How do I calculate concrete for a round slab?

Use π × radius² × thickness. Keep radius and thickness in feet to get cubic feet, divide by 27 for cubic yards, and add a waste allowance.

How much concrete is needed for a 12-foot round slab?

At 4 inches thick, a 12-foot-diameter slab contains about 37.7 ft³ or 1.40 yd³ net. With 10% waste, the order quantity is about 1.54 yd³.

How do I calculate a concrete ring?

Calculate the outer circle volume, calculate the inner opening volume, and subtract the inner value from the outer value before adding waste.

Round Slab Calculator

Calculate concrete for a circular slab, pad, fire-pit base, or round footing from diameter and thickness. Get cubic yards, bags, weight, form circumference, waste, and estimated ready-mix cost.

Circular Slab Dimensions

Use the inside diameter of the circular form.

Diameter

Thickness

Identical slabs

slabs

Waste allowance

Ready-mix price

$/yd³

Order quantity

1.54yd³ with waste

Total slab area

113.1ft²

Net volume

37.7ft³

Form circumference

37.7ft per slab

60 lb bags

93bags

80 lb bags

70bags

Concrete weight

6,220lb · 3.1 tons

Ready-mix estimate

$306material only

Round Slab Volume Formula

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.

Circular slab

ft³ = π × [diameter (ft) ÷ 2]² × [thickness (in) ÷ 12]

Divide cubic feet by 27 for cubic yards, then add the selected waste allowance.

Measuring a Circular Concrete Slab

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.