How is concrete volume calculated for a slab?

Volume equals length times width times thickness. The tool converts every dimension to a common unit first, then reports the result in both cubic yards and cubic metres so you can order in either.

Why add a waste factor?

Forms spill, sub-grade is uneven, and some concrete sticks to the truck and tools. A 5 to 10 percent allowance keeps you from running short mid-pour, which is far more costly than a little leftover.

How many bags of concrete do I need?

A standard 80 lb bag yields about 0.6 cubic feet, a 60 lb bag about 0.45 cubic feet, and a 25 kg bag roughly 0.0125 cubic metres. The tool divides your total volume by the bag yield to estimate the count.

When should I order ready-mix instead of bags?

Bags are practical up to roughly one cubic yard or a fraction of a cubic metre. Beyond that the bag count and mixing labour become impractical, and a ready-mix truck delivering by volume is cheaper and more consistent.

How do I handle a column or footing?

Switch the shape. A column uses pi times radius squared times height. A footing is treated as a rectangular prism of length, width, and depth. You can compute each pour separately and sum them yourself for the order.

Concrete Volume Calculator

Ordering the right amount of concrete avoids both an expensive short pour and wasted material. This calculator computes volume for the three most common pour shapes, converts between cubic yards and cubic metres, adds a waste allowance, and estimates how many bags you would need.

How it works

Each shape has a simple volume formula, computed after converting all dimensions to a common unit:

slab   = length × width × thickness
column = π × radius² × height
footing = length × width × depth
with waste = volume × (1 + waste% / 100)

The tool reports volume in both cubic feet and cubic yards (1 yd³ = 27 ft³) and in cubic metres (1 m³ = 35.31 ft³), then divides by the yield of each bag size to estimate the bag count.

Example and notes

A slab 20 ft by 10 ft by 4 in thick is 20 x 10 x 0.333 = 66.7 ft³, which is 2.47 cubic yards, or about 2.7 cubic yards with a 10 percent waste factor. That is well past practical bag mixing, so order ready-mix. For small piers and post footings, bags are fine — an 80 lb bag yields about 0.6 ft³, so a 12 in diameter by 36 in deep pier needs roughly four bags.