Construction · Concrete
Concrete Slab CalculatorVolume, Bags & Installed Cost (UK)
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Example — 6 × 6 m garage floor at 100 mm
3.60
cubic metres
299
bags (25 kg)
100 mm
thickness
£2,100–£3,240
est. installed
A standard 6 × 6 m garage floor at 100 mm needs 3.6 m³ of concrete – around £2,100–£3,240 installed with a brushed finish and A142 mesh. Adding pattern-imprinted concrete and a deeper pour can push that to £6,000+.
Use this UK concrete slab calculator to work out how much concrete you need for any slab project. Enter your length, width, and thickness in metric units to get instant results for m³ volume, 25 kg bag count, and an installed cost estimate in £.
How to calculate concrete for a slab in the UK
In the UK, concrete volume is measured in cubic metres (m³). To work out how much you need, multiply the length, width, and thickness of your slab. Thickness is typically given in millimetres, so divide by 1,000 to convert to metres before multiplying.
Thickness is the most commonly underestimated variable. A 150 mm slab uses 50% more concrete than a 100 mm slab of the same footprint – always confirm your structural requirements before calculating.
For volume-only estimates without cost, use the concrete volume calculator to get m³ before moving on to pricing.
UK concrete slab calculation formula
Formula
Volume (m³) = Length (m) × Width (m) × (Thickness (mm) ÷ 1,000)
Dividing the thickness in millimetres by 1,000 converts it to metres. Multiply all three dimensions to get cubic metres. Always add 10% for waste before placing your order.
3×3 m, 100 mm thick
0.90 m³
≈75 bags
6×6 m, 100 mm thick
3.60 m³
≈299 bags
6×6 m, 150 mm thick
5.40 m³
ready-mix
Bagged concrete vs ready-mix in the UK
For small garden slabs under 1 m³, bagged concrete from a builders’ merchant is practical. For anything larger, ready-mix delivered by truck is faster, cheaper per m³, and produces a more consistent mix.
Bagged concrete (DIY)
83 bags per m³
25 kg bags — ≈0.012 m³ each
Ideal for slabs under 1 m³ – fence posts, steps, and small pads. 25 kg bags cost £4–£7 each at builders’ merchants, working out to around £330–£580 per m³. For larger pours, hand-mixing is impractical and uneconomical.
Ready-mix (truck delivery)
£80–£110 per m³
Plus delivery charge, typically £100–£200
The right choice for anything over 1–2 m³. Consistent mix to spec (C20, C25, C30 or fibre-reinforced), faster pour, and lower cost at scale. A 6×6 m garage floor at 100 mm (≈3.6 m³) is a typical minimum ready-mix order.
Need an exact bag count? Use the concrete bag calculator \u2192
Concrete slab thickness guide (UK)
Getting thickness wrong is the costliest concrete mistake. Use this as your quick reference before entering dimensions above.
75–100 mm
Patios & footpaths
Light foot traffic only
100–150 mm
Residential driveways
Standard cars and vans
100–150 mm
Garage floors
Add A142 mesh for vehicles
150–200 mm
Heavy loads
HGVs, commercial, plant
A 150 mm slab uses 50% more concrete than a 100 mm slab of the same area. For structural slabs and ground-bearing floors, always get your thickness specification from a structural engineer or follow Building Regulations Approved Document A.
How much does a concrete slab cost in the UK?
Installed concrete slab costs in the UK run £60–£120 per m² for a standard brushed finish, depending on thickness, reinforcement, and local labour rates. Use the table below as a quick reference, then use the calculator above for a tailored estimate.
| Slab area | 100 mm thick | 150 mm thick |
|---|---|---|
| 9 m² (3×3 m) | £540–£1,080 | £810–£1,620 |
| 20 m² (4×5 m) | £1,200–£2,400 | £1,800–£3,600 |
| 36 m² (6×6 m) | £2,160–£4,320 | £3,240–£6,480 |
| 50 m² (5×10 m) | £3,000–£6,000 | £4,500–£9,000 |
| 100 m² | £6,000–£12,000 | £9,000–£18,000 |
What affects concrete slab cost in the UK?
Size and thickness
The primary cost driver. A 100 mm slab uses 0.1 m³ per m². Moving to 150 mm adds 50% more material for the same footprint.
Reinforcement
A142 steel mesh is standard for driveways and garage floors, adding roughly £3–£8/m². T10 rebar is required for structural ground-bearing slabs.
Finish type
Brushed is cheapest. Exposed aggregate adds £15–£25/m². Pattern-imprinted (block-paving effect) adds £50–£80/m².
Ground preparation
Breaking out existing hardstanding, excavation, and laying a compacted MOT Type 1 sub-base can add £10–£30/m² depending on depth and access.
Location
Labour costs vary significantly. London and the South East typically run 20–40% higher than the Midlands or the North.
Access & drainage
Restricted access for a ready-mix truck can add £300–£1,000. Front driveways over 5 m² may require permeable edging or a drainage gulley under permitted development rules.
Frequently asked questions
How much concrete do I need for a slab in the UK?
Multiply length (m) × width (m) × (thickness in mm ÷ 1,000) to get cubic metres. A 6×6 m slab at 100 mm thick needs 3.6 m³. Always add 10% for waste before ordering.
How many 25 kg bags of concrete do I need for a slab?
One 25 kg bag yields roughly 0.012 m³, so you need around 83 bags per m³. A 6×6 m slab at 100 mm thick (3.6 m³) needs about 299 bags — 329 bags including 10% waste. For slabs over 1–2 m³, ready-mix is more practical.
What thickness should a concrete slab be in the UK?
Patios and footpaths: 75–100 mm. Residential driveways: 100–150 mm. Garage floors: 100–150 mm with A142 mesh. Heavy commercial or agricultural use: 150–200 mm minimum. Always confirm with a structural engineer for load-bearing applications.
How do you calculate cubic metres of concrete?
Formula: Length (m) × Width (m) × (Thickness mm ÷ 1,000). For example, a 6×6 m slab at 100 mm = 6 × 6 × 0.1 = 3.6 m³. Add 10% for waste: order 3.96 m³.
How much does a concrete slab cost per m² in the UK?
A standard brushed concrete slab costs £60–£100 per m² installed in 2026. Exposed aggregate adds roughly £15–£25/m²; pattern-imprinted concrete adds £50–£80/m². London and the South East typically cost 20–40% more.
Do I need planning permission for a concrete slab in the UK?
Most rear garden patios do not require planning permission. A front driveway over 5 m² of impermeable concrete does, unless you install adequate drainage or use a permeable surface. Always check with your local planning authority before starting.
This calculator provides estimates for planning purposes only. Actual concrete quantities depend on ground conditions, form accuracy, pour technique, and material yield. Cost figures are approximate UK national averages for 2026 and may not reflect local pricing, VAT, or skip hire. Always confirm quantities and costs with your supplier and contractor before ordering.