Concrete for a Shed Slab in Sydney

Start with the slab details, not just the shed label

A shed sold as 3 × 3 m does not automatically need a slab exactly that size. Check the slab drawing for external dimensions, rebates, thickened edges, anchor positions and separate footings before measuring concrete.

For example, Absco’s garden-shed guidance describes a particular 3 × 2.26 m model with a 100 mm slab and a 25 mm deep, 75 mm wide rebate. Those details belong to that model; use the drawing supplied for your own shed.

Thickness and grade do different jobs. A stronger mix does not establish that a thinner slab, different mesh or smaller footing will suit the shed. If the shed will hold a vehicle, machinery or a hoist, have those loads included in the design before requesting concrete.

Concrete volume for common shed footprints

Volume = length × width × thickness in metres. Divide millimetres by 1,000 first: 125 mm becomes 0.125 m. The table is calculated from this formula, with volumes rounded to three decimal places.

The thickness columns are examples, not recommendations. Use a column only if it matches the thickness supplied for your project. These are flat-slab quantities before beams, footings, rebates or allowance.

Each link fills the footprint into the calculator’s size step. Replace its default depth with your specified thickness and review the allowance and mix settings before using the estimate. A size preset does not select an approved shed design.

Turn the slab volume into an order quantity

1. Confirm the actual slab dimensions and specified thickness.
2. Calculate the flat slab using length × width × thickness.
3. Add the additional volume of beams, pads or piers. Count only the part outside the volume already measured.
4. Account for rebates or other detailed geometry with your concreter.
5. Agree an allowance for the measured site, then confirm the supplier’s order increments and minimum quantity.

A percentage allowance cannot replace measured footings. For a 3 × 6 m slab, an extra 10 mm of depth across the whole footprint uses another 0.18 m³ (3 × 6 × 0.01), which is why checking the prepared base matters before booking.

Worked example: a 3 × 3 m garden-shed slab in Blacktown

Suppose your supplied details call for a 3 × 3 m flat slab at 100 mm, with no additional beam or footing volume in this example. Your concreter agrees to a 10% allowance for estimating.

Open this 3 × 3 m calculation and change it to your dimensions. The 10% allowance is an example, not a universal order rule; a designed edge beam would need its own quantity added first.

Should you mix bags or order delivery?

Choose bags only when you can mix, move and place the complete quantity with the required product and crew. Otherwise, arrange delivery with an agreed discharge method. A narrow side gate alone does not settle the decision: delivered concrete may still reach the slab by wheelbarrow or pump.

As a product-specific example, Cement Australia’s Concrete Mix data sheet specifies 110 × 20 kg bags per cubic metre. Check the current packaging for the product you buy; bag yields vary, and bags of cement are not bags of complete concrete mix.

For the Blacktown example, 0.99 m³ would require 109 bags at that yield, rounded up. That is 2,180 kg of dry product to source and handle, plus water, mixing equipment and the work of placing and finishing.

Access questions before booking shed concrete

A 0.99 m³ order for a backyard shed in Marrickville needs a different placement plan from the same quantity beside an open driveway in Blacktown. Send measurements and photos of the actual route; the suburb and load size cannot establish whether a truck will fit.

Boral’s delivery requirements include a minimum 3 m clear access-road width. That is one supplier requirement, not a guarantee of access: confirm the actual vehicle, height, turning, ground support and discharge position with the supplier serving your order.

Get a price for the concrete you actually need

Once the quantity and access plan are clear, compare the complete supply options. For bags, multiply the rounded-up bag count by your current bag price and add transport and equipment; for delivery, request the price for your address, quantity, specified mix and date.

If the Blacktown example’s specification calls for N25 and you order 1.0 m³, MixHub’s published Sydney material range gives an indicative material component of $340–370. This excludes delivery, short-load charges, service fee, pumping, waiting time and GST; it is not a delivered quote or an installed shed-slab price.

Ask what the quote includes, the minimum billed quantity, allowed discharge time and any access-related truck charge. Keep excavation, formwork, reinforcement, placing and finishing in the concreter’s scope so a concrete-supply price is not mistaken for the whole shed base.

Calculate your order using your slab details, then check Sydney small-load pricing and delivery options. Have the specified mix and access answers ready before you book.