The basic rebar spacing formula is: spacing = 1,000 × bar area ÷ steel area required per metre. With 10M bars (100 mm²) and a requirement of 400 mm² per metre, that gives 1,000 × 100 ÷ 400 = 250 mm. Two other formulas cover the rest of everyday spacing work: the number of bars at a given spacing (length minus edge distances, divided by spacing, rounded up, plus one), and the spacing of a fixed number of bars across a beam or footing. Whatever you calculate has to sit between the CSA A23.1 minimum clear spacing and the CSA A23.3 maximum for that element.
This post sets out each formula with worked metric examples. The designer sets the steel area and spacing on structural work; these calculations are for checking drawings, estimating and laying out.
The three rebar spacing formulas
| You know | You want | Formula |
|---|---|---|
| Steel area needed per metre (As, mm²/m) and bar size | Spacing | s = 1,000 × Ab ÷ As |
| Spacing and the width to fill | Number of bars | n = (L minus 2e) ÷ s, rounded up, plus 1 |
| Number of bars and the width available | Centre-to-centre spacing | s = (width available minus db) ÷ (n minus 1) |
Where Ab is the area of one bar, L the length or width being filled, e the edge distance to the first bar, n the number of bars and db the bar diameter.
Bar areas and diameters from CSA G30.18:
| Bar | Area (Ab) | Diameter (db) |
|---|---|---|
| 10M | 100 mm² | 11.3 mm |
| 15M | 200 mm² | 16.0 mm |
| 20M | 300 mm² | 19.5 mm |
| 25M | 500 mm² | 25.2 mm |
| 30M | 700 mm² | 29.9 mm |
| 35M | 1,000 mm² | 35.7 mm |
Spacing from the steel area you need
Slabs, walls and footings are designed in steel area per metre width. Once the designer has a required area, the bar size and spacing follow.
Example: minimum steel in a 200 mm slab. CSA A23.3 (Clause 7.8.1 in the 2004 edition) requires slabs to have at least 0.002 times the gross area in each direction.
- Required area: 0.002 × 200 mm × 1,000 mm = 400 mm² per metre
- With 10M: 1,000 × 100 ÷ 400 = 250 mm
- With 15M: 1,000 × 200 ÷ 400 = 500 mm
Both layouts carry the same steel. 10M @ 250 uses twice as many bars and controls cracking better; 15M @ 500 is quicker to place but sits right at the 500 mm cap. Our comparison of 10M vs 15M for slabs goes further into that choice.
Round down, not up. If the formula gives 222 mm, a spacing of 220 or 200 mm provides at least the required area; 250 mm does not. Drawings normally use round figures.
The reverse check. To see how much steel a callout provides: As = 1,000 × Ab ÷ s. So 15M @ 300 gives 1,000 × 200 ÷ 300 = 667 mm² per metre. Our rebar area and spacing table lists these values for common layouts.
Number of bars and actual spacing across a width
Once the spacing is known, the bar count follows. Bars stop short of the forms to keep edge cover, so subtract the edge distance at both ends first.
Example: a 4.5 m wide slab with 10M @ 300, bars 50 mm in from each edge.
- Width to fill: 4,500 minus 2 × 50 = 4,400 mm
- Spaces: 4,400 ÷ 300 = 14.7, round up to 15
- Bars: 15 + 1 = 16 bars
- Actual spacing: 4,400 ÷ 15 = 293 mm
Rounding up the number of spaces means the actual spacing is always equal to or a little less than the specified spacing, never more. That is the right direction: a callout of "10M @ 300" is a maximum.
Example: transverse bars in a strip footing. A 10 m long footing with 15M transverse bars @ 300 and 75 mm end cover:
- Length to fill: 10,000 minus 150 = 9,850 mm
- Spaces: 9,850 ÷ 300 = 32.8, round up to 33
- Bars: 34, at an actual spacing of 9,850 ÷ 33 = 298 mm
For converting bar counts into metres and kilograms, see our rebar estimating guide.
Bar spacing across a beam or column face
In beams and columns the question is usually the opposite: the number of bars is fixed by the design, and you need to know whether they fit.
The width available to bar centres is the member width minus the cover on each side, minus the stirrup or tie on each side. Then:
- Centre-to-centre spacing = (width available minus db) ÷ (n minus 1)
- Clear spacing = centre-to-centre spacing minus db
Example: bottom bars in a 300 mm wide beam, 40 mm cover to 10M stirrups, 20M bars, 20 mm maximum aggregate.
