In civil engineering field, calculation of concrete quantity for slab is basic but important calculation. Wrong calculation causes site concrete deficiency or extra ordering of concrete which affects cost as well as material wastage.
Here we will understand how to calculate concrete quantity for RCC slab with a simple formula and a practical example.
What is Concrete Quantity for Slab?
It is slab volume of concrete for given slab area and thickness. Usually, concrete quantity is calculated in cubic meter.
How to calculate slab concrete volume?
The volume of slab mainly depends on three things i.e., length of slab, width of slab and thickness of slab.
If all these are given then it is easy to calculate concrete quantity for slab with simple formula.
Formula to Calculate concrete quantity for slab:
We can calculate concrete quantity for slab by using simple formula as:
Concrete Quantity = Length × Width × Thickness
Where,
Length = slab length in meter.
Width = slab width in meter.
Thickness = slab thickness in meter.
If slab thickness is given in centimeter or millimeter then first convert it into meter.
For example: slab thickness is 150 mm = 150 ÷ 1000 = 0.15 m
Now, let us proceed for a practical example to understand this.
How to calculate concrete quantity for slab?
Assume that we have a RCC slab with following dimensions:
Length = 5 m
Width = 4 m
Thickness = 150 mm
First convert slab thickness into meter:
150 mm = 0.15 m
Now apply formula:
Concrete Quantity = 5 × 4 × 0.15
Concrete Quantity = 3.00 m³
Hence, the theoretical volume of concrete for this slab is 3 cubic meter.
How to calculate slab concrete in cubic feet?
Sometimes site measurements are available in feet. In that case concrete quantity can be calculated in cubic feet by applying similar formula.
Concrete Volume = Length × Width × Thickness
Here, Length, width and thickness should be in feet.
For example, consider a slab with following dimensions:
Length = 16.40 ft
Width = 13.12 ft
Thickness = 0.492 ft
Therefore,
Concrete Volume = 16.40 × 13.12 × 0.492
Volume ≈ 105.8 cubic feet.
To convert this volume in cubic meter:
1 m³ = 35.3147 ft³
Therefore,
105.8 ÷ 35.3147 ≈ 3.00 m³
Hence, by both methods we get almost same value of concrete quantity.
How to convert slab thickness from mm to meter?
Generally slab thickness is shown in drawing in millimeter and common thickness are 100 mm, 125 mm, 150 mm, 175 mm. So the formula to convert thickness in mm to meter is:
Thickness in meter = Thickness in mm ÷ 1000
For example:
- 100 mm = 0.10 m
- 125 mm = 0.125 m
- 150 mm = 0.15 m
- 175 mm = 0.175 m
- 200 mm = 0.20 m
How to calculate concrete quantity for slab with following dimensions?
Assume we have a slab in a building with following dimensions.
Length = 8 m
Width = 5 m
Thickness = 125 mm
Step 1: Convert thickness in meter.
125 ÷ 1000 = 0.125 m
Step 2: Calculate slab area.
Slab Area = mm = 0.150__ m²
Step 3: Calculate concrete volume.
Concrete Quantity = 40 × 0.125
Concrete Quantity = 5.00 m³
Hence, this slab will need 5 m³ quantity of concrete.
Do we need to add extra concrete for site requirements?
After calculating theoretical quantity little extra quantity should be added for wastage and site requirements. There may be differences in theoretical concrete, site batched concrete due to various reasons like concrete handling, level variation, uneven surface, pump line retention etc.
For example, if calculated quantity is 5.00 m³ and in your project extra 2% quantity is been allowed than extra quantity will be:
Extra = mm = 0.151__ m³
Therefore,
Total = mm = 0.152__ m³
How to find concrete quantity for slab with multiple panels?
Sometimes slab is of irregular shape. In such cases to make calculation easy, divide the slab into simple rectangular panels and calculate quantity separately.
For example, take following slab:
Panel A: 4 m × 3 m × 0.15 m
Concrete = mm = 0.153__ m³
Panel B: 5 m × 3 m × 0.15 m
Concrete = mm = 0.154__ m³
Total concrete = mm = 0.155__ m³
Do we add quantity of beams along with slab?
If slab and beam quantities are being calculated separately than beams should not be added in slab. Beam quantity is generally calculated by:
Beam Volume = Length × Width × Depth
However, while calculating quantity in general it is always advised to check if per drawing or measuring method, there is no possibility to get slab-beam junction quantity duplicated.
How does the amount of steel in the slab affect the concrete quantity?
Though reinforcement is present in RCC slab, but normally, the basic slab concrete volume is calculated in standard way without considering volume occupied by steel bars. Though in detailed site calculation or specific project requirements, necessary deductions or adjustments are made as per specifications.
Theoretical concrete quantity is usually calculated as:
Length × Width × Thickness
How to find slab concrete quantity? What are important points to consider while doing this calculation?
Few important points while calculating concrete quantity for slabs are:
- Convert all given dimensions to same unit.
- Check slab thickness properly.
- Verify slab area measurements against structural drawings.
- Calculate various slab panels separately.
- Make sure that beam quantities are not included in slab quantity.
- Check openings/cut-outs in slab for quantity deduction.
- Make distinction between theoretical quantity and quantity for ordering.
- Apply wastage allowance or additional quantity as per project requirements.
Concrete Quantity Calculator Formula
You can remember a simple formula to calculate concrete quantity for slab as:
Slab Concrete Quantity = Length × Width × Thickness
If slab thickness is given in millimeter than,
Concrete Quantity = Length × Width × (Thickness ÷ 1000)
For example:
mm = 0.156__ m
mm = 0.157__ m
Thickness = 150 mm
Therefore,
Concrete = 10 × 6 × (150 ÷ 1000)
= mm = 0.158__ m³
So, theoretical concrete volume is 9 cubic meters.
How to find concrete quantity for slab?
The basic of concrete quantity calculation for slabs is very simple. You should just know the length, width and thickness of slab and apply simple formula of length × width × thickness. It is necessary to check structural drawings for slab dimensions. If slab thickness is given in millimeter, than divide it with 1000 to get thickness in meter. The main formula to calculate concrete quantity for slabs is:
Concrete Quantity = Length × Width × Thickness
If we have a slab with thickness of 150 mm, than this thickness will be converted to 0.15 m for calculation.
Accurate calculation helps in ordering proper concrete at proper cost, but for final quantity of concrete for slab, other factors like openings, beams, site conditions, wastage allowance and project requirements should also be considered.
This calculation is frequently used in civil engineering field and is very important for site engineers, supervisors and quantity estimators.