The slab is the biggest single concrete pour in a house, and its thickness drives a large share of the cement, sand, aggregate and steel you buy. It is also a structural decision, not a matter of preference. Here is how thickness is set and what it means for your material order.
What Thickness Is Typical?
| Slab | Typical thickness |
|---|---|
| Toilet, small store, short spans | 100 – 115 mm |
| Bedrooms, living room (3 – 4.5 m spans) | 125 – 150 mm |
| Larger halls, longer spans | 150 mm or more, or beams to break the span |
| Roof slab | Same as floor slab, plus waterproofing and screed above |
| Sunshade (chajja) | Tapered, about 75 mm at the edge to 100 mm at the wall |
How Does IS 456 Set Slab Thickness?
Slab thickness is controlled mainly by deflection — how much the slab sags under load. IS 456 limits the ratio of span to effective depth (depth to the centre of the bottom steel):
| Support condition | Basic span / effective depth |
|---|---|
| Simply supported | 20 |
| Continuous | 26 |
| Cantilever | 7 |
These basic values are then adjusted for the amount and grade of steel. For two-way slabs on normal residential spans and loads, IS 456 gives simpler span-to-overall-depth ratios, which, after the adjustment for Fe 500 steel, typically land the slab at 100–150 mm.
Worked example: a continuous slab spanning 3.6 m with a basic ratio of 26 needs an effective depth of about 138 mm before modification factors; with typical steel percentages and modification, the overall thickness often comes out around 125–150 mm including cover.
How Does Thickness Change Your Material Order?
For each 1,000 sq ft (92.9 m²) of slab in M20:
| Thickness | Concrete | Cement (≈8.06 bags/m³) | Sand | Aggregate |
|---|---|---|---|---|
| 100 mm | 9.3 m³ | about 75 bags | 3.9 m³ | 7.8 m³ |
| 125 mm | 11.6 m³ | about 94 bags | 4.9 m³ | 9.8 m³ |
| 150 mm | 13.9 m³ | about 112 bags | 5.8 m³ | 11.7 m³ |
Each extra 25 mm on 1,000 sq ft adds about 19 bags of cement and nearly 2.3 m³ of concrete — and its weight, which the beams, columns and footings must also carry.
When Should a Slab Be Thicker?
- Longer spans, unless beams are added to shorten them.
- Heavier loads, such as a terrace garden, water tanks or a future floor of rooms on the roof.
- Concealed services that reduce the effective depth.
- Better sound separation between floors.
Can You Make the Slab Thinner to Save Money?
Not on site. Thickness, steel and span are designed together; reducing thickness without redesign increases deflection and cracking, and can leave the slab under-strength. If you want to save, ask the engineer whether a beam layout that shortens spans would allow a thinner slab — that is a design decision with a real basis.
The Short Version
Expect 125–150 mm for a typical house slab, set by the engineer from the span. Each 25 mm per 1,000 sq ft is about 19 bags of cement. Do not thin a slab on site; if you want a lighter slab, change the design, not the pour.
