Updated August 11th, 2020

We often need to calculate slab concrete quantity in our projects either for ordering concrete from ready mix suppliers or purchasing the concrete ingredients.

Calculating concrete quantity for a slab is actually easy. You just need to get the volume of some geometric shapes.

But the problem is we often don’t have that simple slabs in our project.

They come with beams, voids, openings, as well as various thicknesses.

In this post, I’ll show you how to calculate concrete quantity for a slab of a building.

First, I’ll show you how to calculate the concrete quantity for a simple slab.

Then we’ll see the calculating process of a slab with beams, opening, and various thicknesses.

So, let’s discuss it without further ado.

## What Is A Concrete Slab?

As Wikipedia says, a concrete slab is a structural element of modern buildings, consisting of a flat, horizontal surface made of cast concrete.

Often Reinforcement is used in concrete slabs.

It’s designed by structural engineers. The thickness and reinforcement vary depending on the load on the slab. Structural engineers calculate the loads following building codes. And depending on that they define the thickness of the slab and use reinforcement as required.

## How To Calculate The Concrete Quantity For A Rectangular Slab?

A simple rectangular slab looks like:

This is an example of a slab without any supports.

So, calculating the concrete quantity for this slab is easy. You just need to know the concrete volume formula. And that is,

= Length × Width × Thickness

So the concrete quantity of our example slab is,

= 14′ × 10′ × 6″ [6″ = ½′]

= **70 Cubic feet**.

Very simple.

But in our building projects, we often don’t get that simple slab.

At least, I never found, lol.

So, let’s see…

## How To Calculate Concrete Quantity For A Slab Of A Building?

Slabs of a building often come with beams, openings, and various thicknesses. That looks like the image below –

A **slab layout plan** of the structural drawing looks something like this.

We’ll calculate the concrete quantity for this slab.

For that, go through the following steps –

### Step-1: Keep Drawings And Required Things With You

As you can see, there are no dimensions in the drawing. We often don’t find dimensions in the **structural slab layout plan**.

You need to collect them from other drawings.

So, keep both the **architectural **and the **structural **drawing book with you.

Also keep a pen, paper, and calculator.

With that let’s…

### Step-2: Calculate The Slab Concrete

You’ll often find a separate sheet in the architectural drawing book named **slab layout plan**.

The slab layout plan in the architectural drawing book and in the structural drawing book is different.

The plan that you get in the structural drawing book often doesn’t show the dimension of the slab. It just shows the thickness of the slab.

Even if the slab layout plan of the structural book shows dimensions, you should follow the architectural drawing for accuracy.

But you won’t find slab thickness in the architectural slab layout plan.

You’ll find them in the **structural slab layout plan**.

As you can see in the image above, we have three slab panels with **6″** thickness and one panel with **5″** thickness.

So, I divided the slab into two parts – all **6″** panels together as **part-1 **and the **5″** panel as **part-2**.

Let’s calculate…

#### Slab Concrete Quantity For The Part-1:

Applying the formula, the slab concrete quantity for the **part-1** is,

= Length × Width × Thickness

= (40′-5″) × (18′-2″) × 6″

= **367.21 **cubic feet

#### Slab Concrete Quantity For The Part-2:

Similarly, for **part-2**,

= (6′-9½″) × (18′-2″) × 5″

= **51.82 **Cubic feet.

There is also a void in the slab.

So, we need to…

#### Deduct The Void:

The concrete quantity for the void is,

= (8′-3½″) × (7′-3″) × 6″

= **30.05 **cubic feet.

So, the concrete quantity for the slab (only) is,

= 367.21 + 51.82 – 30.05

= **388.98 cubic feet.**

This is the only slab concrete quantity.

Let’s calculate the…

### Step-3: Concrete Quantity For Beams

Most of the time we get the cross-sectional dimension of beams from the beam layout plan. Which is looked like…

You also need the length of the beams. But the beam layout plan doesn’t show them.

But you can collect them from the column layout drawing.

Now you can calculate the length of beams. Which will look like:

You have now everything to calculate beams’ concrete quantity.

Applying the concrete volume formula, which is,

= length × width × height,

the concrete quantity for,

**B1 **= {(16′-11½″) + (20′-5½″)} × 10″ × 14″ [ Actual beam height is 20″. But we’ve taken 6″ while calculating the slab concrete]

= 36.34 cubic feet.

**B2 **= (21′-7″) × 10″ × 14″

= 20.96 cubic feet

**B3 **= {(15′-8″) + (10′-11″)} × 10″ × 14″

= 25.82 cubic feet.

**B4 **= 7′ × 10″ × 14″

= 6.80 cubic feet.

**B5 **= (6′-1½″) × 10″ × 15″ [as we have taken 5″ while calculating the slab concrete quantity.]

= 6.35 cubic feet.