- Width available: 300 minus 2 × 40 minus 2 × 11.3 = 197.4 mm
- Four 20M bars: (197.4 minus 19.5) ÷ 3 = 59.3 mm centre to centre, so 39.8 mm clear
- Five 20M bars: (197.4 minus 19.5) ÷ 4 = 44.5 mm centre to centre, so 25.0 mm clear
The minimum clear spacing here is the largest of 1.4 × 19.5 = 27 mm, 1.4 × 20 = 28 mm and 30 mm, so 30 mm. Four bars fit; five do not. The designer would use two layers, a wider beam or larger bars. In practice the bend at each stirrup corner pushes the corner bars slightly inward, so leave some margin.
Check the answer against the code limits
A calculated spacing still has to respect the limits for the element. From CSA A23.3 (2004 edition clause numbers) and CSA A23.1:
| Limit | Value |
|---|---|
| Maximum spacing, principal bars in one-way slabs and walls | Smaller of 3 × thickness and 500 mm (A23.3 Cl. 7.4.1.2) |
| Maximum spacing, minimum slab reinforcement | Smaller of 5 × thickness and 500 mm (A23.3 Cl. 7.8.3) |
| Minimum clear spacing between parallel bars | Largest of 1.4 db, 1.4 × maximum aggregate size and 30 mm (A23.1) |
| Beams | Crack control check on bar spacing and cover rather than a single number (A23.3 Cl. 10.6.1) |
| Column ties and stirrups | Separate spacing rules based on bar diameters and member size |
For a 150 mm slab, the principal bar limit is 450 mm; for anything 167 mm or thicker, the 500 mm cap governs. Our post on maximum rebar spacing covers each limit, including the separate rules for ties and stirrups.
Slabs on grade in houses are a separate case: Part 9 of the Ontario Building Code does not require reinforcement in ordinary floors on ground, so any spacing there comes from the designer. See rebar spacing for concrete slabs.
Worked example: a two-way slab grid
Slab: 5.0 m by 7.0 m, 200 mm thick, minimum steel each way (400 mm² per metre), 10M bars, 50 mm edge distance.
- Spacing: 1,000 × 100 ÷ 400 = 250 mm. Below the 500 mm limits, so 10M @ 250 each way.
- Bars running the 7.0 m direction (spread across 5.0 m): (5,000 minus 100) ÷ 250 = 19.6, round up to 20 spaces, so 21 bars, each 6.9 m long. Actual spacing 245 mm.
- Bars running the 5.0 m direction (spread across 7.0 m): (7,000 minus 100) ÷ 250 = 27.6, round up to 28 spaces, so 29 bars, each 4.9 m long. Actual spacing 246 mm.
- Total: 21 × 6.9 + 29 × 4.9 = 144.9 + 142.1 = 287.0 m of 10M, or 287.0 × 0.785 = about 225 kg.
A "grid spacing calculator" is just this done twice, once in each direction. The two directions do not have to share the same spacing or bar size; drawings often show different steel each way, especially in one-way slabs.
From spacing to a bar list
Spacing formulas get you the count. A bar list adds laps, hooks, openings, edge bars and dowels, which is where most estimates come in light. We prepare take-offs and bar lists from your drawings, then cut, bend, tag and deliver 10M to 55M rebar across the GTA and Ontario. Send your drawings for a quote, or see our rebar products.
Frequently asked questions
What is the formula for rebar spacing?
When you know the steel area needed per metre, spacing (mm) = 1,000 × bar area (mm²) ÷ required area (mm² per metre). For example, 400 mm² per metre with 10M bars (100 mm²) gives 1,000 × 100 ÷ 400 = 250 mm.
How do you calculate how many bars fit at a given spacing?
Subtract the edge distance at both ends from the length, divide by the spacing, round up to a whole number of spaces and add one. The actual spacing is then that length divided by the number of spaces.
What is the maximum rebar spacing in a slab?
Under CSA A23.3, principal bars in one-way slabs and walls may be no further apart than three times the thickness or 500 mm, whichever is smaller. Minimum slab reinforcement may be no further apart than five times the thickness or 500 mm.
What is the minimum clear spacing between rebar?
CSA A23.1 sets the minimum clear distance between parallel bars as the largest of 1.4 bar diameters, 1.4 times the maximum coarse aggregate size, or 30 mm. Clear spacing is measured edge to edge, not centre to centre.
