"Hollow bricks" usually means one of two things in India: hollow concrete blocks, made from cement, sand and aggregate (IS 2185 Part 1), or hollow clay blocks, which are fired clay with large cavities (IS 3952). Both are larger and lighter than solid bricks, so walls go up faster with less mortar.
Standard sizes
| Type | Face size (mm) | Thickness (mm) | Common use |
|---|---|---|---|
| Hollow concrete block | 400 × 200 | 200 | Outer walls, boundary walls |
| Hollow concrete block | 400 × 200 | 150 | Outer and inner walls |
| Hollow concrete block | 400 × 200 | 100 | Partition walls |
Hollow clay block sizes vary more between manufacturers. Check the supplier's size before you calculate.
Example: the hollow block we supply at BricksStreet
Naresh Sihag, who writes this site, runs BricksStreet, a brick and block supplier with its own kiln in Rajasthan. Our standard cement hollow block, as an example of what to ask any supplier for:
| Specification | BricksStreet cement hollow block |
|---|---|
| Size | 400 × 200 × 200 mm (other sizes on request) |
| Minimum compressive strength | 40 kg/cm² (about 4 N/mm²), for boundary and partition walls |
| Water absorption | Under 10% |
| Weight | Roughly 40% lighter than a solid concrete block of the same size |
At 40 kg/cm², this block is meant for boundary and partition walls, not load-bearing walls. See bricksstreet.in.
How many blocks you need
For a 400 × 200 mm block with 10 mm joints, each block covers 410 × 210 mm of wall face:
- Per sq ft: about 1.08 blocks
- Per m²: about 11.6 blocks
- 100 sq ft of wall: about 108 blocks, plus 5% for breakage
The count is the same for 100, 150 and 200 mm thick blocks. Only the wall thickness changes.
Hollow blocks vs solid bricks
| Hollow concrete block | Solid clay brick | |
|---|---|---|
| Units per sq ft (one layer) | About 1.1 | About 4.6 |
| Mortar | Much less, fewer joints | More |
| Speed of laying | Fast | Slower |
| Weight on the structure | Lower per sq ft of wall | Higher |
| Fixings and chasing | Needs care, cavities behind the face | Easy |
| Water tightness | Needs good plaster or coating outside | Better on its own |
Where hollow blocks work well
- Infill walls in RCC-frame buildings
- Boundary and compound walls
- Godowns, sheds and factory walls
For load-bearing use, the cavities are usually filled with concrete and steel at corners and openings, as the structural engineer designs. Don't use unfilled hollow blocks as load-bearing walls without a design.
Laying tips
- Lay the first course on a level mortar bed over the plinth beam or DPC.
- Use blocks that have cured fully at the plant. Fresh blocks shrink and crack the plaster.
- Don't soak concrete blocks before laying. Damp is fine, dripping is not.
- Put mortar on the face shells (outer edges), not across the cavities.
- Fill cavities with concrete where the design needs it, and where heavy items like cabinets or geysers will be fixed.
- Use a mesh strip over the joint between blockwork and RCC columns or beams before plastering, to stop cracks.
Checks before buying
- Ask for the plant's compressive strength test results for the batch.
- Look for broken corners and cracked webs. Reject loads with many.
- Check that sizes are consistent. Uneven blocks mean thicker plaster.
- Ask how many days the blocks have been cured.
