Rebar Calculator Australia — Concrete Slab ReinforcingAS 3600
Slab dimensions
Reinforcing bar
Results
Summary
Bar breakdown — by direction
| Direction | Bars | Bar length (m) | Lineal m |
|---|
Inputs used
How to calculate rebar for a concrete slab in Australia
Reinforcing bar (rebar) is placed in a grid pattern within a concrete slab to control cracking and carry tensile loads. The two key variables are bar diameter (which determines strength and weight) and bar spacing (which determines how many bars are needed). For a simple rectangular slab, the number of bars running in each direction is calculated by dividing the slab width (minus cover each end) by the bar spacing, then rounding up. Total lineal metres is bars × bar length; total weight is total LM × the bar's kg/m rating. Australian standard deformed bar designations (N-bars) and plain round bars (R-bars) follow AS 3600 and are available from steel merchants in standard 6 m, 9 m and 12 m lengths.
Rebar calculator — FAQs
What is the difference between R-bar and N-bar in Australia?
R-bar (round bar, e.g. R10) has a smooth surface and is typically used for ligatures, ties and starter bars. N-bar (deformed bar, e.g. N12, N16) has a ribbed surface that bonds to concrete mechanically — it is the standard choice for slab and footing reinforcement. N-bar is specified under AS 3600 for structural applications.
What bar spacing is typical for a residential concrete slab in Australia?
For a standard domestic slab-on-ground, N12 bars at 200 mm centres each way (or the equivalent mesh — SL82 or SL92) is a common starting point. Your engineer will specify the exact arrangement based on soil class, slab thickness and loading. Some engineers specify 150 mm centres in reactive soil areas (Class M, H1, H2, E).
How do lap lengths work in rebar calculations?
Steel bars come in standard lengths (6 m, 9 m, 12 m). Where two bars need to join, they overlap — this is called a lap splice. AS 3600 typically requires a minimum lap of 25–40 × bar diameter (e.g. 300 mm for N12). This calculator lets you add a fixed lap length to each bar run as a conservative allowance for slabs where bar lengths don't align exactly with slab dimensions.
What concrete cover is required for rebar in Australia?
AS 3600 sets minimum cover based on exposure classification. For residential slabs: 20 mm (A1 — interior, protected), 30 mm (A2 — sheltered exterior), 40 mm (B1 — near coast or direct soil contact). NCC Volume Two also references these values for Class 1 buildings. This calculator uses cover to determine the net length of each bar within the slab.
Should I order extra rebar for wastage?
Yes. Steel merchants typically supply in standard bar lengths so cuts are unavoidable. A 10% wastage allowance is common for simple rectangular slabs; increase this to 15–20% for irregular shapes or where a lot of cutting around penetrations is needed. Use the wastage checkbox above to include 10% in your estimate automatically.
How to use this calculator
- Enter the slab length and width in metres. Use the overall slab dimensions — the calculator subtracts cover from each end to get the net bar lengths.
- Select the bar size. N12 at 200 mm is the most common starting point for residential slabs. Your structural engineer will specify the exact size.
- Select bar spacing. 200 mm centres (c/c) is common for residential slab-on-ground. 150 mm is used on reactive soils. For custom spacing, select "Custom" and enter the value.
- Enter lap length. Where bars join end-to-end, they must overlap. 300 mm is a conservative default for N12. Set to 0 if your slab dimensions are shorter than the bar stock length you're buying.
- Enter concrete cover. 40 mm is typical for ground-contact applications. 20 mm for interior protected locations.
- Click Calculate. Results show total lineal metres, weight in kg, and a breakdown by bar direction.
Worked example: A 6.0 m × 4.0 m slab with N12 bars at 200 mm c/c, 40 mm cover, 300 mm lap. Net slab dimensions: 5.92 m × 3.92 m. Bars along width (L-bars): floor(3.92 ÷ 0.2) + 1 = 21 bars, each 5.92 + 0.3 = 6.22 m. Bars along length (W-bars): floor(5.92 ÷ 0.2) + 1 = 31 bars, each 3.92 + 0.3 = 4.22 m. Total LM = (21 × 6.22) + (31 × 4.22) = 130.6 + 130.8 = 261.4 LM. Weight: 261.4 × 0.888 = 232 kg.
