Ordering the right amount of concrete can be the difference between a smooth pour and an expensive mistake. Too little and you're mixing emergency bags in a panic. Too much and you've wasted money. Here's how to calculate exactly what you need.
The Core Formula
Concrete volume is calculated in cubic metres (m³) or cubic yards (yd³):
Volume = Length × Width × Depth
All measurements must be in the same unit before multiplying.
Calculating for a Concrete Slab
Example: A patio slab, 4m × 3m, 100mm thick.
Convert depth to metres: 100 mm = 0.1 m
Volume = 4 × 3 × 0.1 = 1.2 m³
Add 5–10% for waste and spillage:
1.2 × 1.1 = 1.32 m³ (with 10% added)
Order 1.4 m³ to be safe.
Quick Reference: Common Depths
| Application | Typical depth |
|---|---|
| Path (foot traffic) | 75–100 mm |
| Domestic patio | 100 mm |
| Driveway (cars) | 100–150 mm |
| Garage floor | 100–150 mm |
| Lorry/heavy traffic | 150–200 mm |
| Foundation strip (small building) | 150–225 mm |
| Shed base | 75–100 mm |
Converting to Cubic Yards
For US projects:
1 cubic metre = 1.308 cubic yards
1 cubic yard = 0.765 cubic metres
Example: 1.32 m³ × 1.308 = 1.73 cubic yards
Calculating How Many Bags You Need
Ready-mix bags come in different sizes. Most commonly:
| Bag size | Volume of concrete |
|---|---|
| 25 kg | ~0.012 m³ |
| 20 kg | ~0.009 m³ |
| 40 lb | ~0.011 m³ |
| 60 lb | ~0.017 m³ |
| 80 lb | ~0.022 m³ |
Formula:
Bags needed = Volume (m³) / Volume per bag
Example: 1.32 m³ ÷ 0.012 m³ per 25 kg bag = 110 bags
For larger projects, ready-mixed concrete delivered by truck is far more economical.
Ready-Mix vs Bagged Concrete
| Ready-mix | Bagged | |
|---|---|---|
| Min order | Usually 0.5–1 m³ | Any amount |
| Best for | >0.5 m³ | Small repairs, under 0.3 m³ |
| Cost per m³ | Lower (£80–130) | Higher (£150–250 in bags) |
| Quality | Consistent | Variable by mixing |
| Labour | Less mixing work | More mixing work |
Rule of thumb: For anything larger than a small shed base or garden step, get ready-mix delivered.
Volume for Other Shapes
Circular Slab (Pond base, round patio)
Volume = π × radius² × depth
Example: Circular slab, 3m diameter, 100mm thick:
Radius = 1.5m
Volume = 3.14159 × 1.5² × 0.1 = 3.14159 × 2.25 × 0.1 = 0.707 m³
Footing (Rectangular trench)
Volume = Length × Width × Depth
Example: Strip foundation, 8m long, 600mm wide, 300mm deep:
Volume = 8 × 0.6 × 0.3 = 1.44 m³
Column/Post Hole
Volume = π × radius² × depth
Example: Post hole, 300mm diameter, 600mm deep:
Radius = 0.15m
Volume = 3.14159 × 0.15² × 0.6 = 3.14159 × 0.0225 × 0.6 = 0.0424 m³
For 6 post holes: 0.0424 × 6 = 0.255 m³ (about 22 × 25 kg bags)
Multiple Areas
If you're pouring a driveway with different depths or an L-shaped area, calculate each section separately and add them:
Total volume = Area 1 + Area 2 + Area 3...
Mix Ratios for DIY Concrete
If mixing by hand (for small jobs only):
| Mix (cement:sand:aggregate) | Use |
|---|---|
| 1:2:3 | General purpose, foundations |
| 1:1.5:2.5 | Higher strength slabs |
| 1:3:6 | Mass fill, less strength needed |
Water ratio: aim for about 0.5 parts water per part cement. Too much water weakens concrete significantly.
Finishing Tips
- Always add 10% waste to your calculated volume
- Wet the ground before pouring to prevent the earth absorbing water from the concrete
- Cure properly — cover with polythene for 3–7 days; don't allow to dry too fast
- Avoid freezing temperatures — concrete should not be poured below 4°C without precautions