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.