Concrete Volume Calculator
Slabs, footings, columns, beams, and steps. Mix design with bag counts or ready-mix volumes.
10% is typical for slabs. Use 15% for irregular shapes, 5% for ready-mix where you can order exact m².
Set individual quantities per section below.
Pour Sections
Mix Design
A 25 kg bag yields ~0.012 m²; a 40 kg bag yields ~0.019 m² of finished concrete.
Site-mix proportions per m² of finished concrete (approximate):
Job Costing (optional)
UK Market PricesFills in the material cost fields with current UK market prices. You can review and edit each price before applying.
Results
Net Volume
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m²
With Waste
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m² to order
Bags (pre-mix)
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x 40 kg bags
Ready-mix
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m² (order)
Pour Breakdown
| Section | Type | Qty | Net vol (m²) | With waste (m²) |
|---|
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Frequently Asked Questions
How do I calculate how much concrete I need?
Multiply length x width x depth to get the volume in m³. A 4 m x 3 m x 0.15 m slab = 1.8 m³. Add 5-10% waste and round up. For multiple pours, calculate each separately and total them. The calculator above handles slabs, footings, columns and beams and converts volume to bags or readymix lorry loads automatically.
How deep do concrete foundations need to be under UK Building Regulations?
Under Approved Document A, strip foundations for domestic buildings must be at least 600 mm deep in most soils. In shrinkable clay soils, 900 mm to 1 m or more is typically required. Always consult a structural engineer or building control for site-specific depths - ground conditions vary significantly.
What concrete mix should I use for foundations and slabs?
Domestic strip foundations: C25/30 (formerly GEN3). Ground floor slabs: C25/30 or C28/35. Garden paths and lightly loaded pads: C20 (GEN2/ST2). In aggressive ground (high sulphate levels), use sulphate-resisting cement. Order readymix to the relevant BS 8500 designation for your application.
How many 25 kg bags of concrete do I need per cubic metre?
A 25 kg bag yields approximately 0.012 m³ when mixed, so you need approximately 83 bags per m³. A 100 mm thick 1 m x 1 m slab (0.1 m³) needs around 9 bags. For larger pours, readymix ordered by the m³ is more economical than bagged product.
What waste allowance should I use for concrete?
Allow 5-10% waste for straightforward pours; 10% for complex shapes, steps or many column bases. Readymix is typically ordered in 0.5 m³ increments, so round up after applying your waste allowance. For bagged concrete, order to the nearest full bag above your calculated quantity plus waste.
How long does concrete take to cure?
Concrete reaches approximately 70% of its 28-day strength within 7 days at 10-20°C. Full structural strength arrives at 28 days. Light foot traffic is possible after 24-48 hours; vehicles after at least 7 days. Do not pour below 2°C - concrete can freeze before curing. Protect pours in cold weather with insulating blankets.