Concrete Volume Calculator
Cubic feet
26.40
80lb bags
59
How it works
Calculating concrete volume correctly is critical for construction projects — ordering too little means an unfinished job, and ordering too much wastes expensive material. Ready-mix concrete is typically sold in cubic metres (m³) or cubic yards, and ordered in quantities requiring advance notice. The Concrete Volume Calculator handles slabs, footings, columns, stairs, and round piers.
**Slab calculation** Volume (m³) = length (m) × width (m) × thickness (m). For a 4m × 3m slab 100mm (0.1m) thick: V = 4 × 3 × 0.1 = 1.2 m³. Always add 5–10% for waste, spillage, and slight thickness variations: order 1.32–1.44 m³.
**Footing/foundation** Continuous footing: volume = length × width × depth. Point footing (square): width × length × depth. Cylindrical footing (sonotube): π × r² × depth.
**Unit conversions** 1 cubic metre = 1.308 cubic yards. 1 cubic yard = 0.7646 m³. Concrete density: approximately 2400 kg/m³ for standard ready-mix. 1 m³ ≈ 2400 kg ≈ 2.4 tonnes.
**Bag concrete equivalent** 1 m³ requires approximately: 60-kg bags at 1:2:3 mix ≈ 40 bags. Pre-mixed 40-lb (18kg) bags: approximately 130 bags per cubic metre. Bagged concrete is practical for small jobs (< 0.5 m³); larger projects should use ready-mix delivery.
**Slump and mix design** Standard slump for slabs: 75–100mm. High slump (100mm+): easier to place but reduces compressive strength. Specify the right mix: C25 (residential slabs), C30 (driveways, structural slabs), C35 (structural elements). The calculator outputs m³ and equivalent bag quantities for any project shape.
Privacy: all calculations run in the browser. No data is transmitted.
Frequently Asked Questions
- 1 cubic yard = 0.7646 m³ = 27 cubic feet. Standard 60-lb bags yield about 0.45 cubic feet; 80-lb bags yield about 0.60 cubic feet. Bags per cubic yard: 60-lb bags: 27/0.45 ≈ 60 bags. 80-lb bags: 27/0.60 = 45 bags. Pre-mixed 40-lb bags (common metric equivalent in UK): approximately 75 bags per cubic metre. Bagged concrete is economical for small jobs (< 0.5 m³/0.65 cy); for larger pours, ready-mix delivered by truck is significantly more cost-effective (minimum load typically 0.5–1 m³).
- For driveways and patios: C25 or C30 strength class (25–30 MPa compressive strength at 28 days). Standard mix ratios by volume: 1:2:3 (1 cement : 2 sand : 3 aggregate) produces approximately C25. 1:1.5:3 produces approximately C30–C35. For freeze-thaw resistance (driveways in frost-prone areas): use C35 with air entrainment (an admixture that introduces microscopic air bubbles to resist ice expansion). Water-to-cement ratio: ≤ 0.5 for durability. Lower w/c = stronger, less permeable concrete but harder to work with. Don't add extra water to improve workability — it dramatically reduces strength.
- Minimum thickness guidelines: foot traffic patio: 75–100mm (3–4 inches). Vehicle driveway: 100–125mm (4–5 inches). Light commercial vehicles: 150mm (6 inches). Heavy vehicles or forklifts: 200mm+ (8 inches+). The slab should also be reinforced with A142 mesh (6mm bars at 200mm grid) for vehicle slabs to prevent cracking. Residential pathways: 75mm unreinforced is adequate. Post holes: pour concrete 'collar' around post from bottom of hole upward, sloping away from post at surface for drainage.
- Concrete gain of strength: 24 hours — initial set (can walk on carefully). 3 days — approximately 40% of 28-day strength. 7 days — approximately 70% of 28-day strength. 28 days — design strength (industry standard cure time). 1 year — concrete continues slowly gaining strength for years. Practical guidance: foot traffic: 24–48 hours. Vehicle traffic: 7 days minimum, 28 days recommended. Full load-bearing capacity: 28 days. Keep concrete damp for at least 7 days (curing) to prevent premature drying which reduces strength and causes surface cracking. Don't pour in temperatures below 5°C or above 30°C without cold/hot weather precautions.