Rebar Spacing Calculator for a 4 Inch Slab
Find how many 10M or 15M bars a slab needs, spaced the Canadian way, in feet or metres.
Measure your job
We keep bars 75 mm (3 in) in from each edge. A formed edge needs 50 mm, so this is on the safe side. Laps: 450 mm for 10M and 650 mm for 15M, the code's numbers for ICF walls. Check your drawings.
Your answer
10M rebar. A grid every 18 in, both ways.
We cut short pieces from whole bars. We don't join cut-off ends again, so you may have a few spare ends.
Or use wire mesh
The usual light mesh in Canada is 6×6 10/10 (152×152 MW9.1×MW9.1), at 1.02 kg per m². For 37.2 m² (400 sq ft), that is about 38 kg of mesh.
This answer is a guide. For a slab under trucks or heavy loads, check your local building rules or ask a builder or engineer.
What it costs
Middle price per bar at Canadian building-supply stores (Oct 2026), before tax. 10M: 5 stores for 20 ft, only 2 for 10 ft. 15M: only 3.
Quick answers
- Does a 4 in slab need rebar?
- Not for a home driveway or garage floor, says Concrete Manitoba.
- Rebar spacing for a 4 in slab
- If you use it: 45–60 cm (18–24 in) both ways, says Concrete Alberta.
- What size bar?
- Concrete Alberta doesn't say. With 60 mm on top, a 10M leaves about 29 mm of concrete under it.
- 20 × 20 ft slab
- 28 bars of 10M, 6 m long, at 18 in (45 cm). About $245 before tax.
How to work it out
- Take the slab's length and width in feet. Keep bars 3 in (75 mm) in from each edge.
- Bars along the length: (width in inches − 6) ÷ 18. Drop the part, then add 1.
- Do the same the other way.
- A run longer than one 6 m bar needs a second bar, lapped 450 mm.
- Add both ways together. Tie every second place where bars cross.
20 ft = 240 in. (240 − 6) ÷ 18 = 13, so 14 bars each way
Each bar is 19.5 ft, so it fits in one 6 m (19 ft 8 in) bar
14 + 14 = 28 bars
196 crossings ÷ 2 = 98 ties
Does a 4 inch driveway or garage slab need rebar?
Canadian concrete groups use 100 mm (4 in) for a car driveway. Concrete Manitoba says steel is not required for home driveways and garage floors.
Steel does not stop cracks. Concrete Manitoba says it can mean fewer control joints, or joints spaced wider.
The building code (Part 9) has no rebar rule for slabs. Its slab minimum is 75 mm.
Joints do the work
Without steel, cut control joints both ways. Concrete Manitoba and Alberta say no more than 2.5 m (8.2 ft) apart in a 100 mm slab.
Cut them ¼ of the slab deep. For trucks or anything heavy, check your local building rules or ask a builder or engineer.
Rebar spacing for a 4 inch slab
If you do add bars, Concrete Alberta gives the only Canadian spacing we found. Space them 45–60 cm both ways. Tie every second crossing.
Bars need 60 mm of concrete on top where road salt is used. In a 100 mm slab, that leaves about 29 mm under a 10M bar, and 24 mm under a 15M. That is our sum, not a rule.
| Slab | Bars at 18 in (45 cm) | Bars at 24 in (60 cm) | Ties at 18 in | Bar cost at 18 in |
|---|---|---|---|---|
| 10 × 10 ft | 8 | 6 | 25 | $70 |
| 12 × 20 ft | 22 | 16 | 56 | $193 |
| 20 × 20 ft | 28 | 20 | 98 | $245 |
| 24 × 24 ft | 44 | 32 | 128 | $385 |
| 30 × 40 ft | 98 | 73 | 270 | $858 |
Bars 3 in from each edge, laps of 450 mm. Cost: $8.75 a bar, the middle of 5 stores, before tax.
Welded wire mesh in Canada
Concrete Alberta says wire mesh or fibres may be used in place of bars. The usual light mesh is 6×6 10/10, also called 152×152 MW9.1×MW9.1.
It weighs 1.02 kg per m². The calculator shows the kg for your slab.
| Mesh (mostly 6 × 6 in grid) | Covers | Price (sellers) | Per sq ft |
|---|---|---|---|
| 4 × 8 ft sheet, 10 gauge | 32 sq ft | $7.93 (2) | $0.25 |
| 6 × 200 ft roll, 10 gauge | 1,200 sq ft | $219.95–$289.95 (2) | $0.18–$0.24 |
| 8 × 20 ft sheet, 6 gauge | 160 sq ft | $101.80 (3) | $0.64 |
Ontario only, with 2 or 3 sellers each, so rough. We found no 10 gauge 8 × 20 ft price.
Ontario big-box stores sell 9 gauge mesh for about $0.83 per sq ft (only 2 stores). Sheets overlap where they meet. We found no Canadian rule for how much.
Questions people ask
Is rebar needed for a concrete slab?
Not all slabs need it. Concrete Manitoba says home driveways and garage floors don't. Part 9 of the building code doesn't ask for it.
What is the rebar spacing for a 4 inch slab?
Concrete Alberta says 45–60 cm (18–24 in) both ways, if you use bars. Tie every second crossing.
What size rebar do I need for a 4 inch slab?
Concrete Alberta doesn't say. With 60 mm on top, a 10M bar leaves about 29 mm under it. A 15M leaves about 24 mm. That is our sum, not a rule.
How much rebar for a 30 × 40 ft slab?
About 98 bars of 10M, 6 m long, at 18 in (45 cm) both ways. That is about $858 before tax.
Does rebar stop concrete cracking?
No. Concrete Manitoba says steel does not stop cracks. It can let you use fewer control joints.
How deep should rebar sit in a slab?
With 60 mm of concrete over it where road salt is used, says Concrete Alberta. In a 100 mm slab, that is about ⅔ of the way down.
Can I use wire mesh instead of rebar?
Yes, Concrete Alberta says mesh or fibres can be used. The usual mesh is 6×6 10/10, at 1.02 kg per m².
Where our numbers come from
- Driveways and garage slabs: Concrete Manitoba (thickness, base, steel)
- Driveways, joints and slope: Concrete Alberta flatwork guide
- Concrete strength and site mixes: BC Building Code 2018, Section 9.3 (Part 9)
- Floor slabs: BC Building Code 2018, Section 9.16 (at least 75 mm)
- Rebar sizes and mesh: OCCDC Reinforced Concrete Reference Guide (2021)
- Rebar cover: City of Nanaimo engineering standards, Section 11
- Cover and air: Canadian Farm Builders Association concrete guide
- Rebar prices: Canadian building-supply stores (Oct 2026)
- Wire mesh prices: Ontario stores (Oct 2026; few prices)
- Store prices: Ontario Home Depot, RONA, Home Hardware and Chamberlain Timber Mart (Oct 2026)
Made by BuildersCount. Last checked 4 Oct 2026.
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