Circle Concrete Calculator
This circle concrete calculator handles round slabs — fire-pit surrounds, tank pads, gazebo bases and circular patios. Enter the diameter and thickness; the πr² math is built in.
How to Use This Calculator
- Enter your dimensions — feet for lengths, inches for thickness.
- Adjust the options — waste percentage, bag size and your local concrete price.
- Read your results — volume, bags, weight and cost update live as you type.
The Formula
Circle volume = π × (diameter ÷ 2)² × thickness Worked example: A 10 ft round pad at 4 in: π × 5² × 0.333 = 26.2 ft³ ÷ 27 ≈ 0.97 yd³.
A concrete calculator round pours need starts from area: A = πr². Measure the diameter (the full width across the center) — it's easier to tape than the radius and this tool halves it for you.
Rule of thumb from the concrete calculator circle math: a round slab uses 78.5% of the concrete of the square that contains it. A 12 ft circle ≈ 0.785 × a 12 × 12 ft square.
For vertical round elements — columns, piers, sonotubes — use the column or sonotube calculators, which extend this same circle area into height.
Frequently Asked Questions
Common questions about the circle concrete calculator.
How much concrete for a round slab?
π × (diameter ÷ 2)² × thickness. A 12 ft circle at 4 in: π × 36 × 0.333 = 37.7 ft³ ≈ 1.40 yd³.
Do I enter diameter or radius?
Diameter — the full distance across. The calculator divides by 2 internally to get the radius.
How do I mark out a circular slab?
Drive a pin at the center, tie a string at half your diameter, and scribe the circle. Form with hardboard or flexible plastic edging.
What about a half-circle pour?
Calculate the full circle here and divide the result by two — waste allowance unchanged.
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