Core answer: concrete volume = member volume × 1.02 (waste); C30 takes ~380kg cement, 0.5m³ sand and 0.85m³ gravel per m³; rebar weight per meter = diameter²×0.00617 (12mm ≈ 0.888kg/m); a comfortable staircase satisfies 2×riser+tread ≈ 600-640mm with risers under 175mm.
Concrete volume and mix ratios
Base formula: volume = length×width×thickness × 1.02. A 5m×4m slab at 12cm: 5×4×0.12×1.02 = 2.45m³. Ready-mix usually starts at 0.5m³ steps; self-build orders round up to full m³.
Mix table by grade
| Grade | Cement per m³ | Sand | Gravel | Use |
|---|---|---|---|---|
| C15 | ~280kg | 0.55m³ | 0.85m³ | bedding layers |
| C25 | ~350kg | 0.52m³ | 0.85m³ | beams/slabs (homes) |
| C30 | ~380kg | 0.50m³ | 0.85m³ | structures, garages |
| C40 | ~450kg | 0.46m³ | 0.85m³ | high-strength members |
A cement bag is 50kg — C25 takes ~7 bags per m³.
Rebar weight quick table
Formula: kg/m = d² × 0.00617 (d in mm).
| Diameter | kg/m | Meters per tonne |
|---|---|---|
| 6mm | 0.222 | 4505m |
| 10mm | 0.617 | 1621m |
| 12mm | 0.888 | 1126m |
| 16mm | 1.58 | 633m |
| 20mm | 2.47 | 405m |
The comfortable-stair formula
Blondel: 2h+b ≈ 600-640mm (h=riser, b=tread). Residential code: h≤175mm, b≥260mm; public: h≤160mm, b≥280mm. Steps = floor height ÷ riser — round to an integer, then back-solve the true riser.
Example: slab materials list
Slab 8m×6m×0.12m, C25:
- Volume: 8×6×0.12×1.02 = 5.88m³ → order 6m³
- Cement: 6×350 = 2100kg = 42 bags
- Sand 3.1m³, gravel 5.1m³
- Rebar 12mm@150 both ways: ~638m → ×0.888 ≈ 567kg
Example: stairs for 3m floor height
Try 17 risers → h=3000/17=176mm, over limit. 18 risers → h=166.7mm ✓; tread b=620−2×166.7≈287mm (≥260 ✓). Total run 17×0.287≈4.9m — if the opening is short, split into an L-shaped two-flight with a landing.
Common pitfalls
- “Order the net volume”: mixing, transport and vibration all waste material — multiply by 1.02-1.05. Running short costs more (color mismatch, crew idle) than leftovers.
- “More cement means stronger”: excess cement cracks — water-cement ratio governs strength; follow the grade’s mix.
- “Thicker rebar is always safer”: over-reinforced members fail brittle, worse than properly designed ones — sizing belongs to calculations, not instinct.
- “Any riser under 175mm works”: back-compute exactly; legs sense a 5mm inconsistency between steps and trip.