Getting the right amount of concrete is one of the most important parts of any building project. Order too little and you’ll have to stop mid-pour and wait for more. Order too much and you’re paying for material you don’t need. This guide covers everything you need to know about calculating, ordering, and working with concrete.
Understanding Concrete Volume
Concrete is measured in cubic metres (m³) in the UK and most of the world, or cubic yards in the US. One cubic metre is 1,000 litres — roughly enough to fill a box 1m × 1m × 1m. For a typical garden shed base (2m × 3m × 100mm thick), you’d need 0.6m³.
The formula is simple: Length × Width × Depth = Volume. But real-world projects are rarely that straightforward — you need to account for irregular shapes, sub-base displacement, and waste.
Always Add a Waste Factor
Professional builders typically add 5-10% extra to their calculated volume. Concrete gets spilled, sticks to tools, fills unexpected dips in the sub-base, and formwork is never perfectly precise. For DIY projects, add 10% to be safe. It’s far better to have a barrow-load left over than to run short during a pour.
Ready-Mix vs Bagged Concrete
For small jobs (under 0.5m³), bagged concrete from a builders’ merchant is practical. Each 25kg bag makes roughly 0.012m³ of concrete. For anything larger, ready-mix delivery is more economical and produces a more consistent result. Most ready-mix suppliers deliver a minimum of 1m³.
Common Concrete Mix Ratios
- General purpose (C20): 1 part cement : 2 parts sand : 4 parts aggregate. Suitable for foundations, paths, garden walls, and shed bases.
- Strong mix (C30): 1 : 2 : 3. Used for structural work, lintels, and load-bearing elements.
- Foundation mix (C25): 1 : 2.5 : 3.5. Common for house foundations, retaining walls, and driveways.
- Post mix: Pre-blended bags designed for setting fence posts. Just add water to the hole — no mixing needed.
Concrete Thickness Guide
How thick should your concrete be? It depends on the load:
- Garden path: 75-100mm
- Shed base: 100mm
- Patio: 100mm
- Driveway (cars): 100-150mm
- Driveway (heavy vehicles): 150-200mm
- Garage floor: 100-150mm
- House foundations: As specified by structural engineer (typically 300mm+ strip foundations)
Tips for a Successful Concrete Pour
- Prepare your sub-base: Compact the ground and add a layer of MOT Type 1 or similar aggregate. Poor sub-base is the number one cause of concrete cracking.
- Check the weather: Don’t pour in heavy rain or freezing temperatures. Ideal conditions are 10-20°C. If it’s hot, keep the concrete damp while curing.
- Have enough help: Concrete waits for no one. Once it arrives (or you start mixing), you need to work quickly. Have enough people and wheelbarrows ready.
- Allow curing time: Concrete reaches about 65% strength in 7 days and full strength in 28 days. Don’t drive on a new driveway for at least a week, preferably two.