For a structural slab designed to CSA A23.3, principal reinforcement can be spaced no farther apart than three times the slab thickness, and never more than 500 mm. That rule appears for one-way slabs in Clause 7.4.1.2 and for two-way slabs in Clause 13.10.4. At the other end, CSA A23.1 sets a minimum clear gap between bars of 1.4 bar diameters, 1.4 times the coarse aggregate size, or 30 mm, whichever is largest. Everything between those two limits is the designer's call.
This post sets out the actual limits, element by element, so you can sanity-check a drawing or a placed mat. For how slab spacing callouts are written and laid out, see our slab rebar spacing guide.
Maximum spacing in slabs
The slab limit is simple to apply because it only depends on thickness:
Maximum spacing = the smaller of 3 × slab thickness and 500 mm
| Slab thickness | 3 × thickness | Maximum principal bar spacing |
|---|---|---|
| 100 mm (about 4 in) | 300 mm | 300 mm |
| 125 mm (about 5 in) | 375 mm | 375 mm |
| 150 mm (about 6 in) | 450 mm | 450 mm |
| 175 mm (about 7 in) | 525 mm | 500 mm |
| 200 mm (about 8 in) | 600 mm | 500 mm |
| 250 mm and thicker | 750 mm or more | 500 mm |
The 500 mm cap takes over at about 167 mm of slab thickness. Thicker slabs do not earn wider spacing.
A few points that catch people out:
- "Principal" reinforcement means the main flexural bars, the ones carrying bending. Shrinkage and temperature steel, which runs the other way in a one-way slab, has its own separate spacing rule in A23.3. The designer applies it, and it is shown on the drawings like any other bar.
- Two-way slabs have a tighter band near columns. In flat plates and flat slabs, part of the top steel over the columns is banded, and that band can be no wider apart than 1.5 times the slab thickness or 250 mm (Clause 13.10.4).
- The maximum is a ceiling, not a target. Most slabs are spaced closer than the limit because the amount of steel required, crack control or the bar size chosen pushes the spacing down.
Slabs on grade are a different case. Part 9 of the Ontario Building Code does not require reinforcement in ordinary floors on ground, as our rebar code requirements overview explains, so the spacing on a reinforced slab on grade comes from whoever designed it. The one prescriptive slab spacing in Part 9 is for small reinforced slabs over basement cold rooms: 10M bars at no more than 200 mm each way.
Limits for walls, beams, columns and stirrups
Slabs are only one part of A23.3. Each element type has its own spacing rule, and they are not interchangeable:
| Element | What controls spacing | Where it comes from |
|---|---|---|
| Walls | A maximum spacing for vertical and horizontal bars, plus rules on when two layers are needed | A23.3 Clause 14.1.8 |
| Beams (main bars) | Crack control: bar spacing and cover feed a crack control parameter, z, limited to 30,000 N/mm for interior exposure and 25,000 N/mm for exterior exposure | A23.3 Clause 10.6.1 |
| Deep beams | Skin reinforcement along the sides when the member is deeper than 750 mm | A23.3 Clause 10.6.2 |
| Beam stirrups | 0.7 dv or 600 mm, halved where shear or torsion is high | A23.3 Clause 11.3.8 |
| Column ties | The least of 16 longitudinal bar diameters, 48 tie diameters and the least column dimension | A23.3 Clause 7.6.5.2 |
For beams, there is no flat "maximum bar spacing" number in the way there is for slabs. The crack control check limits how far apart tension bars can be for a given bar stress and cover, which is why heavily loaded beams with large cover end up with more, smaller bars. Our post on stirrup spacing in beams and columns works through the stirrup and tie limits with examples, and walls are covered in our guide to concrete wall rebar spacing.
Minimum spacing: the other limit
Bars too close together are as much of a problem as bars too far apart. Concrete has to flow between them and fully surround each bar, or the bond that the whole design relies on is lost.
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 size of coarse aggregate, or
- 30 mm.
