How to estimate cement, sand, stone chips and bricks

Material estimates follow one method: find the volume of work, apply the mix proportions, then add wastage. The same method works for a footing, a slab or a wall.

Concrete: from volume to bags

Concrete is mixed by volume of cement : sand : stone chips. A common structural mix is 1 : 1.5 : 3. Because dry materials fill gaps when wet, you multiply the wet volume by about 1.54 to get the dry volume.

1. Find the wet volume. For a slab 20 ft x 15 ft x 5 in thick: 6.1 m x 4.6 m x 0.127 m, about 3.5 m³.
2. Dry volume = 3.5 x 1.54, about 5.4 m³.
3. The mix total is 1 + 1.5 + 3 = 5.5 parts. Cement is 1/5.5 of the dry volume, about 0.98 m³.
4. A 50 kg bag is roughly 0.035 m³, so cement is about 28 bags.
5. Sand is 1.5/5.5 of the dry volume, about 1.5 m³ (about 52 cft). Stone chips are 3/5.5, about 2.9 m³ (about 103 cft).

Add 5 to 10% for wastage and spillage. In round numbers, a cubic metre of 1 : 1.5 : 3 concrete uses about 8 bags of cement.

Brick walls

A standard first-class brick wall of 5 in thickness needs roughly 450 to 500 bricks per cubic metre of wall including mortar joints. Subtract the area of doors and windows before you multiply by the wall thickness. Mortar for the wall is usually 1 : 4 cement to sand.

Plaster

Plaster 12 to 15 mm thick on both faces of a wall uses a 1 : 4 cement-sand mortar. Estimate the volume as wall area x thickness, then use the same dry-volume method.

Let the planner do the arithmetic

The cost screen in the PlanCraft 3D planner carries these steps through your whole drawing and lists the totals in bags, cft and pieces, so you can compare it with a contractor's number.

These are estimation rules. Mix design, site conditions and the engineer's drawings decide the quantities you actually order.

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