How Much Concrete Do You Need for a Shed Slab? A Step-by-Step Guide

Ordering concrete for a shed slab sounds simple until you realise being wrong is annoying in two different ways. Order too little and your pour turns into a sweaty panic. Order too much and you pay for a truck full of regret.

The good news is the maths is straightforward. The slightly less fun news is the maths is only half the job. Thickness, waste allowance, base prep and delivery minimums matter too, because a cheap slab that cracks is not really a cheap slab.

Here is the practical Australian version, without the usual internet chest-beating from blokes who have apparently never made a mistake in their life.

The formula is simple

To work out how much concrete you need, use:

Length (m) × Width (m) × Thickness (m) = Concrete volume in m³

So if your shed slab is 6 metres long, 4 metres wide and 100mm thick:

6 × 4 × 0.1 = 2.4m³

That 100mm thickness needs to be converted into metres, which is 0.1m. If you use 125mm, that becomes 0.125m. If you use 150mm, that becomes 0.15m.

If you want the number fast, use the SmartKoala concrete calculator and let the calculator do the boring bit.

Always add a waste buffer

Do not order the exact calculated number unless you enjoy living dangerously for no real reason.

For most residential slab pours, adding 5% to 10% is sensible. That covers small variations in formwork, uneven sub-base, a bit left in the chute, and the general reality that building projects are not precision surgery.

Using the 2.4m³ example above:

• 5% extra = 2.52m³
• 10% extra = 2.64m³

For a simple rectangular shed slab with decent prep, many concreters will aim closer to 5%. If the site is rough, access is awkward, or you are not fully confident in the base levels, 10% is the safer call.

How thick should a shed slab be?

This depends on what the slab needs to carry, not what you wish it could carry.

For many backyard sheds, 100mm is the normal starting point. If you are building something larger, storing heavier gear, or your soil is reactive clay, thicker slabs and reinforcement may be worth it. Final slab design can also depend on soil class, shed manufacturer engineering, vehicle loads, and whether thickened edges or footings are required. That is where paying for proper advice can save you from a cracky mess later.

Reinforcement matters too

Concrete is strong in compression but not brilliant at handling movement and tension on its own. That is why reinforcement mesh often goes into slabs, especially bigger ones.

For residential shed slabs, reinforcing mesh like SL72 or SL82 is common. Exact requirements depend on slab size, loading and site conditions. If the shed is substantial or the ground is questionable, check the shed manufacturer specs and local council or engineer requirements instead of relying on your mate who "reckons it'll be fine".

Worked examples

Example 1: Small garden shed

Size: 3m × 3m
Thickness: 100mm (0.1m)

3 × 3 × 0.1 = 0.9m³

Add 5% to 10% waste and you are ordering roughly 0.95m³ to 1.0m³.

Example 2: Mid-sized workshop shed

Size: 6m × 4m
Thickness: 100mm (0.1m)

6 × 4 × 0.1 = 2.4m³

With waste allowance, order around 2.5m³ to 2.65m³.

Example 3: Vehicle-capable slab

Size: 6m × 6m
Thickness: 125mm (0.125m)

6 × 6 × 0.125 = 4.5m³

With waste allowance, order around 4.75m³ to 5.0m³.

Premix bags or readymix truck?

This is where people try to save money and accidentally sign up for a terrible Saturday.

Premix bags can make sense for tiny jobs, like a post footing or a very small slab. But once you get into anything close to a real shed slab, bags become expensive, slow and painful.

A 20kg bag of premixed concrete often yields roughly 0.01m³ once mixed, give or take by product. That means 1 cubic metre takes about 100 bags. For a 2.4m³ slab, that is around 240 bags. Which is not a fun DIY project. That is a punishment. Check the yield on the exact product data sheet before buying because bagged mixes do vary.

Readymix delivery is usually the sensible option for larger slabs. In Australia, small residential deliveries often come with:

• a minimum order or short-load fee
• a delivery fee depending on location
• time limits for unloading before waiting charges kick in

As a rough guide, once you move beyond a very small pour, readymix often starts making more sense. The break-even point depends on local supplier minimums, delivery fees, short-load charges and how much you value not spending half your day mixing bags.

Rough concrete cost in Australia

Concrete pricing moves around by region, supplier, strength and access, but as a broad 2026 consumer guide, plain readymix often lands somewhere around $350 to $450+ per m³ before extra charges. Remote areas, difficult access, pumps, engineered mixes and smaller loads can push the real cost higher again.

So for a 2.6m³ order, the concrete itself might land somewhere around $910 to $1,170+ before delivery, pump fees or extras. The slab cost is never just the concrete, either. You still need base prep, formwork, mesh, vapour barrier if required, and probably a coffee afterwards while you reconsider how heavy everything is.

The prep mistakes that cost more than the concrete

If you get the cubic metres right but ignore the base, you can still end up with a bad slab.

• Poor sub-base preparation: Soft or uneven ground leads to movement and cracking.
• Wrong thickness: A thin slab under heavy loads is asking for trouble.
• No fall or drainage plan: Water pooling around or under the shed is a long-term headache.
• Ignoring local requirements: Some sheds, slabs or sites may need planning checks with council, building approval through a private certifier or building surveyor, or engineered details depending on your state and setup.
• Ordering exact volume with no margin: This is how you turn a calculator win into a site loss.

What to do before you order

1. Measure length and width in metres.
2. Confirm slab thickness based on actual use.
3. Calculate volume in m³.
4. Add 5% to 10% extra.
5. Check supplier minimums, delivery fees and whether you need a pump.
6. Confirm any slab or engineering requirements from your shed supplier, local council planning rules, and where required a private building certifier or building surveyor.

If you are also costing the rest of the structure, the timber calculator is handy for framing and formwork estimates, and the paint calculator helps later when the shed stops being a grey box and starts looking intentional.

The simple rule of thumb

For a normal backyard shed slab, the workflow is:

measure it, calculate it, add a buffer, then check the load and site conditions before you order.

That gets you most of the way there. The last 20% is not glamourous, but it matters: proper base prep, the right thickness, and not getting cute with the waste allowance.

If you want a quick starting point, 100mm thick for a standard storage shed and a 5% to 10% buffer is often reasonable. If vehicles, heavy equipment, or dodgy soil are involved, get more specific advice before you pour.