags instantly.
Estimates only. Bag yields per manufacturer specs (80 lb ≈ 0.60 ft³, 60 lb ≈ 0.45 ft³, 40 lb ≈ 0.30 ft³). Ready-mix trucks carry ≈ 10 yd³. Always round up and confirm with your supplier.
Hot tub pads, water tank bases and round patios need the circle formula, not the square one. The free circular slab calculator below converts diameter and thickness into cubic yards, bags and cost.
How to use the circular slab calculator
- Measure the diameter across the full circle in feet.
- Enter the slab thickness in inches.
- Add 10 percent waste for edge over-pour.
- Read cubic yards, cubic metres, bags and truck loads.
Formula
Volume = π × radius² × (thickness ÷ 12) ÷ 27, giving cubic yards.
Practical tips
- A 10 ft diameter pad at 4 inches thick needs 0.97 cubic yards (44 bags of 80 lb).
- Mark the circle with a string line from a centre peg for a true round edge.
- Thicken the edge to 6 inches where the pad meets traffic or heavy loads.
- Radius is half the diameter: a 10 ft circle has a 5 ft radius.
Frequently asked questions
How do I work out concrete for a circle?
Square the radius, multiply by 3.14, multiply by thickness in feet, then divide by 27 for cubic yards. The calculator does it for you.
How thick for a hot tub pad?
Four inches of concrete on compacted gravel suits most hot tubs. Check the tub manual for weight limits.
Circle or square: which uses less concrete?
A circle uses about 21 percent less concrete than the square that contains it.
Try the concrete calculator hub or estimate project costs.
Worked examples
Example 1: 10 ft diameter, 4 in thick. Volume = 0.97 cubic yards. With 10% waste: 1.07 cubic yards (48 x 80 lb bags).
Example 2: 12 ft diameter, 6 in thick. Volume = 2.09 cubic yards. With 10% waste: 2.30 cubic yards (104 x 80 lb bags).
Circular slab reference table
Cubic yards before waste; 80 lb bags include 10% waste.
| Diameter x thickness | Cubic yards | 80 lb bags |
|---|---|---|
| 8 ft x 4 in | 0.62 | 31 |
| 10 ft x 4 in | 0.97 | 48 |
| 12 ft x 4 in | 1.40 | 70 |
| 12 ft x 6 in | 2.09 | 104 |
More circular slab questions
What is the formula for a round slab?
Volume = pi x radius^2 x thickness. Divide cubic feet by 27 for cubic yards.
How thick should a round patio be?
Four inches suits patios. For vehicles, pour 6 inches.