Circular Slab Calculator

Round concrete slabs show up more often than you might expect in residential projects. Fire pit pads, hot tub bases, flagpole footings, circular patio sections, column footings, and round stepping stones all require a circular pour. The math is slightly different from a rectangular slab since you are working with the area of a circle rather than simple length times width. This calculator uses the standard circle area formula (pi times radius squared) multiplied by your depth, then converts to cubic yards which is how ready-mix concrete is sold. Enter the full diameter of your circle in feet and the thickness in inches to get your estimate. For most circular pads like fire pit bases or hot tub pads, 4 inches of thickness is sufficient. If the slab will support significant weight like a large water feature or heavy equipment, increase to 6 inches and consider adding rebar or wire mesh reinforcement. Forming a circular pour requires flexible form material. Thin plywood strips, metal landscape edging, or purpose-made flexible concrete forms all work. Stake the form material every 12 to 18 inches around the perimeter to hold the shape under the weight of wet concrete. For small circles under 3 feet in diameter, you can use a cardboard concrete tube form like a Sonotube cut lengthwise. As with any concrete project, order 5 to 10 percent more than your calculated amount. Uneven ground, slight form bulging, and spillage all eat into your material. For small circular pours under one cubic yard, premixed bags are usually more practical than ordering a truck delivery.