How the Circular Slab Estimate Works
The calculator halves the diameter to find the radius, then uses π × radius² × thickness. A 2m diameter slab entered at 100mm thick has a geometric volume of π × 1² × 0.1 = approximately 0.314m³. With 10% selected, the planning quantity is approximately 0.346m³, or 38 bags under the stated yield assumption.
Measure diameter and depth carefully
Measure across the widest internal part of the circular formwork. Use the actual designed thickness, converted to metres by the calculator. If the shape is an annulus or contains large voids, this solid-circle calculator will overestimate unless those volumes are calculated and subtracted separately.
Use a designed thickness
The calculator does not choose thickness, reinforcement, concrete strength or ground preparation for fire pits, water tanks or other loads. Use project documentation or advice from an appropriately qualified professional.
See the complete methodology and source list.