Concrete Quantity Calculator
Estimate cement, sand and aggregate needed for a concrete slab.
Assumptions used in this calculation
- Mix ratio: Assumes M20 grade concrete (1:1.5:3) with a 1.54× dry volume factor and 50kg cement bags.
About this calculator
Ordering concrete materials by guesswork almost always means either an overpriced surplus sitting on site or a mid-pour run to the supplier, and the underlying math, wet-to-dry volume conversion plus splitting a mix ratio into three separate material quantities, is easy to get wrong by hand. This calculator takes a slab's length, width and thickness, converts the wet volume to dry volume using the standard 1.54× factor (concrete shrinks as it's mixed and compacted from its loose dry ingredients), then divides that dry volume by the M20 mix ratio (1:1.5:3, cement:sand:aggregate) to output cement bags, sand and aggregate needed. Entering three dimensions instead of manually working through volume conversion and ratio math removes a common source of costly over- or under-ordering on a construction site.
Worked example
20 × 15 ft slab, 4-inch thick
Result: ≈ 27 cement bags
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