Shed Slab Calculator Australia — Concrete, Bags & Mesh
Slab dimensions
Edge thickening
Results
Concrete volume
Volume breakdown
Bags (if using premix)
Reinforcing mesh
How to calculate concrete for a shed slab in Australia
A standard Australian shed slab uses a flat central panel with a thickened edge beam running around the perimeter. The flat slab volume is length × width × thickness. The edge thickening adds extra concrete along all four sides: perimeter × edge width × (edge depth − slab thickness). Add 10% for wastage, spillage and over-excavation. Mesh quantity is the slab area divided by the sheet size (6 × 2.4 m = 14.4 m²) plus 10% for laps and offcuts. For slabs above about 0.5 m³, a ready-mix truck is more economical than bags.
Shed slab calculator — FAQs
How thick should a shed slab be in Australia?
For a garden shed or small utility shed storing light equipment, 75–100 mm is typically adequate on stable, well-compacted ground. For a garage or workshop with vehicle access, 100–125 mm is standard. Heavy machinery or loaded vehicles require an engineer-designed thickness, typically 125–150 mm or more. AS 2870 and your local NCC requirements will govern the minimum thickness based on soil classification — Class H or reactive soils often require deeper edge beams and heavier reinforcement.
What mesh should I use for a shed slab in Australia?
SL72 (7.6 mm bars at 200 mm centres each way) is the most common residential mesh for shed and garage slabs in Class A and S soil conditions. SL82 or SL92 may be specified for Class M or reactive soils. Trench mesh (TM) is used in the edge beams in many slab designs. Your engineer or certifier will specify the mesh type — do not substitute a lighter mesh without approval as it affects the slab's ability to control cracking in reactive soil conditions.
Do I need a building permit for a shed slab in Australia?
Yes, in most cases. A concrete slab that forms the base of a shed or structure typically requires a building permit, especially if the shed is above a certain floor area (commonly 10 m² in Victoria, 25 m² in Queensland). Even if the shed itself is exempt from a permit in some states, a structural slab poured to support it may require one. Always check with your local council and, in most states, engage a registered building surveyor before starting.
How much does a shed slab cost in Australia?
A basic 6 × 3 m shed slab (18 m²) typically costs $1,500–$3,500 installed, depending on soil conditions, reinforcement requirements and your location. Ready-mix concrete is usually $150–$250 per m³ plus a short-load fee for small pours. Labour, forming, mesh, and base preparation (compacted road base) add the bulk of the cost. Getting 3 quotes from licensed concreters is recommended — prices vary significantly by region and current demand.
How long before I can use a shed slab after pouring?
Concrete reaches approximately 70% of its design strength after 7 days and 95%+ at 28 days (the standard curing period for strength testing). You can typically walk on the slab after 24–48 hours and drive a standard vehicle on it after 7 days in warm conditions. In cold weather (below 10°C) curing slows significantly — extend these times. Keep the slab moist for at least 7 days by covering with plastic sheeting or wet hessian; this dramatically improves surface hardness and reduces cracking.
How to use this calculator
- Enter the shed length and width in metres — the overall outside dimensions of the slab footprint.
- Enter the slab thickness in mm. 100 mm is standard for a residential shed floor. Use 125 mm if vehicles will park inside.
- Enter the edge thickening details. Edge depth is the total depth from the top of the slab to the bottom of the edge beam (typically 200–300 mm). Edge width is how wide the thickened strip is along each side (typically 250–300 mm).
- Check the wastage box (recommended — 10% accounts for uneven excavation, spillage, and pour variation).
- Click Calculate. The results show the flat slab volume, edge beam volume, total concrete, bag counts, and a mesh sheet estimate.
