Circular Concrete Slab Calculator

A circular slab pour is common for round patios, fire-pit pads, fountain bases, garden features and round pump pads. The math is π × radius² × depth — easy to get wrong with a calculator on site, especially when you also need to convert between feet, inches and cubic yards.

How to measure

The trick with circular slabs is finding the centre and getting an accurate diameter. Two approaches:

String and peg. Drive a peg at the centre and tie a string the length of the radius (half the diameter). Trace the perimeter — that’s your formwork line. Diameter is the rope length doubled.
Direct measure. If the formwork is already in place, measure the longest straight line across the circle through the centre. That’s the diameter.

For depth, measure from the top of the formwork down to the prepared subgrade at four or more points around the circle. Average them or use the deepest reading and accept slight over-pour.

How the calculation works

Volume is π × (diameter ÷ 2)² × depth. The calculator converts every input into millimetres internally, computes the cubic-millimetre volume, then converts to your chosen output unit:

Metric: cubic metres (m³)
Imperial: cubic yards (yd³) and cubic feet (ft³)

A 10% buffer is added — the ACI 318 industry standard for waste, spillage and uneven subgrade. Order to the buffered figure.

Worked example

A 3 m diameter fire-pit pad, 150 mm deep:

radius = 1.5 m
volume = π × 1.5² × 0.15 = 1.06 m³
with 10% buffer = 1.17 m³

That’s just over a single ready-mix bay (most plants quote in 0.2 m³ increments) — order 1.2 m³ and you’ll have a comfortable margin.

Ordering tips

Round shapes are particularly prone to short-pours because the formwork bows outward under the wet concrete. Always order the buffer, and have a small disposal plan for the leftover (a wheelbarrow’s worth makes useful stepping stones, footings or a small landing pad).

On the jobsite

The Concrete Calculator app for iOS has this calculator plus four others (rectangular slabs, irregular slab area, circular columns and post holes), saved projects, and full offline support. Get it on the App Store.

Frequently asked questions

How do I calculate concrete for a circular slab?

Use the formula π × (diameter ÷ 2)² × depth. The calculator handles unit conversion and adds a 10% buffer per ACI 318. Common circular pours include round patios, fire-pit pads and circular pump pads.

How do I measure diameter accurately on the ground?

Drive a peg at the centre and use a string the length of the radius. Mark the perimeter at four points 90° apart. Diameter is twice the radius — measure the actual maximum distance across the marked circle, not just the rope.

What if my slab isn't a perfect circle?

For ovals, octagons or rough shapes, use the irregular Slab Area calculator (in the iOS app) and enter the measured area. The circular calculator only works when the shape is genuinely round.

Why add a 10% buffer?

A 10% waste factor accounts for uneven subgrade, formwork bow, spillage during the pour, and the small amount lost in chutes. The American Concrete Institute (ACI 318) recommends 5–10%; 10% is the conservative default.

What units does this calculator output in?

Metric mode shows cubic metres (m³). Imperial mode shows cubic yards (yd³, the unit ready-mix is sold in) plus cubic feet for cross-reference.