Understanding your results
The total lineal metres (LM) figure is what you need to order from your steel merchant. Rebar is priced and sold by the lineal metre or by weight (kg or tonne). Steel merchants in Australia typically stock 6 m, 9 m, and 12 m bar lengths — request the length closest to your bar run to minimise waste. For most residential slab work, 6 m bars suit spans up to about 5.7 m with one lap.
The weight figure is useful for estimating freight costs and for checking your structural engineer's specification — the engineer's drawings will typically show a reinforcement schedule in kg or tonnes per element.
Common mistakes: This calculator assumes a single-layer grid (one layer of bars each way). For two-layer reinforcement (top and bottom mesh), double the totals. Forgetting to specify deformed N-bars when ordering (not R-bars, which are plain round bars only suitable for ties and ligatures); and placing mesh on the ground without bar chairs — mesh on the ground provides no tensile reinforcing benefit in a positive-moment slab.
Rebar and reinforcing mesh in Australian concrete construction
Steel reinforcement in concrete works because concrete is strong in compression but weak in tension. When a slab bends under load, the bottom of the slab is in tension — and that is where reinforcing bars carry the tensile forces that would otherwise crack the concrete.
Bar types in Australia: The Australian market uses two main categories. N-bars (deformed bars with ribs) provide mechanical bond to concrete and are used for structural reinforcement in slabs, footings, columns, and beams. Common sizes are N12, N16, and N20 for residential work, scaling up to N32 for commercial. R-bars (plain round bars) are used for ligatures, ties, and stirrups — they rely on concrete cover rather than mechanical bond and are not suitable as primary slab reinforcement. Both are available from steel distributors including OneSteel (InfraBuild) and BlueScope Distribution.
Reinforcing mesh vs. loose bars: Mesh (e.g. SL72, SL82, SL92) is pre-welded bar in sheets and is faster to install than individual loose bars for flat slabs. Loose bars (cut and bent to order) are used for complex shapes, beams, columns, and where specific spacing or bar sizes are required by an engineer. For simple rectangular slabs, mesh is more economical and reduces on-site labour.
Cover requirements under AS 3600: Cover protects the steel from corrosion. The required cover depends on the exposure classification: 20 mm for interior protected elements (A1), 25–30 mm for sheltered exterior (A2), 40 mm for ground contact or near-coastal (B1), 50–65 mm for marine zones (B2/C). Insufficient cover is the leading cause of long-term concrete cracking and structural degradation in Australian buildings, particularly in coastal areas.
Lap splices: Where bars are joined (because the slab is longer than standard bar stock), they must overlap by a minimum distance as specified by AS 3600 — typically 25–40 times the bar diameter. For N12 this is 300–480 mm. Always lap in zones of lower stress if possible (away from midspan), and stagger laps in adjacent bars.
Important: This calculator is for quantity estimation only. All structural reinforcing design must be specified by a licensed structural engineer and comply with AS 3600.
Australian standards and references
- AS 3600:2018 — Concrete Structures: the primary Australian standard for reinforced and prestressed concrete design, including bar sizes, spacing, cover, and lap splice requirements.
- AS/NZS 4671:2019 — Steel Reinforcing Materials: specifies mechanical properties for N-bars and R-bars, including yield strength (500 MPa for N-bars) and ductility classifications.
- NCC Volume 1 (Building Code of Australia) — structural performance requirements for Class 2–9 buildings; Volume 2 for Class 1 residential buildings.
- AS 3008.1.1:2017 — Weldability of reinforcing steel (relevant when bars need to be welded rather than lapped).
- InfraBuild (formerly OneSteel) reinforcing bar data sheets — bar mass per metre (kg/m) for standard Australian N-bar and R-bar sizes.