About this calculator
Estimates quantities for a monolithic (cast in-situ) suspended/elevated concrete floor slab: concrete volume, soffit formwork area, and prop count on a grid. Quantities only — slab thickness, reinforcement, and prop spacing must be sized by a qualified engineer against the actual span and loads.
Formula
area = slab length × slab width
concrete volume (incl. waste) = area × thickness × (1 + waste% / 100)
soffit formwork area = area
props (X) = ceil(slab length / prop spacing X) + 1
props (Y) = ceil(slab width / prop spacing Y) + 1
props (total) = props (X) × props (Y)
Worked example
Defaults: 6 m × 5 m slab, 200 mm thick, 1.2 m prop grid both directions, 5% waste.
area = 6000 × 5000 = 30,000,000 mm²
concrete volume = 30,000,000 × 200 × 1.05 = 6,300,000,000 mm³ (≈ 6.3 m³)
props = (ceil(6000/1200)+1) × (ceil(5000/1200)+1) = 6 × 6 = 36
FAQ
Why is this different from the slab-on-ground calculators? A slab cast on the ground bears directly on the subgrade — it needs perimeter formwork at most, no soffit shuttering or props. A suspended/elevated floor slab is cast on formwork held up by props until the concrete cures, which this calculator's soffit area and prop count account for.
Does this calculator check whether the prop spacing is adequate? No. This is a quantity take-off only — an info diagnostic reminds you that slab thickness, reinforcement, and prop spacing must be verified by a qualified engineer against the actual span and loads.
Why doesn't the calculator cite a standard?
EN 1992-1-1 governs concrete structural design, not a generic formwork/prop quantity
take-off, so meta.standards is left empty.
Assumptions and limits
Props are counted on a plain grid, rounded up on each axis — not a real propping layout, which would avoid doubling up near slab edges or beams. This is a quantity take-off only: slab design must be verified by a qualified engineer.