Rebar spacing for a concrete slab is set by the designer together with the bar size, and it is written on the drawings as a callout such as 10M @ 400 EW (10M bars, 400 mm centre to centre, each way). On residential and light commercial slabs in Ontario it is common to see 10M or 15M bars somewhere in the 300 to 450 mm range, but the right number depends on slab thickness, loads, and whether the slab sits on the ground or spans between supports. Structural slabs designed to CSA A23.3 also have to stay inside minimum and maximum spacing limits.
How to read a slab spacing callout
Slab reinforcement on Canadian drawings follows a short, consistent notation. Once you can read it, you can check what is delivered and placed:
| Callout | Meaning |
|---|---|
| 10M @ 400 | 10M bars at 400 mm centre to centre |
| EW | Each way: the same bars run in both directions |
| T&B | Top and bottom: two separate mats, one near each face |
| EF | Each face: used for walls and thick elements |
| LW / SW | Long way / short way: bars running parallel to the long or short side |
| c/c or o/c | Centre to centre, the same as "@" |
Spacing is always measured centre to centre of the bars, not as the gap between them. A callout of "10M @ 300 T&B EW" means four layers of bar in total: two directions in the bottom mat and two in the top mat.
If a drawing shows different spacing in different zones (tighter over supports, around openings, or at thickened edges), those zones are usually dimensioned or shown with a range line. Read the general notes too: they often set the default slab reinforcement, and the plan shows only the exceptions.
The limits on spacing
Spacing is a design choice, but it is not unlimited in either direction.
Minimum spacing. Bars need enough room between them for concrete to flow and consolidate around each bar. CSA A23.1 sets the minimum clear distance between parallel bars as the largest of 1.4 times the bar diameter, 1.4 times the maximum coarse aggregate size, or 30 mm. In ordinary slabs with 10M or 15M bar this rarely governs, but it matters where bars bunch up at laps, around openings, or in heavily reinforced zones.
Maximum spacing. For structural slabs, CSA A23.3 limits how far apart bars can be, with the limit expressed as a multiple of the slab thickness and capped at 500 mm. The exact multiple depends on the type of slab and whether the bars are main flexural steel or shrinkage and temperature steel, so the designer applies it, not the site crew. A23.3 also sets a minimum amount of steel based on the slab's cross-sectional area, which is why a thicker slab needs more steel even when the loads are light.
Slabs on grade are a little different: they are supported continuously by the ground, and the reinforcement is often there mainly to hold cracks tight rather than to carry load across a span. Part 9 of the Ontario Building Code does not require reinforcement in ordinary floors on ground at all, so when a slab on grade is reinforced, the spacing comes from the designer's drawings. It still has to make sense with the slab thickness and the joint layout. The one prescriptive slab spacing in Part 9 is for small reinforced slabs over basement cold rooms (Section 9.40), which call for 10M bars at no more than 200 mm each way.
Spacing and slab thickness
Searches for "rebar spacing for a 4 inch slab" or "6 inch slab" are common, but thickness alone does not give you a spacing. What thickness does change is how much steel the slab needs and how the bars fit inside it:
| Nominal slab | Metric | What changes as the slab gets thicker |
|---|---|---|
| 4 in | about 100 mm | Usually a single mat if reinforced at all. Little room, so 10M is the usual bar and cover is tight. |
| 5 in | about 125 mm | Single mat. Common for garage and exterior slabs where the design calls for it. |
| 6 in | about 150 mm | Single mat for most slabs on grade; heavier loads may get 15M or closer spacing. |
| 8 in | about 200 mm | More steel needed for the same minimum ratio. Structural slabs at this thickness often get top and bottom mats. |
For a thin slab on grade, the bar position matters as much as the spacing. Industry guidance for slabs on ground (ACI 302.1R in the US is the commonly cited reference) puts crack control steel in the upper part of the slab, because shrinkage cracks start at the top surface. Your drawings set the exact position by specifying the concrete cover. Our concrete cover guide explains how cover is chosen.