Worked example: A 6.0 m × 4.0 m shed slab, 100 mm thick, with edge beams 200 mm deep × 300 mm wide. Flat slab: 6.0 × 4.0 × 0.1 = 2.40 m³. Extra edge depth: 200 − 100 = 100 mm = 0.1 m. Perimeter: 2 × (6 + 4) = 20 m. Edge volume: 20 × 0.3 × 0.1 = 0.60 m³. Net total: 3.00 m³. With 10% wastage: 3.30 m³ to order. Mesh: 6 × 4 = 24 m² × 1.1 ÷ 14.4 = 2 sheets.
Understanding your results
The volume breakdown separates the flat slab from the edge thickening so you can verify each component. The edge beam volume is often larger than expected — on a 6 × 6 m slab with 300 mm deep × 300 mm wide edges, the edge beams alone account for about 1.3 m³ of concrete. Ignoring the edge thickening is a common and costly mistake when ordering concrete for shed slabs.
The mesh sheet count assumes standard 6.0 m × 2.4 m sheets with 10% added for laps and offcuts. For slabs requiring two-layer mesh (common on reactive soils), double the mesh count. Mesh sheets are stocked at most Bunnings, Mitre 10, and steel merchant outlets — SL72 is the standard grade for residential shed and garage slabs in Class A and S soils.
Common mistakes: Entering dimensions in centimetres instead of metres; forgetting that edge depth is the total depth, not the extra depth; and ordering without checking whether ready-mix is more economical — for most shed slabs above 1.5 m³, ready-mix is more cost-effective and produces a more consistent result than bagged product.
Shed and garage slab construction in Australia
A shed slab is one of the most common concrete projects in Australia, and the thickened-edge slab design used by this calculator is the standard residential approach. The slab consists of a relatively thin flat panel (100 mm typical) with a deeper, wider beam around the perimeter that acts as a foundation. This is sometimes called an "L-edge" or "edge-thickened" slab, and it is the design referenced in the Australian standard construction manuals for Class A and S soil conditions.
Soil preparation: The ground beneath the slab must be compacted and free of organic material. In most Australian states, a minimum of 100 mm of compacted road base (DGB20 or equivalent) is placed on top of the prepared subgrade to provide a stable, draining base. In termite-prone areas (most of mainland Australia), a physical or chemical termite barrier must be installed before the slab is poured under AS 3660.1.
Formwork: The perimeter formwork (often F17 plywood or timber boards) is set to the slab level and braced. The thickened edge is formed by the excavated trench walls — in stable soils this can be poured directly against the ground with no forming. In loose or sandy soils, boxing may be needed to hold the concrete in place during the pour.
Reinforcing: The flat slab panel is typically reinforced with SL72 mesh. The edge beams are reinforced with trench mesh (TM) — a long strip of welded mesh in a U-profile that sits in the trench. Your engineer or building certifier will specify the exact reinforcement — do not substitute a lighter mesh without approval.
Ordering concrete: For shed slabs above about 1.5 m³, ordering ready-mix from a local batch plant (Boral, Holcim, Hanson) is almost always more economical than buying bags. Standard specification for a shed slab is N25 concrete, 100 mm slump. Always have concrete in the truck tested with a slump test before accepting delivery — if the truck is overwatered on site, reject it and request a replacement batch.
Curing: Apply a spray curing compound immediately after finishing, or cover with plastic sheeting. Cure for 7 days minimum. Do not allow vehicles on the slab for 7 days; heavy loads should wait 28 days for full strength.
Australian standards and references
- AS 2870:2011 — Residential Slabs and Footings: the governing standard for shed and garage slab design in Australia, including soil classification and edge beam requirements.
- NCC Volume 2 (Building Code of Australia) — Part 3.2 sets out deemed-to-satisfy provisions for residential footing and slab construction.
- CSIRO Acceptable Construction Practice for Residential Buildings — provides construction details for standard shed slabs in Class A, S, M, and H soil conditions.
- AS 3660.1:2014 — Termite Management: New Building Work — relevant for all slabs in termite-prone zones (most of mainland Australia).
- AS/NZS 4671:2019 — Steel Reinforcing Materials: specification for SL72, SL82, and trench mesh used in shed slab construction.