What Size Rebar Do I Need? A Practical Selection Guide
Rebar size is one of those decisions that feels technical but mostly comes down to what you're building and how much load it will carry. For most residential DIY work, the range of sensible options is narrow. For anything structural, the decision belongs with an engineer — not a general guide.
How rebar is sized
Rebar is sized by its nominal diameter. The two systems in common use are metric (diameter in millimetres) and US Imperial (bar numbers, where the number refers to the diameter in eighths of an inch).
| US bar size | Nominal diameter | Metric equivalent |
|---|---|---|
| #3 | 9.5mm (⅜") | ~10mm |
| #4 | 12.7mm (½") | ~12mm |
| #5 | 15.9mm (⅝") | ~16mm |
| #6 | 19.1mm (¾") | ~20mm |
| #8 | 25.4mm (1") | ~25mm |
Typical sizes by application
The table below covers common residential and light commercial applications. These are general starting points, not engineered specifications.
| Application | Common bar size | Typical spacing |
|---|---|---|
| Residential slab (100–150mm) | 10–12mm (#3–#4) | 200–300mm |
| Driveway | 10–12mm (#3–#4) | 300mm |
| Footing / strip foundation | 12–16mm (#4–#5) | Per engineer spec |
| Fence post hole | 10mm (#3) | N/A |
| Structural column or beam | 16mm+ (#5+) | Engineer required |
Rebar spacing
Spacing controls how well the reinforcement distributes load and controls cracking. For residential slabs, 200–300mm centre-to-centre spacing is standard. ACI 318 sets a maximum spacing for temperature and shrinkage reinforcement at the smaller of three times the slab thickness or 450mm — so for a 150mm slab, the maximum would be 450mm, but 200–300mm is more typical in practice.
For anything beyond a simple residential slab, spacing should be specified by a structural engineer based on loads, soil conditions, and the concrete grade being used.
When to involve an engineer
General guidance covers non-structural slabs, driveways, and light footings. For foundations, retaining walls, structural beams, columns, or any element where failure would pose a safety risk, rebar size and spacing must be designed by a licensed structural engineer. Local building codes typically require this for permitted work.
Putting it together
Once you know the bar size and spacing you need, our rebar calculator can work out the total weight by diameter and length — useful for estimating material costs and ordering quantities.
FAQ
What size rebar should I use for a concrete slab?
For a standard residential slab (100–150mm thick), 10mm or 12mm rebar at 200–300mm spacing is commonly used. Thicker slabs or those carrying heavier loads may require larger bar sizes. For any structural slab, consult a structural engineer for the correct specification.
What is the difference between #4 and 12mm rebar?
These are measurements from two different systems. #4 rebar is a US Imperial designation referring to a bar with a nominal diameter of 12.7mm (roughly half an inch). The metric 12mm bar has a nominal diameter of 12mm. They are close in size but not identical and are not directly interchangeable in engineered designs.
How far apart should rebar be spaced?
For residential concrete slabs, 200–300mm spacing is typical. ACI 318 specifies that the maximum spacing for temperature and shrinkage reinforcement should not exceed 3 times the slab thickness or 450mm, whichever is smaller. Structural applications require engineer-specified spacing.
Do I need rebar in a concrete driveway?
Rebar is not always required in a residential driveway, but it significantly reduces the risk of cracking under load and improves long-term durability. Wire mesh is a common alternative for thin slabs, while rebar is preferred for driveways that will carry heavy vehicles or are in areas with expansive soils.