About this calculator
Work out how much concrete a slab, patio, shed base or driveway needs, then see it as a ready-mix order in cubic metres, a bag count, or a site-mix recipe. Add the result to a job and it carries straight into your quote.
How it’s calculated
Volume = length × width × thickness. A circular slab uses π × radius² × thickness. The waste allowance is then added on top.
Bag yields used: a 25 kg bag of post-mix or pre-mixed concrete makes roughly 0.0115 m³ and a 20 kg bag roughly 0.0095 m³. Check the yield on your brand’s bag.
Worked example
With these inputs:
- Slab shape
- Rectangle / square
- Length
- 6 m
- Width
- 4 m
- Thickness
- 100 mm
- Number of slabs
- 1
- Supply as
- Pre-mixed bags
- Bag size
- 25 kg
- Waste allowance
- 10%
the result is:
- Concrete needed
- 2.64 m³
- Slab area
- 24 m²
- Net volume (no waste)
- 2.4 m³
- Ready-mix order (to 0.25 m³)
- 2.75 m³
- 25 kg bag
- 230 bags
Tips from the trade
- Measure thickness at several points on the sub-base. A slab that is 10 mm thicker than planned across the whole area adds up fast.
- 5–10% waste is normal for a formed slab on a level base. Use 10–15% if the ground is uneven or the edges are hand-dug.
- Ready-mix suppliers usually deliver in 0.25 m³ steps. Check for a part-load charge under about 3–4 m³.
- Past about 1 m³ (around 90 bags), ready-mix or a mix-on-site lorry is usually cheaper and much faster than bags.
Common questions
How thick should a concrete slab be?
Patios and shed bases are typically 100 mm on 100 mm of compacted sub-base. Driveways are usually 150 mm, and garage floors 100–150 mm with mesh. Building Control requirements take priority.
How many bags of concrete do I need for a slab?
Divide the volume by your bag yield. The calculator shows every common bag size at once so you can compare prices.
Results are estimates for planning and pricing. Check quantities against your drawings, the manufacturer’s data and the Building Regulations before ordering.