Every cement and aggregate estimate for the frame starts with concrete volume. The formulas are simple; the mistakes are in double counting and forgetting members.
Formulas by Member
| Member | Volume |
|---|---|
| Slab | Length × width × thickness |
| Beam | Length × width × depth below slab |
| Column | Width × depth × height |
| Rectangular footing | Length × width × depth |
| Sloped footing | Rectangular base + frustum: h/3 × (A1 + A2 + √(A1·A2)) |
| Plinth beam | Length × width × depth |
| Staircase waist slab | Inclined length × width × waist thickness, plus steps (½ × riser × tread × width per step) |
Worked Examples
Column: 230 × 300 mm, 3.0 m high → 0.23 × 0.30 × 3.0 = 0.207 m³
Beam: 230 × 450 mm overall, 125 mm slab above, 4.0 m long → depth below slab 0.325 m → 0.23 × 0.325 × 4.0 = 0.299 m³
Rectangular footing: 1.2 × 1.2 × 0.3 m → 0.432 m³
Sloped footing: base 1.5 × 1.5 × 0.3 m, sloping 0.3 m up to a 0.5 × 0.5 m top:
- Base: 1.5 × 1.5 × 0.3 = 0.675 m³
- Frustum: A1 = 2.25, A2 = 0.25, √(A1·A2) = 0.75 → 0.3 ÷ 3 × (2.25 + 0.25 + 0.75) = 0.325 m³
- Total: 1.0 m³
Slab: 3.6 × 4.0 × 0.125 m → 1.8 m³
From Volume to Materials
| Grade | Cement per m³ | Sand | Aggregate |
|---|---|---|---|
| M15 (1:2:4) | 6.3 bags | 0.44 m³ | 0.88 m³ |
| M20 (1:1.5:3) | 8.06 bags | 0.42 m³ | 0.84 m³ |
| M25 (1:1:2) nominal | 11.1 bags | 0.385 m³ | 0.77 m³ |
For the sloped footing above in M25 (nominal): 1.0 × 11.1 = about 11 bags of cement.
Avoiding Double Counting
- Measure slabs over their full area and beams below the slab only.
- Measure columns floor to slab soffit, or between beams, consistently.
- Count every footing type separately with its own count.
- Add 3–5% wastage, more for small, scattered pours.
The Short Version
Length × width × depth for most members, a frustum for sloped footings, beams below the slab only. Multiply by cement per m³ for the grade, add a few percent, and you have a cement order you can check against a supplier's quote.
