Rebar Calculator (Metric)
Enter your slab size in metres and grid spacing in centimetres to get the number of rebar bars each way and the total linear metres.
What your result means
Total rebar is what you order, usually in 6 m bars, so the bar count rounds that up for you. The two bar counts are how many run each direction in the grid. This is for a standard residential mat; lap any joints about 40 bar diameters and set the bars on chairs so they sit mid-slab.
How to use this calculator
- Measure slab dimensions in metres.
- Standard residential rebar spacing is 30 cm OC (on centre).
- Heavier loads use 20 cm OC.
- Read the number of bars in each direction plus total linear metres.
- Order standard 6 m rebar lengths from your supplier.
The formula
Each direction gets a bar every spacing interval across the opposite dimension, plus one (spacing in centimetres, slab in metres). Multiply each bar count by the length it spans and add the two for total linear metres.
Worked example
A 6 by 6 m slab on a 30 cm grid is 21 bars one way and 21 the other, 42 bars total, for 252 m. At 6 m a bar, that's 42 bars.
Rebar size and spacing
| Use | Bar size | Spacing |
|---|---|---|
| Residential slab | 12 mm | 30 cm OC |
| Driveway | 16 mm | 30 cm OC |
| Non-load-bearing | 12 mm | 40 cm OC |
Tips & gotchas
- Standard residential bar is 12 mm diameter.
- 30 cm on-centre spacing is standard for slabs up to about 15 cm thick.
- 40 cm on-centre is fine for a non-load-bearing slab.
- Tie the grid at every other intersection, minimum.
- Set the bars up on chairs so they end up in the middle of the slab, not on the ground.
Frequently asked questions
How much rebar do I need for a slab?
A standard residential mat is a 30 cm grid of 12 mm bar. Enter your slab and spacing above for the exact count.
What size rebar should I use?
12 mm for residential, 16 mm for a driveway, and 20 mm or bigger for structural work.
Do I need rebar in a path?
It's recommended but not always required. Welded wire mesh is an acceptable alternative.
How do I cut rebar?
A rebar cutter, bolt cutters, or an angle grinder all work.
How much should I overlap rebar at a joint?
At least 40 times the bar diameter, so about 50 cm for 12 mm bar.
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