How RCC slab concrete is calculated
The concrete in a reinforced cement concrete (RCC) slab is its length × width × thickness. A 6 m × 5 m slab that's 150 mm thick contains 4.5 m³ of concrete. This calculator returns that volume in cubic metres and cubic feet and, if you choose a mix ratio, estimates the cement bags, sand and aggregate to batch it.
The thickness field defaults to millimetres because that's how slab thickness is usually written on drawings. Length and width default to metres, and any of the three can be switched to cm, in, ft or yd.
The formulas
Slab volume = Length × Width × Thickness
For the material estimate the calculator uses the same method as the general concrete calculator:
- Dry volume = slab volume × 1.54, to allow for voids between dry particles.
- Wastage is added to the dry volume, 5% by default.
- The dry volume is split by the mix ratio. A 1:1.5:3 mix has 5.5 parts, so cement is 1 ÷ 5.5 of it.
- Cement bags = cement volume × 1,440 kg/m³ ÷ 50 kg.
The reinforcement bars displace a small amount of concrete, and the calculator doesn't deduct it. That keeps the estimate on the safe side.
Worked example
A 6 m × 5 m roof slab, 150 mm thick, in 1:1.5:3 with 5% wastage:
- Thickness in metres: 150 mm = 0.15 m.
- Volume: 6 × 5 × 0.15 = 4.500 m³ (158.916 ft³).
- Dry volume with wastage: 4.5 × 1.54 × 1.05 = 7.277 m³.
- Cement: 7.277 ÷ 5.5 = 1.323 m³, × 1,440 ÷ 50 = 38.10 bags.
- Sand: 7.277 × 1.5 ÷ 5.5 = 1.985 m³. Aggregate: 7.277 × 3 ÷ 5.5 = 3.969 m³.
Same slab, different mixes
For the 6 m × 5 m × 150 mm slab above with 5% wastage:
| Mix | Cement (50 kg bags) | Sand (m³) | Aggregate (m³) |
|---|---|---|---|
| 1:1.5:3 | 38.10 | 1.985 | 3.969 |
| 1:2:4 | 29.94 | 2.079 | 4.158 |
The 1:1.5:3 mix is usually associated with M20 nominal mix in Indian practice. IS 456:2000 permits nominal mixes only up to M20, and many specifications now call for a design mix or ready-mix concrete for suspended slabs. Use whatever grade the structural drawings specify.
Slab thickness and the design code
IS 456 controls deflection partly through span-to-depth ratios. Clause 23.2.1 gives basic span to effective depth ratios of 20 for simply supported and 26 for continuous members with spans up to 10 m, and clause 24.1 gives simpler span to overall depth ratios for short two-way slabs. These are adjusted for the reinforcement used and are only one of the checks an engineer does, alongside bending, shear, cover and fire resistance. They're mentioned here so you know where the thickness on your drawings comes from, not as a way to choose one.
Estimating a full slab pour
- Take the slab's outer dimensions from the drawing. For an L-shaped slab, split it into rectangles and calculate each.
- Enter thickness in millimetres. If the slab has drops or a thicker section, calculate that part separately.
- Add the volume of beams below the slab with the beam calculator, using the beam depth below the slab only.
- Estimate reinforcement weight with the steel weight calculator from the bar schedule.
- Choose a mix ratio if mixing on site, or order the total volume as ready-mix.
For other rectangular pours, the general concrete calculator does the same job with depth in metres. The RCC stair calculator handles waist slabs and steps.
This is a material-quantity estimate. Slab thickness, concrete grade and reinforcement must be designed or approved by a qualified structural engineer under your local building code.
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