How the slab calculation works
Slab concrete volume is simply length × width × thickness. The material split then follows the chosen grade and the 1.54 dry-volume factor — the same method as the main calculator, shown in full on the method page. For an M20 slab, every cubic metre of concrete works out to roughly eight 50 kg cement bags, 0.42 m³ of sand and 0.84 m³ of aggregate, with about 200 litres of water at a 0.5 water-cement ratio.
The steel figure is an estimate using a typical reinforcement density of about 80 kg per cubic metre for slabs. Actual steel depends on span, loading, support conditions and your structural design — treat it as a planning number for budgeting and ordering, not a substitute for a detailed bar bending schedule. When you need real cut lengths and bar-wise weights, use the bar bending schedule calculator instead.
One-way vs two-way slabs
A one-way slab spans in a single direction and is common for verandahs, walkways and rooms where one pair of sides is much longer than the other (a length-to-breadth ratio above two). A two-way slab spans in both directions and suits roughly square rooms, distributing load more efficiently and usually needing slightly less concrete depth for the same span. The concrete volume from this calculator is the same either way — what changes is the reinforcement layout, which is why the steel figure here is only a guide.
Typical slab thicknesses
- Residential floor slab: 100–150 mm
- Roof slab: 100–125 mm
- Heavily loaded or longer spans: 150 mm and above (engineer-designed)
- Thin exterior paving / sunshade: 75–100 mm
Tips for ordering
Always add a wastage allowance — this tool uses 5% on cement by default, which covers normal site loss during mixing and placing. Order aggregate and sand in brass (100 cft) if your supplier quotes that way; the other calculators and the BOQ let you switch units freely. For a continuous pour, having materials staged before you start avoids cold joints in the slab.