Concrete Calculator UK | Bags & Volume for Any Pour
Free UK concrete calculator. Work out concrete volume in m³ and number of 25kg bags for slabs, footings, posts and pads. Includes waste allowance.
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Results are estimates only and may vary depending on site conditions, material specifications and waste. EasyEstimate and its affiliates accept no responsibility for any loss or damage arising from reliance on these figures. Always verify quantities with your supplier before ordering.
How to Calculate Concrete Volume
Calculating how much concrete you need is one of the most common construction calculations. Whether you are laying a slab, digging foundations, or setting fence posts, the formula is the same.
The Formula
Volume (m³) = Length (m) × Width (m) × Depth (m)
Then add a waste allowance (typically 10%) to account for uneven ground, overspill, and any variation in thickness:
Order quantity = Volume × (1 + waste%)
Common Concrete Depths
| Application | Typical depth |
|---|---|
| Garden path / patio | 75–100mm |
| Garage floor | 100mm |
| Driveway (cars) | 100–150mm |
| Strip foundation | 200mm minimum |
| Pad foundation | 300mm minimum |
Worked Example: 3m × 3m Patio Slab
- Volume = 3 × 3 × 0.1 = 0.9 m³
- Add 10% waste = 0.9 × 1.1 = 0.99 m³ → order 1 m³
- Bags of 25kg concrete: 1 × 12.6 = ~13 bags
Concrete Mix Ratios
| Mix | Cement : Sand : Aggregate | Typical use |
|---|---|---|
| C10 | 1 : 3 : 6 | Blinding, non-structural fill |
| C20 | 1 : 2 : 4 | Domestic slabs, patios |
| C25 | 1 : 1.5 : 3 | Foundations, driveways |
| C30 | 1 : 1 : 2 | Heavy duty, exposed surfaces |
Materials per m³ of Concrete (C20 mix)
- Cement: 320 kg (~6.5 bags of 50kg)
- Sand (sharp): 640 kg
- Coarse aggregate: 1,280 kg
- Water: approx. 160 litres
Ready-Mix vs Bagged
| Ready-mix | Bagged | |
|---|---|---|
| Best for | >0.5 m³ | <0.5 m³ |
| Quality | Consistent | Variable |
| Cost | Lower per m³ | Higher per m³ |
| Convenience | Delivery required | Mix on site |
For most domestic projects over a cubic metre, ready-mix concrete (ordered by volume) is the practical choice. The concrete calculator above gives you your order quantity directly.
From Concrete Volume to Full Estimate
This calculator gives you volume and bag count. A complete groundworks estimate also includes excavation, formwork, reinforcement (rebar or mesh), DPM, curing, and associated labour.
EasyEstimate’s full concrete calculator includes all of these elements with live UK pricing — from material costs to digger hire.
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Frequently Asked Questions
- How do I calculate how much concrete I need?
- Multiply length × width × depth to get the volume in cubic metres (m³). Add a waste allowance of 10% for uneven ground and spillage. For ready-mix concrete, this figure is your order quantity. For bagged concrete, multiply by 12.6 to get the number of 25kg bags per m³ (approximately).
- How many 25kg bags of concrete per m³?
- Approximately 12.6 bags of 25kg ballast mix are needed per m³ of concrete. In practice, most suppliers round this to 13 bags. For a C20 mix using separate cement, sand and aggregate, you will need roughly 400kg of cement per m³.
- What concrete mix should I use for a slab?
- For a domestic floor slab or patio, a C20 mix (1:2:4 cement:sand:aggregate) is standard. For a driveway subject to vehicle traffic, use C25 or C30. For foundations, C25 is typically specified. For exposed or heavy-duty applications, consult a structural engineer.
- What depth of concrete do I need for a slab?
- For a domestic patio or footpath: 75–100mm. For a garage floor or light vehicle: 100mm. For a driveway with car access: 100–150mm. For strip foundations: minimum 200mm deep × 600mm wide (check Building Regulations for your specific project).
- Should I order ready-mix or use bagged concrete?
- For volumes over 0.5 m³, ready-mix concrete is usually more cost-effective and produces a consistent mix. For smaller pours (fence posts, footings, repairs), bagged concrete or ballast with separate cement is more practical. Ready-mix has minimum order quantities — typically 0.5 or 1 m³.
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