**B6 **= (6′-9½″) × 10″ × 15″

= 7.04 cubic feet.

**B7 **= {(6′-11″) + (7′-7″)} × 10″ × 14″

= 14.08 cubic feet.

**B8 **= {(7′-1″) + (7′-7″)} × 10″ × 14″

= 14.24 cubic feet.

**B9 **= (15′-6″) × 10″ × 14″

= 15.05 cubic feet.

The total concrete quantity in beams,

= 36.34 + 20.96 + 25.82 + 6.80 + 6.35 + 7.04 + 14.08 + 14.24 + 15.05

= **145.68 cubic feet.**

As you see we’ve left the column portion.

So, let’s calculate the…

### Step-4: Concrete Quantity For Column-Heads

For calculating the concrete quantity for column-heads, you’ll need the cross-sectional dimensions of columns.

Most of the time, You can get them from the column layout drawing.

If you don’t find them there look in the column schedule drawing which looks something like:

So, let’s calculate the concrete quantity for the column,

**C1 **= 12″ × 20″ × 14″ [ the height of the column-head will be same as the beam hight which is 14″].

= 1.95 cubic feet.

**C2 **= 12″ × 20″ × 14″

= 1.95 cubic feet.

**C3 **= 12″ × 20″ × 14″

= 1.95 cubic feet.

**C4 **= 12″ × 20″ × 14″

= 1.95 cubic feet.

**C5** = 10″ × 46″ × 14″

= 3.74 cubic feet.

**C6 **= 12″ × 20″ × 14″

= 1.95 cubic feet.

**C7** = 12″ × 25″ × 14″

= 2.44 cubic feet.

**C8 **= 12″ × 25″ × 14″

= 2.44 cubic feet.

**C9 **= 12″ × 20″ × 14″

= 1.95 cubic feet.

The total concrete quantity for column-heads is,

= 1.95 + 1.95 + 1.95 + 1.95 + 3.74 + 1.95 + 2.44 + 2.44 + 1.95

= **20.32 cubic feet**.

There is a difference in the height of the column head of C3.

When we took the slab thickness, half portion (almost) of the column (C3) is taken as 5″ and the rest is 6”.

So, when we calculate the column-head height, we should take the half portion (almost) of the column head height **15 ″** and the rest is

**14″**. But I ignored that as it is a small portion.

I think you get it.

Our calculation is almost finished.

We just need to sum up all three concrete quantities.

That is,

= **The quantity of slab concrete + The quantity of beam concrete + The quantity of column head concrete**

= 388.98 + 145.68 + 20.32

= 554.98 cubic feet.

Say, **555 cubic feet**.

This is the exact concrete quantity required for our example slab.

You can order this quantity from a ready-mix supplier.

If you want to manually prepare the concrete on your project then you have to purchase concrete ingredients separately.

Let’s see…

## How We Can Calculate Concrete Ingredients?

Let’s say, the slab concrete ratio is **1:1½ :3**.

We know the required concrete quantity, that is **555 **cubic feet.

This is the wet volume.

The **dry volume of concrete** is,

= 555 × 1.54

= 854.7 cubic feet.

You may want to know how I calculate the dry volume. I’ve talked about this before on the post What is The Dry Volume of Concrete and How to Calculate It.

Now sum up the ratio of concrete ingredients. That is,

= 1 + 1½ + 3

= 5½

Finally, get the quantity of concrete ingredients.

**Cement quantity**= (854.7 ÷ 5½) × 1

= 155.4 cubic feet

You may want to get the cement quantity as bags.

In that case, you need to know the volume of a cement bag.

In my place, it’s 1.25 cubic feet.

So, we need,

= 155.4 ÷ 1.25

= 124.32

Say, **125 bags of cement**.

**Fine aggregate**(sand) quantity is,

= (854.7 ÷ 5½) × 1½

= 233.10

Say, **234 cubic feet**.

**Course aggregate**(stone or brick chips) quantity is,

= (854.7 ÷ 5½) × 3

= 466.20

Say, **467 cubic feet**.

That’s it.

** Read More:** How to Calculate Materials For Different Ratio Concrete?

**Conclusion**

Slabs come in different sizes and shapes. Sometimes you need to cast the slab with the beams. Sometimes beams need to be cased separately. Depending on the situation, just use your common sense to know how you’re going to calculate the concrete quantity for the slab.

*Your Turn:*

*In this post, I’ve calculated the slab concrete quantity all through. Later I deducted the slab thickness from the beam when calculating the beam concrete.*

*Now please tell,*

*Is this a easy way?*

Or

*Is calculating the beam concrete first then the slab concrete panel by panel the easiest way?*

*Please share your opinion in the comments below…*

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