The 6x4m shed is one of the most popular shed sizes in Brisbane and SEQ. It's big enough for a serious garden shed, a tool shed with a workbench, or storage for a ride on mower. Before you ring the concrete plant though, you need one number: how many cubic metres of concrete to order. Get it wrong by even a small margin and you'll either pay for concrete you didn't need or, worse, run out halfway through the pour.
This guide gives you the exact volumes for a 6x4m slab at every common thickness, how much extra to add for waste, what it costs in Brisbane, and which thickness to choose for your particular shed. If you want to skip the maths, jump straight to our shed slab calculator.
The Simple Concrete Volume Formula
Concrete volume is just length times width times thickness, all in metres. The result is the volume in cubic metres (m³).
Volume (m³) = Length (m) x Width (m) x Thickness (m)
The trick most people get wrong is the thickness. Slabs are usually quoted in millimetres (100mm, 125mm, 150mm). To use them in the formula, convert to metres by dividing by 1,000:
- 100mm = 0.1m
- 125mm = 0.125m
- 150mm = 0.15m
Worked Example: 6x4m Slab at 100mm
Plug the numbers in:
6 x 4 x 0.1 = 2.4 cubic metres
That's it. A 6m long by 4m wide slab at 100mm thick needs 2.4 cubic metres of concrete. The total footprint is 24 square metres, which is a nice big shed slab without being huge.
Common Thicknesses and Volumes for a 6x4m Slab
Here are the three thicknesses we pour most often for a 6x4m shed slab, along with the calculated volume of concrete for each.
| Slab Thickness | Calculation | Volume Needed | Typical Use |
|---|---|---|---|
| 100mm | 6 x 4 x 0.1 | 2.4 m³ | Standard garden or tool shed |
| 125mm | 6 x 4 x 0.125 | 3.0 m³ | Heavy duty shed, ride on mower, small vehicle storage |
| 150mm | 6 x 4 x 0.15 | 3.6 m³ | Workshop, heavier machinery, vehicle storage |
These are the raw calculated volumes. Before you order from the plant, you need to add a bit on top for waste.
Why You Order 5 to 10 Percent Extra
The calculated volume is the volume of the finished slab. The real world is messier than that. You should order 5 to 10 percent more than the raw number for a few reasons:
- Uneven ground: Even after prep, the base is never perfectly flat. Concrete fills low spots and adds depth in patches.
- Formwork thickness: Your formwork sits inside the marked area and the inside face has to be the right dimension, but small variations in setup can mean a fraction more concrete is needed.
- Spillage and dropped loads: Concrete that lands outside the form, gets stuck in the truck chute, or is lost cleaning up tools.
- Over-excavation: If the base was dug a touch deeper than planned, the slab takes more concrete to come up to finished level.
- Running short is a disaster: Once the truck leaves, a second truck is expensive and slow, and the joint between the two pours is a permanent weak spot. A small leftover is cheap insurance.
Run extras on the smaller end (5 percent) for very flat, well-prepared sites and the higher end (10 percent) for typical backyard pours on average ground.
Final Order Amounts Including Waste
Here are the numbers to actually quote to the concrete plant for a 6x4m slab.
| Thickness | Raw Volume | Order (5% extra) | Order (10% extra) |
|---|---|---|---|
| 100mm | 2.4 m³ | 2.5 m³ | 2.6 m³ |
| 125mm | 3.0 m³ | 3.1 m³ | 3.3 m³ |
| 150mm | 3.6 m³ | 3.7 m³ | 4.0 m³ |
Most concrete plants in SEQ deliver in 0.2 m³ increments, so round up to the nearest tenth when you place the order.
What Concrete Costs Per Cubic Metre in Brisbane
Ready mix concrete in Brisbane and SEQ generally runs between $250 and $350 per cubic metre delivered, depending on the mix strength, the supplier, and the distance from the plant.
Typical mixes for shed slabs:
- N20 (20 MPa): Cheapest. Suitable for light duty slabs without vehicles. Around $250 to $290 per m³.
- N25 (25 MPa): Most common choice for shed slabs. Around $270 to $310 per m³.
- N32 (32 MPa): Heavier duty, used for garage slabs and workshops. Around $290 to $350 per m³.
Minimum delivery fees, short load fees (under 4 m³), and after hours pours all push the per-cube cost up. The plant will quote a per-cube rate but also a per-truck minimum, so always confirm both.
Total Concrete Cost for a 6x4m Slab
Using the order volumes above and a per-cube range of $250 to $350, here's what the concrete alone costs for a 6x4m slab.
| Thickness | Order Volume | Concrete Cost (low) | Concrete Cost (high) |
|---|---|---|---|
| 100mm | 2.5 m³ | $625 | $910 |
| 125mm | 3.1 m³ | $775 | $1,155 |
| 150mm | 3.7 m³ | $925 | $1,400 |
That's just the concrete. It's the single biggest line item but it's not the whole bill.
What Else Goes Into the Final Job Price
The cost of the concrete is roughly a third of the total cost of a finished slab. The rest goes into:
- Site prep and excavation: Stripping topsoil, levelling, and compacting the base.
- Crushed rock base: 50 to 100mm of road base compacted under the slab.
- Formwork: Timber boards and pegs to hold the wet concrete in shape until it sets.
- Reinforcement mesh: SL72 or SL82 sheets cut and tied with bar chairs to hold them at the right height in the slab.
- Vapour barrier: Plastic sheet under the slab where the shed will be enclosed.
