For most reinforced concrete slabs in Canada, the bar is 10M or 15M. Thin residential slabs on grade (100 to 125 mm, about 4 to 5 inches) that get rebar usually use 10M, while thicker or more heavily loaded slabs, such as shop floors, structural slabs and slabs carrying point loads, often move up to 15M. The size on your job should come from the drawings, because bar size only makes sense together with spacing, cover and the load the slab has to carry.
10M vs 15M at a glance
Canadian bar sizes are set by CSA G30.18. The two sizes you will see on almost every slab drawing compare like this:
| Property | 10M | 15M |
|---|---|---|
| Nominal diameter | 11.3 mm (0.445 in) | 16.0 mm (0.630 in) |
| Cross-sectional area | 100 mm² | 200 mm² |
| Weight | 0.785 kg/m | 1.570 kg/m |
| Weight of a 6 m bar | about 4.7 kg | about 9.4 kg |
| Closest US size | between #3 and #4 | #5 |
The simple relationship to remember: one 15M bar has the same area of steel as two 10M bars, and weighs the same as two 10M bars. So a mat of 15M at a given spacing carries the same steel as 10M at half that spacing. For the full size range up to 55M, see our Canadian rebar size chart.
Translating #3, #4 and #5 into metric
A lot of slab advice online is American and talks about #3, #4 and #5 bar. US bar numbers are the diameter in eighths of an inch, and they do not line up neatly with Canadian sizes:
| US size | US diameter | Nearest Canadian bar | Notes |
|---|---|---|---|
| #3 | 9.5 mm (3/8 in) | 10M is larger | #3 has about 71 mm² of area vs 100 mm² for 10M |
| #4 | 12.7 mm (1/2 in) | 10M is smaller | #4 has about 129 mm² vs 100 mm² for 10M |
| #5 | 15.9 mm (5/8 in) | 15M | Close match in diameter and area |
So when a US article says "use #4 at 18 inches", there is no direct Canadian equivalent. Swapping in 10M at the same spacing gives you less steel; swapping in 15M gives you more. If your drawings were prepared to US sizes, or you are pricing a job from an American detail, the designer needs to restate the reinforcement in metric bars. Metric bars are what Canadian mills and fabricators stock.
What actually decides the bar size
Nobody picks a slab bar size on its own. The engineer or designer works out how much steel the slab needs per metre of width, then chooses a bar size and spacing that delivers it and fits the slab. The main inputs are:
- What the slab is for. A basement floor that only resists shrinkage cracking needs far less steel than a garage slab under vehicle loads, a warehouse floor under racking, or a suspended slab spanning between beams.
- Slab thickness. The bar has to fit inside the slab with the required concrete cover around it. In a 100 mm slab, a 10M bar placed near mid-depth leaves reasonable cover top and bottom. A 15M bar in the same slab eats more of the depth.
- Soil and support. Slabs on grade lean on the ground below. Poor or variable subgrade, or a slab bridging over fill, pushes the design toward more steel or a thicker slab.
- Crack control goals. Closer spacing of smaller bars generally controls crack widths better than wide spacing of big bars carrying the same total area.
- Exposure. Garage floors and exterior slabs see road salt and freeze-thaw cycles. That affects concrete quality and cover more than bar size, but it all gets designed together. Our guide to concrete cover for rebar covers that side.
Do not assume every slab needs rebar, either. Part 9 of the 2024 Ontario Building Code sets a minimum thickness for floors on ground (75 mm) but does not require reinforcement in them, so many residential basement floors are built plain or with welded wire mesh. Floors that carry loads from the structure above, suspended slabs and most commercial slabs are engineered, and the reinforcement comes from that design. Our overview of rebar code requirements in Ontario covers the Part 9 rules. If the drawings call for rebar, they will give a size, a spacing and usually a note like "10M @ 400 EW" (each way).
