Round Slab Concrete Calculator
For a circular patio or equipment pad, measure the finished diameter at its widest point and use the planned thickness.
How the calculation works
Volume in cubic feet = π × (diameter in feet ÷ 2)² × (thickness in inches ÷ 12). Divide by 27 for cubic yards. The waste allowance applies after exact volume is calculated.
Worked example
A 12 ft diameter slab at 4 in thick contains about 37.70 ft³, or 1.40 yd³. With 10% extra, the order estimate is 1.54 yd³, equivalent to 70 bags at 0.60 ft³ yield.
Before you order
A circular edge, thickened perimeter or uneven base changes the amount. Measure the finished depth at several points. Thickness and reinforcement are design decisions outside this quantity estimate.
Review the calculation method and source notes, then use the supplier checklist before placing an order.
A circular pad example
Circular area is π × radius². For a 12 ft diameter pad, the radius is 6 ft and area is about 113.10 ft². At 4 in uniform thickness, volume is about 37.70 ft³, or 1.40 yd³ exact. Add your chosen allowance only after calculating the geometric volume.
The radius is half the full diameter, not the distance from the form edge to a nearby stake. If the shape is a ring rather than a solid disk, subtract the inner circle volume from the outer circle; this tool currently estimates a solid disk only.
Questions before you order
Can I use this for a ring-shaped slab?
No. Calculate outer and inner circles separately and subtract their volumes.
What if the edge is thicker?
Calculate the extra edge geometry separately and add it to the order.
Is the diameter measured across the outside of the forms?
Enter the finished concrete diameter inside the forms.
Continue planning
Content and calculator by pourmeasure. Formula and assumption notes: methodology and sources. Updated October 2026. No independent engineering review is claimed.