Steel area per metre width = area of one bar × 1,000 ÷ bar spacing in mm. So 10M at 300 mm gives 333 mm² per metre, 15M at 300 mm gives 667 mm² per metre, and 15M at 200 mm gives 1,000 mm² per metre. The table below lists the area for 10M to 35M bars at the spacings that show up most on Canadian slab and wall drawings.
Area per metre width table
Values are in mm² of steel per metre width of slab or wall, using the nominal bar areas from CSA G30.18 (10M = 100 mm², 15M = 200, 20M = 300, 25M = 500, 30M = 700, 35M = 1,000). Figures are rounded to the nearest mm².
| Size | 100 mm | 125 mm | 150 mm | 200 mm | 250 mm | 300 mm | 350 mm | 400 mm | 450 mm | 500 mm |
|---|---|---|---|---|---|---|---|---|---|---|
| 10M | 1,000 | 800 | 667 | 500 | 400 | 333 | 286 | 250 | 222 | 200 |
| 15M | 2,000 | 1,600 | 1,333 | 1,000 | 800 | 667 | 571 | 500 | 444 | 400 |
| 20M | 3,000 | 2,400 | 2,000 | 1,500 | 1,200 | 1,000 | 857 | 750 | 667 | 600 |
| 25M | 5,000 | 4,000 | 3,333 | 2,500 | 2,000 | 1,667 | 1,429 | 1,250 | 1,111 | 1,000 |
| 30M | 7,000 | 5,600 | 4,667 | 3,500 | 2,800 | 2,333 | 2,000 | 1,750 | 1,556 | 1,400 |
| 35M | 10,000 | 8,000 | 6,667 | 5,000 | 4,000 | 3,333 | 2,857 | 2,500 | 2,222 | 2,000 |
45M and 55M bars are rarely spaced like slab steel, so they are left off. For any size or spacing not shown, use the formula above. If you need the area of a counted group of bars instead, such as 4-20M in a beam, see our rebar area chart.
A callout like 15M @ 300 EW means 15M bars at 300 mm centres in each direction, so the slab has 667 mm² per metre running each way. T&B or EF (top and bottom, each face) means the area applies to each layer separately. Our guide to rebar spacing for concrete slabs explains these callouts in more detail.
Reading the table both ways
The table works in two directions.
From the drawing to the area. Find the bar size in the left column and the spacing across the top. 20M @ 250 is 1,200 mm² per metre. This is useful when you are checking a detail against a design note or comparing two options.
From a required area to bar and spacing. Find the row for the bar you want to use and move along until the value is at least the required area. If the design calls for 900 mm² per metre:
- 15M works at 200 mm (1,000 mm²/m).
- 20M works at 300 mm (1,000 mm²/m).
- 25M at 500 mm also gives 1,000 mm²/m, but a spacing that wide may be ruled out by the maximum spacing for the member.
When the spacing falls between columns, use the formula and round down, never up. For 900 mm²/m with 15M bars: 200 × 1,000 ÷ 900 = 222 mm, so the practical spacing is 200 mm. Rounding up to 250 mm would leave the slab with 800 mm²/m, short of what was asked for. Our rebar spacing calculator guide walks through more of these.
Equivalent bar and spacing pairs
Because Canadian bar areas are round numbers, some combinations give identical steel per metre:
| Area (mm²/m) | Equivalent arrangements |
|---|---|
| 333 | 10M @ 300 |
| 400 | 10M @ 250, 15M @ 500 |
| 500 | 10M @ 200, 15M @ 400 |
| 667 | 10M @ 150, 15M @ 300, 20M @ 450 |
| 1,000 | 10M @ 100, 15M @ 200, 20M @ 300, 25M @ 500 |
| 1,333 | 15M @ 150, 20M @ 225 |
| 2,000 | 15M @ 100, 20M @ 150, 25M @ 250 |
Equal area does not mean equal performance. Smaller bars at closer spacing control crack widths better and keep each bar's development and lap lengths short. Larger bars at wider spacing are faster to place but run into the maximum spacing limits sooner. Any swap on structural work needs the designer's approval.
