How much concrete do I need?

Find poured volume for a rectangular slab or footing or a circular column, then convert that volume into bags using the product’s mixed yield.

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ToolConcrete Calculator
Input
Output
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How it works

Rectangles use length × width × depth, while a circular column uses π × (diameter ÷ 2)² × height. The project allowance increases cubic metres, then multiplication by 1,000 converts to litres before division by label yield rounds up to whole bags.

Formula
column m³ = π × (diameter m ÷ 2)² × height m
  • Bag weight is not used as a volume shortcut because different dry mixes can produce different litres after water is added.
  • Shape, dimensions, yield, and allowance are required rather than filled with a typical shed-base scenario.

Worked example

A 4m by 3m slab, 100mm thick
Rectangular slab with 10% extra for an uneven subbase
Input
											Shape: slab
Length m: 4
Width m: 3
Depth m: 0.1
Bag yield litres: 12.5
Waste percent: 10
Output
												Volume m3: 1.2
Volume with waste m3: 1.32
Bags needed: 106
Bags exact: 105.6

When to use this

Shed-base slabs use plan dimensions and compacted depth, strip footings use trench dimensions, and cylindrical post foundations use diameter with poured height.

Edge cases

  • A circular pour without a diameter is rejected rather than treating absent rectangle dimensions as zero.
  • The entered diameter is halved before squaring; squaring the full diameter would quadruple the cylindrical volume.
  • A result of 105.6 product bags rounds up to 106 because the missing 0.4 bag would leave the pour short.