Calculate the concrete volume for a circular slab from diameter and depth. Imperial and metric units, with a 10% waste buffer per ACI 318.
Free · iPhone & iPad · Save projects, work offlineAll calculations are local — nothing leaves your browser.
Enter the dimensions to see how much concrete you'll need.
V = π × r² × depth
Halve the diameter to get the radius, square it, multiply by pi (3.1416), then multiply by the depth. Keep every measurement in the same unit and convert the result to cubic metres or cubic yards. Add 10% for waste.
Worked example: A 3 m diameter slab poured 100 mm deep is π × 1.5² × 0.1 = 0.71 m³, so order 0.78 m³ with the buffer.
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.
The trick with circular slabs is finding the centre and getting an accurate diameter. Two approaches:
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.
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:
A 10% buffer is added — the ACI 318 industry standard for waste, spillage and uneven subgrade. Order to the buffered figure.
A 3 m diameter fire-pit pad, 150 mm deep:
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.
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).
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.
Pouring a different shape? These cover the rest of the common jobs.
Calculate the concrete volume for a rectangular slab from length, width and depth. Imperial and metric units, with a 10% waste buffer per ACI 318.
Calculate the concrete volume for an irregular slab from a pre-measured area and depth. Imperial and metric units, with a 10% waste buffer per ACI 318.
Calculate the concrete volume for a circular column from diameter and height. Imperial and metric units, with a 10% waste buffer per ACI 318.
Calculate the concrete volume for fence-post holes — by hole, total across all holes, with post-profile displacement and a 10% waste buffer.
Rectangular slabs, circular slabs, irregular slab area, circular columns and post holes — with saved projects, full offline use, and imperial / metric units across the board.
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