Concrete slab weight
Slab weight is arithmetic, not guesswork: multiply volume by density. What varies enormously is the density — normal-weight concrete, lightweight aggregate concrete and foam concrete span a six-fold range, and the tables below turn that range into a weight per square metre and per square foot.
The one formula
Slab weight is the product of volume and density, and once the geometry is measured the only variable left is density:
mass = length × width × thickness × density
Geometry is rarely the hard part; it is a plan area and a specified thickness. Density is where the real range sits. Normal-weight concrete runs around 2400 kg/m3. Lightweight aggregate concrete, made with a porous aggregate such as expanded clay or shale in place of some or all of the natural coarse aggregate, comes down to roughly 1900 kg/m3. Foam concrete, which replaces solid volume with air rather than lightening the aggregate, spans a far wider range — commonly 400 to 1600 kg/m3 depending on the class specified. A slab of given dimensions can therefore weigh anywhere from roughly a sixth to the full weight of its normal-weight equivalent, purely on the density decision.
Slab weight by thickness and density
Weight per unit area at a set thickness is the density multiplied by the thickness, expressed in consistent units. The two tables below cover the same six densities in metric and imperial terms — metric thicknesses computed in kg/m2, imperial thicknesses computed independently in lb/ft2, so each table stands on its own rather than being a rounded conversion of the other.
| Thickness | 2400 | 1900 | 1600 | 1000 | 600 | 400 |
|---|---|---|---|---|---|---|
| 100 mm | 240 | 190 | 160 | 100 | 60 | 40 |
| 150 mm | 360 | 285 | 240 | 150 | 90 | 60 |
| 200 mm | 480 | 380 | 320 | 200 | 120 | 80 |
All figures in kg per square metre; density row headers in kg/m3.
| Thickness | 2400 | 1900 | 1600 | 1000 | 600 | 400 |
|---|---|---|---|---|---|---|
| 4 in | 49.9 | 39.5 | 33.3 | 20.8 | 12.5 | 8.3 |
| 6 in | 74.9 | 59.3 | 49.9 | 31.2 | 18.7 | 12.5 |
| 8 in | 99.9 | 79.1 | 66.6 | 41.6 | 25.0 | 16.6 |
All figures in lb per square foot; density header row in kg/m3 for consistency with the metric table above.
Worked example, metric and imperial
A slab 6 m × 4 m at 150 mm thickness, calculated first at normal-weight density and then at a foam concrete density, to show the scale of the difference a density decision makes.
Metric. Volume = 6 × 4 × 0.15 = 3.6 m3. At 2400 kg/m3: mass = 3.6 × 2400 = 8640 kg (8.64 t). At 600 kg/m3 foam concrete: mass = 3.6 × 600 = 2160 kg (2.16 t) — 75 % lighter for the same footprint and thickness.
Imperial. The same slab: 6 m = 19.69 ft, 4 m = 13.12 ft, 150 mm = 0.49 ft (5.91 in). Volume = 19.69 × 13.12 × 0.49 ≈ 127.1 ft3 (4.71 yd3, matching 3.6 m3 × 1.308 within rounding). At 149.8 lb/ft3 (2400 kg/m3): mass ≈ 19,050 lb (8.64 t). At 37.5 lb/ft3 (600 kg/m3): mass ≈ 4760 lb (2.16 t). Both routes agree, as they must — it is the same multiplication carried out in two unit systems, with the small residual difference coming from rounding the intermediate feet and pounds figures.
The calculator below runs the same formula for any dimensions, thickness and density.
Fresh weight versus oven-dry weight
Every figure above is a design or specification weight, meaning oven-dry density. A freshly cast slab weighs more than that, because it still carries mixing water that has not yet left the element. For conventional concrete the gap is modest, since water is a small fraction of total mass. For foam concrete it is not: the mix design page works two examples in full — a 400 kg/m3 dry target casts at a predicted wet density of 554 kg/m3, a 38 % premium over the dry figure, and a 1000 kg/m3 dry target casts at 1144 kg/m3, a 14 % premium. The proportional gap grows as density falls, because water content per cubic metre falls far more slowly than solids content does.
The practical consequence: size formwork, propping and any other temporary works for the fresh weight, not the design weight, and do not assume a newly placed foam concrete element has reached its specified density until it has actually dried out, which can take weeks rather than days.
When slab weight actually governs a decision
Most slabs are specified for strength and serviceability, and weight is a consequence rather than a driver. It becomes the driver in a specific set of situations:
- Existing structures. Replacing or overlaying a floor or roof deck on an existing frame, where the additional dead load has to fit inside a fixed structural capacity. This is the single most common reason to specify foam concrete over normal-weight or even lightweight aggregate concrete — see the direct comparison of the two.
- Soft or variable ground. A lighter slab imposes less bearing pressure on the ground beneath it, which matters directly on poor ground and indirectly through reduced settlement risk.
- Transport of precast units. Crane capacity and vehicle payload are both hard limits, and a lighter precast element can change what plant is needed to handle it.
- Temporary works. Formwork, falsework and propping are sized to the load they actually carry, which, as above, is the fresh weight, not the design weight.
Outside these cases, weight is worth checking but rarely the number that decides the mix. A new slab on ground with adequate bearing capacity, for instance, is normally specified on strength, durability and thermal performance, with weight simply falling out of whichever density satisfies those; forcing a lighter density in that situation trades away strength for a saving nobody asked for.
Where weight does govern, it is worth stating explicitly in the specification alongside the density class, so the design assumption is on record rather than implied by a density figure elsewhere in the document.
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