Brick, cement and sand formulas
Brick wall
- Wall volume (cft) = length × height × thickness (inches) ÷ 12
- Bricks = volume × 1,728 ÷ 150 (one brick with mortar is 10″ × 5″ × 3″ = 150 cu in)
- Wet mortar = volume − bricks × 117.56 ÷ 1,728 (bare brick 9.5″ × 4.5″ × 2.75″)
- Dry materials = wet mortar × 1.33
- Cement (cft) = dry ÷ (sum of the ratio); bags = cement ÷ 1.25
- Sand = cement (cft) × sand parts
Plaster: wet volume = area × thickness ÷ 12; × 1.2 for filling brick joints and the uneven wall face, then the same × 1.33 rule. Openings: for walls and plaster, area = length × height − total door and window area. Concrete: wet volume × 1.54 = dry materials; in 1:2:4 the sum is 7, so cement = dry ÷ 7, sand twice that, aggregate 4×.
Wastage adds at the same rate to every material. Buying counts round bricks, bags and cft upwards.
Examples
10-inch wall: 20 ft long, 10 ft high, 1:6 mortar, 5% wastage. Volume = 200 × 10 ÷ 12 = 166.67 cft. Bricks = 166.67 × 1,728 ÷ 150 = 1,920, 2,016 with wastage. Wet mortar 36.04 cft; dry = 36.04 × 1.05 × 1.33 = 50.33 cft. Cement = 50.33 ÷ 7 = 7.19 cft = 5.75 bags (buy 6 bags), sand = 7.19 × 6 = 43.14 cft. At Tk 14 a brick, Tk 550 a bag and Tk 45 a cft of sand (example prices): bricks Tk 28,224 + cement Tk 3,300 + sand (44 cft) Tk 1,980 = Tk 33,504. With a 3 × 7 ft door and a 5 × 3 ft window in the same wall (36 sq ft), the area drops to 164 sq ft: 1,654 bricks, 4.72 bags of cement (5 bags) and 35.38 cft of sand.
Roof slab: 20 × 15 ft, 5 inches thick, 1:2:4, 5% wastage. Volume 125 cft; dry = 125 × 1.05 × 1.54 = 202.13 cft. Cement 23.1 bags (24 bags), sand 57.75 cft, aggregate 115.5 cft.
Per 100 sq ft (no wastage)
| Work | Bricks | Cement (bags) | Sand (cft) |
|---|---|---|---|
| 5-inch wall, 1:4 | 480 | 1.92 | 9.59 |
| 5-inch wall, 1:6 | 480 | 1.37 | 10.27 |
| 10-inch wall, 1:4 | 960 | 3.83 | 19.17 |
| 10-inch wall, 1:6 | 960 | 2.74 | 20.54 |
| ½-inch plaster, 1:4 | — | 1.06 | 5.32 |
| ½-inch plaster, 1:6 | — | 0.76 | 5.70 |
| ¾-inch plaster, 1:4 | — | 1.60 | 7.98 |
| 5-inch concrete, 1:2:4 | — | 7.33 | 18.33 (aggregate 36.67) |
What to know in Bangladesh
- Brick size: standard first-class brick 9.5″ × 4.5″ × 2.75″ (240 × 115 × 70 mm), 10″ × 5″ × 3″ with half-inch joints. Many kiln bricks run smaller; then the same wall takes more bricks. Measure a few before buying.
- 5-inch walls are usually inner partitions, mortar 1:4. 10-inch walls are outer or load-bearing, often 1:6 or 1:5. Follow the design.
- A cement bag is 50 kg, about 1.25 cft by volume.
- Sand: coarse sand (Sylhet sand or equal) for masonry and casting, usually medium/fine sand for plaster. Wet sand bulks, so cft measures vary a little.
- Wastage: bricks break on site, mortar drops, joints run thick; allow 5–10%. Size roof slabs and load-bearing quantities to the engineer’s design.
How the math works, in detail
Masonry counts the wall as a box filled with small “brick-with-mortar” blocks. Each brick is 9.5″ × 4.5″ × 2.75″, and with half an inch of mortar around it each block is 10″ × 5″ × 3″ — 150 cu in. One cft holds 1,728 cu in, so each cft of masonry holds 11.52 bricks.
Subtract the bricks’ own volume from the wall volume and the rest is wet mortar — about 0.22 cft per cft of masonry.
Dry sand-cement settles when water is added, and mortar also fills grooves and uneven spots. So dry materials run higher than wet mortar. The 1.33 factor used here is the engineers’ convention in the country. Some use 1.27 plus separate wastage; results land close. Casting also fills stone gaps, hence 1.54 there.
Plaster has one more allowance: a brick wall is never perfectly flat, and plaster fills the joint grooves too. The usual estimating practice adds 20% to the wet plaster volume before the 1.33 factor. On a very uneven wall, raise the wastage a little more.
Then split by ratio. 1:6 means 1 part cement and 6 parts sand out of 7. Cement cft divided by 1.25 gives bags.
This estimates shopping quantities. Real brick size, joint thickness, sand moisture and the mason’s hand all shift actual use a little.