Slab concrete calculator — volume, cement and reinforcement weight
Concrete volume for a floor slab from length, breadth and thickness, plus indicative reinforcement weight at the steel intensity you specify.
slab
Ordering
Wastage is site-specific. Order allowance, breakage and cutting loss differ by material, supplier and workmanship; enter your own figure.
Your assumption, used only for an indicative weight. Replace it with a bar bending schedule before ordering.
Result — calculated from your inputs
Working
Continue to a full site assessment
A calculator answers one question. A DevPartner assessment carries planning capacity, programme, cost, revenue, deal structure and cash flow through one deterministic model and freezes a versioned decision report.
Nothing from this calculator maps into the assessment intake, so you will start from a blank site. No opportunity is created until you save one.
How this calculator works
volume = count × dimension A × dimension B × dimension C order = volume × (1 + wastage %) steel = net volume (cu m) × steel intensity (kg/cu m)
- You enter
- A value you supply. Nothing is assumed for you.
- Calculated
- Arithmetic on your inputs, shown in the working.
- Assumption
- A factor or allowance you can change — mix ratios, wastage, dry-volume factors.
- Approximation
- Geometry or method that approximates reality; labelled wherever used.
- Verify locally
- Planning parameters, statutory rates and structural design must come from the authority or a qualified professional.
Slabs dominate concrete quantity
On a typical framed residential building the suspended slabs account for the largest single share of concrete volume, usually well over half. Getting slab thickness right is therefore the highest-leverage quantity decision on the project: an extra 25 mm across 20,000 square feet of slab adds roughly 46 cubic metres of concrete plus the structure to carry it.
Thickness is a structural design output driven by span, loading and deflection limits. Enter the figure from your structural drawings, not an assumption.
Steel intensity varies more than people expect
Reinforcement intensity in slabs is commonly discussed in kilograms per cubic metre of concrete, and the realistic range is wide — longer spans, higher loads, cantilevers and seismic detailing all push it up. Using a single remembered figure across a whole project is a reliable way to be wrong.
The steel figure here is calculated only when you enter an intensity, and it is labelled as your assumption in the output. For anything past early feasibility, replace it with a bar bending schedule.
Worked example
A 10 m × 5 m slab at 150 mm is 7.5 cubic metres of concrete. Enter a steel intensity of 80 kg per cubic metre — a figure your engineer supplies, not one we publish — and the indicative reinforcement is 600 kg. Leave the intensity blank and the page reports concrete only, and says so.
At 2% wastage the concrete order becomes 7.65 cubic metres.
Mistakes in slab quantities
Using the finished floor thickness instead of the structural slab thickness.
Double counting: the beam calculator and the slab calculator will both claim the overlap unless you decide which one owns it.
Treating a borrowed steel intensity as a design output. Intensity varies enormously with span, loading and system.
This is quantity only, not design
Nothing here sizes a slab, checks deflection, or specifies reinforcement. Thickness and steel ratio are inputs you obtained from a qualified structural engineer.
Use the result for ordering and budgeting, never for construction.
Frequently asked questions
- How do I calculate concrete for a slab?
- Multiply the slab length by its breadth by its thickness in consistent units, then add your wastage allowance. A 30 × 20 ft slab at 5 inch thickness is 250 cubic feet, or about 7.08 cubic metres.
- How much steel goes into a slab?
- It depends entirely on span, loading and design. Feasibility estimates express it as kilograms per cubic metre of concrete, but the correct number for your slab comes from the structural design and its bar bending schedule.
- Should I deduct openings and cut-outs?
- Yes, for staircase voids, lift shafts and service cut-outs of any size. Subtract their plan area from the slab area before using this calculator, or run the calculator once per slab region.
- What steel ratio should I enter for a slab?
- Only the figure your structural engineer gives you for this slab. Intensity swings widely with span, loading, thickness and system, and a borrowed number produces a confident answer with no engineering behind it. Leaving the field blank is a valid choice: the page then reports concrete only and says the steel was not stated.