- Labour: Placing, screeding, floating, edging, and finishing the concrete. This is the time-sensitive bit and needs experienced hands.
- Finish: Broom finish is standard. Steel trowel or exposed aggregate costs more.
- Cleanup and removal: Spoil and offcuts hauled away.
For full cost ranges, see our detailed shed slab cost in Brisbane article and the pricing guide.
Recommended Thickness for Common Uses
The right thickness depends on what's going on the slab. Here's how we'd spec a 6x4m slab for typical uses.
| Shed Use | Thickness | Reinforcement | Concrete Volume |
|---|---|---|---|
| Garden shed | 100mm | SL72 mesh | 2.4 m³ |
| Tool shed with workbench | 100mm | SL72 mesh | 2.4 m³ |
| Ride on mower or small trailer storage | 125mm | SL82 mesh | 3.0 m³ |
| Vehicle storage or small workshop | 150mm | SL82 mesh | 3.6 m³ |
For more detail on choosing thickness, see how thick should a shed slab be.
Reinforcement Mesh: How Much for a 6x4m Slab
Reinforcement mesh adds strength and crack resistance to the slab. Standard sheets come in 6m x 2.4m (so each sheet covers about 14.4 m²). For a 6x4m slab at 24 m², you need 2 sheets to cover the full area with the standard 200mm overlap on the joins.
- SL72 mesh: Lighter mesh, suitable for standard shed slabs. Around $90 to $110 per sheet.
- SL82 mesh: Heavier mesh, suitable for vehicle slabs and workshops. Around $130 to $160 per sheet.
Plus bar chairs (small plastic supports) to hold the mesh at the correct height in the slab (typically the lower third for a slab on ground).
Brisbane and SEQ Specific Tips
Clay Soils Need a Better Base
Most of Brisbane, Logan and Ipswich sits on reactive clay soils that swell when wet and shrink when dry. That movement is what cracks slabs. The fix is a thicker compacted crushed rock base under the slab (75 to 100mm) and slightly heavier reinforcement. For more on this, see what base is needed under a concrete slab.
Time the Pour for Cooler Hours
Brisbane summers are brutal for concrete. Hot weather pulls water out of the slab too fast and causes plastic shrinkage cracks. Pour early morning or in cooler months wherever possible. Cover the slab with wet hessian or plastic for at least the first three days after the pour to slow the curing.
Order From a Local Plant
Concrete is heavy and trucks charge for distance. Use a plant within 15 to 20 kilometres of the site if you can. Major SEQ suppliers include Boral, Hanson, Holcim, and Hymix, plus smaller regional plants. Always get a quote with a per-cube rate and a delivery fee broken out.
Skip the Maths: Use Our Calculator
If you want the numbers without doing the arithmetic yourself, our shed slab calculator works out concrete volume, mesh quantity, and an indicative cost based on your shed dimensions and thickness. It's tuned for Brisbane and SEQ prices and covers all the standard slab sizes, not just 6x4m.
For a real quote, get in touch. We'll come and look at the site, check access, talk through soil and slope, and give you a fixed price for the finished slab.
Important Disclaimer
All prices are indicative starting-from guides only. Final pricing depends on site conditions, access, soil type, and specific requirements.
Volume calculations in this article are based on standard finished slab dimensions. Always confirm your final order quantity with your concrete supplier based on the actual site after prep is complete. Thickness and reinforcement recommendations are general guidance based on standard practice in SEQ. We specialise in small concrete jobs only, including shed slabs, garage slabs, concrete footpaths, concrete pads, and water tank slabs.
Frequently Asked Questions
For a standard garden or tool shed, 100mm with SL72 mesh is the typical thickness for a 6x4m slab. If you'll be storing a ride on mower or a small trailer, step up to 125mm with SL82 mesh. For vehicle storage or a small workshop with heavier loads, go to 150mm with SL82 mesh. Brisbane's reactive clay soils make it worth choosing the heavier spec if you're on the fence. For more detail, see our guide on how thick a shed slab should be.
A 6x4m slab at 100mm thick needs 2.4 cubic metres of concrete, which works out to roughly 240 x 20kg pre-mix bags (each 20kg bag makes about 0.01 m³ of concrete). Mixing that many bags by hand is not practical. For any slab over about half a cubic metre, ready mix from a concrete truck is faster, cheaper, more consistent, and gives you a much better finish than bagged mix.
Yes. Order 5 to 10 percent more than your calculated volume to allow for uneven ground, spillage, the thickness of your formwork, and minor over-excavation. So for a 6x4m slab at 100mm (2.4 m³ calculated), order 2.5 to 2.6 m³. Running short on a pour is a serious problem because the truck cannot wait and a cold joint will weaken the slab. A small amount of leftover concrete is much better than running out.
A 6x4m slab is at the upper limit of what a confident DIYer can manage and only if you have experienced help, the right tools, and good site access. Once a 2.5 cubic metre truck arrives you have around 90 minutes to place, screed and start finishing the concrete. Most people underestimate how physical and time-sensitive that work is. If in doubt, get a quote from a professional concreter so you know what you're really saving by going DIY.
As an indicative guide for Brisbane and SEQ, a finished 6x4m shed slab (24 square metres) at 100mm with SL72 mesh and broom finish typically starts from around $2,300 to $3,000 on a good access site. At 125mm or 150mm with heavier mesh, expect to start from around $2,800 to $3,800. Final pricing depends on site conditions, access, soil type, and specific requirements. See our shed slab cost in Brisbane guide for more detail, or use our calculator for a quick estimate.