Unit Weight of Building Materials (with Load Example)

For structural design in India, take reinforced concrete as 25 kN/m³, plain concrete as 24 kN/m³, steel as 78.5 kN/m³ (7850 kg/m³), burnt clay brick masonry as about 19–20 kN/m³ and water as about 10 kN/m³. IS 875 (Part 1):1987 lists the unit weights of building materials for dead load calculation, and IS 456 adopts 24 and 25 kN/m³ for plain and reinforced concrete.

Density or unit weight?

Density is mass per volume, in kg/m³. Unit weight is weight per volume, in kN/m³, and this is what load calculations use.

Strictly, 1 kN = 1000 ÷ 9.81 ≈ 102 kg. In design practice g is usually rounded to 10 m/s², so 1 kN is taken as 100 kg. That is why steel at 7850 kg/m³ is written as 78.5 kN/m³. Divide kg/m³ by 100 to get approximate kN/m³.

Unit weight table

The values below are the ones commonly used in Indian design and estimating. Where a single value is shown, it is the standard design figure. Where a range is shown, the value depends on the source, moisture, compaction and type of material, so treat it as approximate. For a specific material such as a particular stone or timber species, check the material tables in IS 875 (Part 1).

How unit weight is used in load calculation

Dead load is the unit weight multiplied by the size of the member. The form of the answer depends on the element you are designing:

• Load per m² (slabs, finishes): γ × thickness. For example, a 125 mm slab gives 25 × 0.125 = 3.125 kN/m².
• Load per metre run (walls on beams, beam self-weight): γ × thickness × height, or γ × width × depth.
• Point load (a tank, a column): γ × volume.

Worked example: slab and wall loads in a house

1. Floor slab, 150 mm thick, residential

1. Self-weight = 25 × 0.150 = 3.75 kN/m²
2. Floor finish: assume 1.0 kN/m², a common allowance for screed and tiles
3. Imposed (live) load for a residential floor, from IS 875 (Part 2) = 2.0 kN/m²
4. Total service load = 3.75 + 1.0 + 2.0 = 6.75 kN/m²
5. Factored load for limit state design = 1.5 × 6.75 = 10.125 kN/m²

2. Wall and self-weight on a plinth beam

The wall is 230 mm brick, 3.0 m high, with 12 mm plaster on both faces. The beam is 230 × 450 mm.

1. Brickwork = 20 × 0.23 × 3.0 = 13.80 kN/m
2. Plaster = 20 × (2 × 0.012) × 3.0 = 1.44 kN/m
3. Beam self-weight = 25 × 0.23 × 0.45 = 2.59 kN/m
4. Total dead load on the beam = 13.80 + 1.44 + 2.59 = 17.83 kN/m, which is roughly 1.8 tonnes per metre

If you had used 19 kN/m³ for the brickwork, the wall load would be 13.11 kN/m instead of 13.80. This difference of about 5% is why many designers simply use 20.

3. Water tank on a roof

A tank holding 2 m × 2 m × 1.5 m of water adds 6 m³ × 10 = 60 kN (about 6 tonnes), not counting the tank itself. This is why a "small" roof tank needs a proper beam under it.

The same steel density gives the familiar rebar formula: weight per metre = d² ÷ 162 kg, with d in mm. See how to calculate steel quantity. For concrete quantities and costs, the RCC estimator applies these unit weights automatically.

Common mistakes

• Using 24 for RCC. 24 kN/m³ is for plain concrete. Reinforced members are taken at 25 kN/m³.
• Mixing kg and kN. A load of 2500 "kN/m³" is clearly wrong. Divide kg/m³ by about 100 to get kN/m³.
• Forgetting plaster, finishes and partitions. These easily add 1–2 kN/m² to a floor.
• Using loose bulk density for structural load. Sand fill under a floor compacts. Use the compacted unit weight, not the loose value from a truck.
• Treating a range as exact. For masonry, stone, timber and AAC, check the actual material or the code table. When in doubt, take the higher value for strength design.

Unit weights come up often in PSC and SSC JE papers. Practise them in our free mock tests.