How do you calculate concrete quantity for a slab? | Q & A

Concrete quantity for a slab is calculated based on plan area × slab thickness, with proper unit conversion.

Step-by-Step Method 1. Take Slab Dimensions from Drawing

From structural/architectural plan:

• Length (L)

• Width (B)

• Thickness (T) of slab

Always use written dimensions, not scale.

2. Convert Thickness into Meters

Slab thickness is usually given in mm.

Thickness in meters=Thickness in mm1000\text{Thickness in meters} = \frac{\text{Thickness in mm}}{1000}Thickness in meters=1000Thickness in mm​

Example:
125 mm = 0.125 m

3. Apply the Formula

Concrete Quantity (m³)=L×B×T\boxed{\text{Concrete Quantity (m³)} = L \times B \times T}Concrete Quantity (m³)=L×B×T​

Example Calculation

Given:

• Slab length = 6 m

• Slab width = 4 m

• Slab thickness = 125 mm = 0.125 m

$$\text{Quantity}=6\times 4\times 0.125=3.0\text{ m³}$$

4. Deduction of Openings (If Any)

• Deduct openings larger than 0.4 m² (as per standard practice)

• Examples:

  • Stair opening

  • Lift opening

  • Large ducts

$$\text{Net quantity}=\text{Gross quantity}-\text{Opening volume}$$

5. Add Extra for Wastage / Site Allowance

• Add 2–5% extra for:

  • Spillage

  • Level variation

  • Minor thickness variation

Practical Site Formula (Interview Tip)

Slab concrete = Plan area × slab thickness

Important Points Interviewers Like

• Slab quantity does NOT include beams

• Beam concrete is calculated separately

• Thickness is taken from structural drawing, not architectural

One-Line Interview Answer (Perfect)

Concrete quantity for a slab is calculated by multiplying slab length, width, and thickness (in meters), after deducting major openings and adding a small wastage allowance.

Common Follow-Up Questions

Q: Do we deduct beams from slab quantity?
A: No. Beam concrete is calculated separately.

Q: From where do you take slab thickness?
A: From structural drawings.