Here is what that works out to with a common 20 mm maximum aggregate (1.4 × 20 = 28 mm):
| Bar | Diameter | 1.4 × diameter | Governing minimum clear gap | Minimum centre to centre |
|---|---|---|---|---|
| 10M | 11.3 mm | 16 mm | 30 mm | about 41 mm |
| 15M | 16.0 mm | 22 mm | 30 mm | about 46 mm |
| 20M | 19.5 mm | 27 mm | 30 mm | about 50 mm |
| 25M | 25.2 mm | 35 mm | 35 mm | about 61 mm |
| 30M | 29.9 mm | 42 mm | 42 mm | about 72 mm |
| 35M | 35.7 mm | 50 mm | 50 mm | about 86 mm |
Two practical notes. First, the minimum is a clear distance, edge of bar to edge of bar, while spacing callouts on drawings are centre to centre. Add one bar diameter to the clear gap to compare them, as the last column does. Second, ribs make the real outside diameter of a deformed bar a little larger than the nominal size, so leave some margin where bars are tight.
Minimum spacing rarely governs in ordinary slabs. It matters at lap splices, where two bars sit side by side, around openings where extra bars are added, at column-beam joints, and in heavily reinforced transfer elements. That is also where detailers move to bundled bars, mechanical splices or a larger member size. Our lap splice chart explains why laps need the room.
Is there a standard rebar spacing?
People often search for "the standard spacing for rebar", hoping for one number. There is not one, because spacing is half of a pair. The designer first works out how much steel area each metre of slab or wall needs, then picks a bar size and spacing that deliver it:
| Layout | Steel area per metre |
|---|---|
| 10M @ 400 | 250 mm² |
| 10M @ 300 | 333 mm² |
| 15M @ 400 | 500 mm² |
| 10M @ 200 | 500 mm² |
| 15M @ 300 | 667 mm² |
| 15M @ 200 | 1,000 mm² |
15M @ 400 and 10M @ 200 carry the same amount of steel, but the 10M layout uses twice as many bars and controls cracking better. The 15M layout is faster to place. Which one appears on a drawing depends on the element, the crack control requirement and the designer's preference. Our 10M vs 15M comparison goes further into that trade-off.
What you can say is that the spacing on any engineered element sits between the A23.1 minimum and the A23.3 maximum for that element type, and that the drawings are where the number comes from.
Checking spacing on site
Spacing errors are among the most common items on a rebar inspection. A few checks before you call for the inspector:
- Measure centre to centre, from the middle of one bar to the middle of the next, and check several spots across the mat, not just one.
- Count bars per metre or per bay. If the spacing has crept wider in one area, the total bar count will be short even if each gap looks reasonable.
- Check zones, not just the general note. Tighter spacing over supports, around openings and at slab edges is easy to miss.
- Keep the mat at the right height. Bars pushed down or pulled up during the pour change both cover and effective spacing. Supports should hold them in place.
- Know the tolerance, and do not use it as a target. Placing tolerances exist for bar position and cover; our post on rebar placement tolerances covers them.
Shop-fabricated bar arrives cut to length, counted and tagged to the placing drawings, so the number of bars delivered for each area matches the spacing shown. That makes a spacing check on site much quicker, because a leftover bundle or a missing bar is obvious.
Getting the spacing right on your job
The limits above tell you the boundaries. The spacing that applies to your slab, wall or beam is on the structural drawings, and if a drawing seems to break one of these limits, raise it with the engineer before the bar is ordered.
We turn structural drawings into bar lists and placing drawings with the spacing, counts and laps worked out, then cut, bend and tag the bar to match. Send us your drawings for a free take-off and quote, or see the rebar products we supply, from straight 10M to 55M bar to fabricated stirrups and dowels.
Frequently asked questions
What is the maximum rebar spacing in a slab?
For structural slabs designed to CSA A23.3, principal bars can be no farther apart than three times the slab thickness, and never more than 500 mm. A 150 mm slab therefore tops out at 450 mm, and any slab 167 mm or thicker is capped at 500 mm.
What is the minimum spacing between rebar?
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. It is measured as the gap between bars, not centre to centre.
What is the standard spacing for rebar?
There is no single standard spacing. The engineer or designer picks bar size and spacing together for each element, inside the code's minimum and maximum limits. Common slab callouts in Ontario fall somewhere between 200 and 450 mm, but your drawings govern.
Does the 500 mm maximum apply to a slab on grade?
The A23.3 limits apply to slabs designed under that standard. Part 9 of the Ontario Building Code does not require reinforcement in ordinary floors on ground, so when a slab on grade is reinforced, the spacing comes from the designer, who will normally stay well inside those limits.
