Concrete slab thickness guide
Last reviewed 1 July 2026
Short answer
75 mm (3 in) for footpaths and light shed bases, 100 mm (4 in) for patios and general slabs, and 125–150 mm (5–6 in) for driveways and garage floors, usually with mesh reinforcement. Any slab that carries a building load, retains ground or supports a vehicle heavier than a car needs designing rather than looking up.
Work it out for your project with the concrete slab calculator →
Thickness by use
| Use | Thickness | Sub-base | Reinforcement |
|---|---|---|---|
| Footpath, light foot traffic | 75 mm (3 in) | 75–100 mm | Not usually needed |
| Shed or summerhouse base | 100 mm (4 in) | 100 mm | Optional |
| Patio | 100 mm (4 in) | 100 mm | Not usually needed |
| Car driveway | 125 mm (5 in) | 150 mm | Mesh recommended |
| Garage floor | 150 mm (6 in) | 150 mm | Mesh recommended |
| Occasional heavy vehicle | 150 mm+ (6 in+) | 200 mm | Designed reinforcement |
These are conventional domestic figures for firm, undisturbed subsoil. Soft, made-up or clay ground changes everything.
Thickness is only half the story
A slab does not carry load on its own — it spreads load onto the ground beneath it. A 150 mm slab on soft, poorly compacted fill will perform worse than a 100 mm slab on a well-compacted sub-base.
That is why the sub-base column in the table above matters as much as the thickness. Adding 50 mm of concrete costs more than adding 50 mm of sub-base and does less good.
Safety notice: This is guidance, not a design
Cracking and movement joints
Concrete cracks. The purpose of joints and reinforcement is to control where, not to prevent it.
As a working rule, provide a movement joint every 4–5 m in each direction, and avoid bays with a length-to-width ratio worse than about 1.5:1. Re-entrant corners — the inside corner of an L-shaped slab — crack reliably unless a joint is taken through them.
Mesh reinforcement does not stop a slab cracking. It holds the crack tight so the slab keeps acting as one piece. For it to do that it has to be in the upper part of the slab, on spacers, not lying on the ground.
What goes underneath
- Dig out to firm, undisturbed subsoil. Remove topsoil and any organic material entirely — it will rot and settle.
- Lay and compact the sub-base in layers of no more than 100 mm. Order about 30% more loose material than the finished depth.
- Blind the surface with sand if the sub-base is sharp enough to puncture the membrane.
- Lay a damp-proof membrane where the slab is inside a building or will have a floor covering, lapped 150 mm and taped.
- Set formwork to the finished level with a fall of about 1:80 away from any building.
Safety notice: Material planning only