Bar size and spacing work as a pair. 15M @ 600 has the same steel area per metre as 10M @ 300, and our 10M vs 15M comparison shows how to compare layouts that way.
Laying out a rebar grid
Once the spacing is known, the number of bars in each direction is straightforward:
- Take the slab dimension and subtract the edge distance at each end (the bars stop short of the forms to keep cover at the slab edge).
- Divide by the spacing and round up to a whole number of spaces.
- Add one bar. The number of bars is always one more than the number of spaces.
Worked example: a 30 ft by 40 ft slab (about 9.14 m by 12.19 m) with 10M @ 400 EW. For this example we assume bars stop 50 mm short of each edge. Your drawings set the real edge cover.
- Bars running the 12.19 m direction: (9,144 − 100) ÷ 400 = 22.6, round up to 23 spaces, so 24 bars, each about 12.09 m long. That is about 290 m of bar.
- Bars running the 9.14 m direction: (12,192 − 100) ÷ 400 = 30.2, round up to 31 spaces, so 32 bars, each about 9.04 m long. That is about 289 m of bar.
- Total: about 579 m of 10M. At 0.785 kg/m, that is roughly 455 kg before laps.
Bars that run longer than the stock length you are buying need lap splices, and each lap adds steel. Thickened edges, dowels into adjacent slabs or walls, and extra bars around openings add more. For a fuller method, see our rebar estimating guide.
Keeping the spacing right on site
A slab can be detailed perfectly and still end up with the wrong reinforcement if the mat moves during the pour. The common problems and their fixes:
- Bars on the ground. Rebar lying on the subgrade, or "pulled up" with a hook during the pour, rarely ends up at the right height. Support the mat on chairs or bolsters sized to give the cover on the drawings.
- Spacing drift. Mark the spacing on the forms or on the first bar of each direction and tie to the marks. Wider gaps creep in when bars are placed by eye.
- Mats sagging under foot traffic. Smaller bars need supports at closer centres. Plan walkways or use stiffer supports where crews and pump hoses will cross the mat.
- Missed extras. Openings, re-entrant corners and drains often need additional bars. Check the details before calling the mat ready for inspection.
Pre-cut bar delivered in bundles tagged to the layout helps a lot here, because the crew is not cutting and sorting stock on site. Our post on how to order cut and bend rebar explains how bar lists and tags work.
Getting the slab steel right
If your drawings show the slab reinforcement, we can turn them into a bar list with counts, lengths and laps worked out, then cut and bundle the bar to match. If all you have is a sketch, get the bar size, spacing and cover confirmed by the designer before ordering, particularly for garage, structural or commercial slabs.
Send your drawings for a quote, or see the rebar products we supply for slab work, from straight 10M and 15M bar to dowels and bent edge bars.
Frequently asked questions
How far apart should rebar be in a slab?
There is no single spacing for every slab. The engineer or designer sets bar size and spacing together from the slab's thickness, loads and support, and the drawings state it as something like 10M @ 400 EW. Structural slabs also have maximum spacing limits in CSA A23.3 that depend on slab thickness.
What does 10M @ 300 EW mean?
It means 10M bars spaced 300 mm centre to centre, running each way (both directions), forming a square grid. T&B or EF notes mean the same grid is repeated in a top and bottom layer, or on each face.
How much rebar do I need for a 30 by 40 slab?
It depends on the bar size and spacing on your drawings. As a worked example, a 30 ft by 40 ft slab with 10M @ 400 mm each way needs roughly 580 m of bar before laps, about 455 kg of steel. Your take-off should use your actual layout and add laps.
Should rebar sit on the ground under a slab?
No. Bar lying on the subgrade does nothing useful. It needs to be supported on chairs or bolsters at the height shown on the drawings so it ends up with the specified concrete cover.
