A concrete estimate should separate the geometric volume, the site allowance and the quantity actually ordered. The Concrete calculator does exactly that, whether you are preparing a slab, footing, wall or cylindrical posts.
GuideBest practicesBeginner
How much concrete for a slab, foundation or posts?
Calculate concrete volume, site allowance and the quantity to order for slabs, footings, walls or cylindrical posts.
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Start with the raw volume
For a rectangular slab:
volume = length × width × thickness
A 6 m × 4 m slab that is 12 cm thick gives 6 × 4 × 0.12 = 2.88 m³. The unit conversion matters: 12 cm is 0.12 m.
For cylindrical posts use π × radius² × depth, then multiply by the number of posts. A common mistake is to use the diameter as the radius, which multiplies the cross-sectional area by four.
Add a realistic allowance
A perfect mathematical volume rarely matches a real site. Excavations can be slightly wider, the base can vary, and some concrete is lost during placement. With a 7% allowance, 2.88 m³ becomes about 3.08 m³.
Do not hide the allowance by increasing every dimension “just in case” and then add another percentage. Enter the measured dimensions first and keep the reserve explicit.
Ready-mix, site mixing or premixed bags
For ready-mix concrete, the useful result is the final volume to order. Check the supplier’s minimum load, rounding rules and delivery constraints.
When mixing on site, the volume can be converted into cement and aggregate quantities. At 350 kg of cement per m³, 3.08 m³ requires roughly 1,078 kg of cement before any supplier-specific adjustment.
For premixed bags, use the yield printed on the bag. The calculator rounds to whole bags, which quickly shows when bagged concrete becomes impractical for a large job.
Quantity is not structural design
A quantity calculator does not decide slab thickness, reinforcement, foundation dimensions or load capacity. Those parameters must already be defined for the project.
For a masonry wall, continue with the Concrete block calculator. For outdoor aggregate, use the Gravel calculator, and for levels or drainage use the Slope calculator.
Before ordering
Measure the real dimensions, convert units, calculate the raw volume, add a justified allowance and then apply the supplier’s commercial constraints. This produces a useful purchasing quantity rather than a purely theoretical number.