Round Slab Calculator

The calculator treats the slab as a solid cylinder with one uniform diameter and thickness. It reports net volume, volume with the selected allowance, bag equivalents, estimated concrete weight, form circumference, and a material-only ready-mix estimate.

Calculate center openings, thickened rims, piers, equipment foundations, steps, and grade beams separately. A fire-pit pad is not the same as a fire-rated hearth design, and the quantity result does not verify heat clearances or structural capacity.

A round slab is a short cylinder. Divide diameter by two for radius, square the radius, multiply by π, then multiply by slab thickness in feet.

• Measure the inside diameter of the form through the center of the circle.
• Take diameter measurements in two directions if the form may be out of round.
• Calculate ring-shaped slabs by subtracting the inner-circle volume from the outer-circle volume.
• Calculate thickened edges or piers separately and add them to the slab volume.

For an irregular oval, do not average random edge measurements and call the result a circle. Divide the footprint into measurable shapes or use a surveyed area, then multiply area by thickness.

Bag counts are rounded up independently from manufacturer-published yields. They are useful for comparing options but do not account for a specialty product with a different stated yield.

• Use a true center point or a verified template so the form stays circular and the circumference estimate remains useful.
• Remove organic material and provide uniform support using the base and compaction requirements for the project.
• Plan isolation from existing foundations, columns, or other slabs where independent movement is required.
• Select thickness, reinforcement, and joints from the approved design; the circle formula only determines quantity.
• Confirm drainage, finished elevation, surface texture, exposure, and curing before placement begins.

Add the volumes of the main disk, thickened rim, piers, and other elements before applying the final allowance. Tell the supplier the total quantity, specified mix, placement method, access limitations, and whether the shape makes direct truck discharge impractical.

Round the final order according to the supplier's increment after confirming the forms and subgrade—not by rounding each component separately.

Reviewed September 20, 2026. Primary references are linked directly below. Local codes, approved drawings, project specifications, geotechnical information, and supplier requirements take precedence.

• ACI 360R — Guide to Design of Slabs-on-Ground

  Primary ACI reference for slab support, loading, reinforcement, jointing, and design scope.

• ACI 302.1R — Concrete Floor and Slab Construction, Chapter 5

  Primary ACI construction guidance on reinforcement, joints, and cracking.

• QUIKRETE — Pouring and Finishing Concrete Slabs

  Manufacturer instructions for forms, base preparation, jointing, finishing, and curing.