Concrete Volume Calculation
Before ordering concrete or buying bags, calculate the exact volume needed. Over-ordering wastes money; under-ordering stops work.
Volume Formulas
Slab: V = L × W × D
Footing: V = L × W × D
Column: V = π × r² × H
Example slab 4m × 3m × 0.1m thick:
V = 4 × 3 × 0.1 = 1.2 m³
Add 10% waste: 1.32 m³Standard Mix Ratios (cement:sand:gravel by volume)
M10 (1:3:6) — Non-structural fill, blinding
M15 (1:2:4) — Light pavements, mass concrete
M20 (1:1.5:3) — General slabs, beams, columns ← most common
M25 (1:1:2) — High-strength structural workHow Many Bags?
M20 mix per m³ of concrete:
Cement: ~8 bags (50 kg each)
Sand: ~0.44 m³
Gravel: ~0.88 m³
1.2 m³ slab (M20):
Cement: 1.2 × 8 = 9.6 → 10 bags
Sand: 1.2 × 0.44 = 0.53 m³
Gravel: 1.2 × 0.88 = 1.06 m³Pre-mixed Bag Coverage (40kg bag ≈ 0.012 m³)
- 0.1 m thick slab 1m² → ~8 × 40kg bags
- Fence post hole (200mm × 600mm deep) → ~2 bags
- 1 m³ total → ~83 × 40kg bags
Calculate concrete volume: Free Concrete Mix Calculator
Standard Concrete Mix Ratios
- C10 (M10): 1:3:6 (cement:sand:aggregate) — blinding, non-structural fill
- C20 (M20): 1:1.5:3 — general structural use, slabs, foundations
- C25 (M25): 1:1:2 — reinforced concrete, columns, beams
- C30 (M30): 1:0.75:1.5 — high-strength structural, water-retaining
- C40: designed mixes used for bridge decks, marine structures
Water-Cement Ratio
The water-cement ratio (w/c) is the most critical factor in concrete strength: lower w/c = higher strength. A w/c of 0.45 produces ~35 MPa at 28 days; 0.65 produces ~20 MPa. Excess water evaporates and leaves voids (capillary pores) that weaken the matrix and increase permeability. However, too little water makes the mix unworkable. Plasticisers (superplasticisers) allow low w/c ratios with adequate workability. Workability is measured by slump test: target slump for pumped concrete ~100–180 mm; stiff hand-placed concrete ~25–75 mm.
Frequently Asked Questions
How long does concrete take to reach full strength?
Concrete gains strength through hydration — a chemical reaction between cement and water that continues for months. At 7 days, concrete typically reaches ~65% of its 28-day strength. At 28 days it reaches the design strength. After 90 days it may be 10–15% stronger still. Full cure for practical purposes (formwork removal, loading) is typically 28 days, but stripping times depend on temperature, section thickness, and structural requirements.
What causes concrete to crack?
Plastic shrinkage cracks (within hours): rapid evaporation of surface water before concrete sets — prevent with curing membranes or wet burlap. Drying shrinkage cracks (after hardening): concrete shrinks ~0.04% as it dries; control with shrinkage reinforcement and construction joints. Thermal cracking: heat of hydration in large pours causes differential expansion/contraction; prevent with low-heat cement and temperature control. Structural cracks: inadequate reinforcement or overloading.
What is the difference between concrete and mortar?
Mortar is cement + sand + water (no coarse aggregate). It is used to bond bricks and blocks, and for rendering. Concrete adds coarse aggregate (gravel, crushed stone) for much greater compressive strength and is used for structural elements. Grout is cement + water (no aggregate) for filling gaps and post-tensioning ducts.