Concrete is sold by the cubic metre, but slabs are measured in feet and square metres, which is why slab pricing confuses people. This guide converts between the two, gives the volume and concrete cost for the common slab sizes, and is honest about what the concrete does not cover.
How Much Concrete Does a Slab Need?
Volume is area multiplied by thickness. At 100mm thick every square metre takes 0.1 cubic metres of concrete, and at 150mm it takes 0.15 cubic metres. Everything below follows from that one line.
The common slab sizes, converted from feet:
| Slab size | Area | Volume at 100mm | Volume at 150mm |
|---|---|---|---|
| 8ft x 6ft | 4.5 m2 | 0.45 m3 | 0.67 m3 |
| 10ft x 8ft | 7.4 m2 | 0.74 m3 | 1.12 m3 |
| 10ft x 10ft | 9.3 m2 | 0.93 m3 | 1.39 m3 |
| 10ft x 12ft | 11.2 m2 | 1.12 m3 | 1.68 m3 |
| 12ft x 12ft | 13.4 m2 | 1.34 m3 | 2.00 m3 |
| 16ft x 12ft | 17.8 m2 | 1.78 m3 | 2.68 m3 |
| 20ft x 20ft | 37.2 m2 | 3.72 m3 | 5.57 m3 |
Standard advice is to add 5 to 10 percent for uneven ground and spillage. With metered volumetric supply you can skip the padding and simply mix more if the dig takes it, because you are billed on what is discharged rather than what was ordered.
What Is the Concrete Price Per m2?
Because concrete is priced by volume, the per square metre figure only means anything once you fix the thickness. Using 2025 rates of roughly 90 to 120 pounds per cubic metre for C20 and around 110 pounds for C30:
- 100mm thick: about 9 to 12 pounds per square metre in C20.
- 150mm thick: about 13.50 to 18 pounds per square metre in C20.
- 200mm thick: about 18 to 24 pounds per square metre in C20.
Those figures are the concrete delivered, nothing else. Full grade-by-grade rates are in our concrete price per cubic metre guide.
What Does It Actually Cost to Lay a Concrete Slab?
This is where the quote and the concrete price part company. On a typical domestic slab the concrete is often only a quarter to a third of the total, because the job also carries:
- Excavation and muck-away. Digging out to depth and removing the spoil, which on a small site can mean a grab lorry or a skip.
- Hardcore sub-base. At least 100mm of well compacted hardcore. Most slab failures are ground failures, so this matters more than an extra strength grade.
- Formwork. Timber shuttering set to level, plus pegs and bracing.
- Damp proof membrane where the slab is inside or under a building.
- Reinforcement. A142 mesh on spacers for most domestic work, or fibre in the mix. See our reinforcement guide.
- Labour for placing, levelling, floating and finishing, which is time-critical once the concrete arrives.
Supplied and laid, a straightforward domestic slab commonly works out somewhere around 60 to 120 pounds per square metre. The spread is wide because access drives it hard: a slab a wagon can discharge straight into is a very different job from one needing barrowing or a line pump through a house.
For a front driveway there is a drainage rule to settle first: see planning permission for a concrete driveway.
Which Grade and Thickness Does Your Slab Need?
Paying for a thicker slab on poor ground is worse value than a correctly specified slab on a properly compacted sub-base. As a starting point:
- Shed and garden building bases: 100mm at C20, or C25 for a heavier workshop.
- Patios: 100mm at C20.
- Driveways for cars: 125 to 150mm at C25, air entrained for freeze-thaw resistance.
- Driveways taking vans: 150 to 200mm at C25 to C30.
- Garage and workshop floors: 150mm at C25, stepping up to C30 for heavy storage.
Our grades guide covers the full range, and the shed base guide walks through a small slab step by step.
Getting an Accurate Price for Your Slab
Measure the area, decide the thickness, and multiply for the volume. Our concrete calculator does the sum including waste allowance.
One thing worth knowing before you ring round: most ready-mix suppliers apply a part-load surcharge on orders under about 3 cubic metres, which almost every domestic slab is. Because volumetric mixers batch on site and meter what comes out of the chute, there is no full-load minimum and no surcharge on a 1.34 cubic metre slab. Call 0800 001 6147 with your measurements and we will price the actual volume.