Comparing layouts by steel per metre
When you are weighing two options, or checking a substitution request, compare the area of steel per metre of slab width, not just the bar size. The arithmetic is simple: bar area × 1000 ÷ spacing in mm.
| Layout | Steel area per metre of width |
|---|---|
| 10M @ 450 mm | about 222 mm²/m |
| 10M @ 400 mm | 250 mm²/m |
| 10M @ 300 mm | about 333 mm²/m |
| 15M @ 600 mm | about 333 mm²/m |
| 15M @ 400 mm | 500 mm²/m |
| 15M @ 300 mm | about 667 mm²/m |
The table is arithmetic, not a recommendation. It shows why "10M @ 300" and "15M @ 600" are equal on paper, and why the engineer might still prefer one over the other: the closer layout with smaller bars controls cracking better and is easier to keep in position, while the wider layout means fewer bars to place and tie. Spacing also has maximum limits that depend on slab thickness, which we cover in rebar spacing for concrete slabs.
Practical differences on site
Beyond the numbers, the two sizes behave differently for the crew:
- Handling. A 6 m length of 10M weighs about 4.7 kg, so one person can carry several. 15M is twice the weight per bar.
- Staying in position. 15M is stiffer and sags less between chairs, so it holds its height better when people walk the mat during the pour. 10M mats need chairs or supports at closer centres to stay at the right depth.
- Bending. Both bend easily in a shop. Field bending 15M by hand is hard work and easy to get wrong, which is one reason contractors order bent corner bars and dowels pre-fabricated.
- Laps. Where bars run longer than one stock length, they are lapped. Lap lengths grow with bar diameter, so a 15M lap uses more steel than a 10M lap. The lap length itself comes from the drawings.
A quick weight comparison
Rebar is ordered and shipped by weight, so it helps to see how the choice affects tonnage. Take a 6 m by 6 m slab and two layouts that give the same steel area per metre:
- 10M @ 300 mm each way: about 21 bars per direction × 6 m × 2 directions = 252 m of bar, × 0.785 kg/m = roughly 198 kg.
- 15M @ 600 mm each way: about 11 bars per direction × 6 m × 2 directions = 132 m of bar, × 1.570 kg/m = roughly 207 kg.
The weights are close, as you would expect from equal steel area, but the 10M layout has almost twice as many bars to place and tie. Real take-offs also account for cover at the slab edges, laps, and any bent bars at edges or thickened sections. Our rebar cost for concrete slabs guide explains how those quantities turn into a quote.
Getting the right bar for your slab
If your drawings already show the bar size and spacing, the job is just to supply it accurately: straight bar cut to length, plus any bent bars, dowels or corner bars the slab needs. If you have US-sized details or only a rough sketch, get the reinforcement confirmed by the designer or engineer before ordering, especially for garage, structural or commercial slabs.
Send us your drawings and we will prepare a take-off and quote for the slab steel. You can also see the rebar products we supply, from straight 10M and 15M bar to fabricated dowels, stirrups and bent shapes.
Frequently asked questions
What size rebar is used for a 4 inch slab?
A 4 inch (100 mm) slab on grade that is reinforced usually gets 10M bar, because a 10M bar leaves room for cover in a thin slab. Many plain residential slabs on grade are not reinforced at all, so check what your drawings or permit call for.
Is 10M rebar the same as #4 rebar?
No. 10M is 11.3 mm in diameter with an area of 100 mm². A US #4 bar is 12.7 mm with about 129 mm². A US #3 bar is smaller than 10M. Substitutions need the designer's approval.
Is 15M the same as #5 rebar?
They are very close. 15M is 16.0 mm and 200 mm²; #5 is 15.9 mm and about 200 mm². They are still made to different standards, so follow the system the drawings use.
Is bigger rebar always better in a slab?
No. A larger bar needs more room for cover, longer laps and more weight per metre. What matters is the steel area per metre and the bar position, both of which come from the design.