Spacing limits that cap the table
Not every cell in the table is usable for every member. CSA A23.3 puts upper limits on bar spacing, and in practice they rule out the right-hand columns for thinner slabs and walls.
In the 2004 edition of CSA A23.3, which we used to check these values:
- Principal reinforcement in walls and one-way slabs may be spaced no farther apart than the smaller of three times the thickness or 500 mm (Clause 7.4.1.2).
- Minimum reinforcement in slabs may be spaced no farther apart than the smaller of five times the slab thickness or 500 mm (Clause 7.8.3).
So a 125 mm slab cannot carry its principal steel at 400 mm, even if the area would be enough. Our post on maximum rebar spacing covers these limits and the tighter ones for beams, columns and two-way slab bands. Your engineer works to the current edition of the standard, so confirm the governing limits on the drawings.
Worked example: minimum steel in a structural slab
CSA A23.3 (Clause 7.8.1 in the 2004 edition) requires a minimum reinforcement area of 0.002 times the gross concrete area in each direction of a structural slab. Here is how the table turns that into bars.
A 200 mm thick slab, per metre width:
- Gross area of a 1 m strip: 200 mm × 1,000 mm = 200,000 mm².
- Minimum steel: 0.002 × 200,000 = 400 mm² per metre, in each direction.
- From the table, 400 mm²/m is 10M @ 250 or 15M @ 500.
- Check spacing for minimum reinforcement: the smaller of 5 × 200 = 1,000 mm and 500 mm, so 500 mm. Both options pass, with 15M @ 500 sitting right at the limit.
A 150 mm thick slab:
- Gross area: 150 × 1,000 = 150,000 mm².
- Minimum steel: 0.002 × 150,000 = 300 mm² per metre.
- 10M @ 300 gives 333 mm²/m, which meets it. 10M @ 350 gives 286 mm²/m, which does not.
This is a check on the minimum only. The steel a suspended slab actually needs comes from the structural design, and it is often well above the minimum. Slabs on grade are a separate case: Part 9 of the Ontario Building Code does not require reinforcement in ordinary floors on ground, so any steel there is set by the designer. See slab-on-grade rebar requirements for that side.
Walls, footings and imperial details
The same table works for walls. Vertical and horizontal bars are each given as area per metre of wall, and if the wall has two curtains of steel the callout tells you whether the spacing applies to each face or to the total.
For footings, steel is usually specified as a count of bars across the footing width (for example 4-15M continuous), not as a spacing, so the bar count table in our area chart is the one to use.
Working from a US detail? US drawings give steel area in square inches per foot of width. To convert, multiply in²/ft by about 2,117 to get mm²/m, or divide mm²/m by 2,117 to go the other way. Converting the area tells you how much steel the detail wants, but the bar size and spacing should still be confirmed by the engineer, because US and Canadian bar sizes do not line up exactly.
Turning spacing into a bar list
Spacing on a drawing is only half the story for an order. The bar list needs the number of bars across each pour, their lengths, laps and any bent ends, which is where most estimating errors creep in. The bar count for a run is the length divided by the spacing, plus one, and edge bars, openings and laps add to it.
If you have slab or wall drawings, send them over for a free take-off and quote. We detail, cut, bend and tag rebar from 10M to 55M and deliver across the GTA and Ontario. See our rebar products for what we supply.
Frequently asked questions
How do you calculate rebar area per metre?
Multiply the area of one bar in mm² by 1,000 and divide by the spacing in mm. For 15M at 300 mm: 200 × 1,000 ÷ 300 = about 667 mm² per metre width.
Is 10M at 200 mm the same as 15M at 400 mm?
Both give 500 mm² of steel per metre, so the area is the same. They are not automatically interchangeable, because spacing limits, crack control and bar development also matter. The designer has to approve the change.
What is the steel area of 15M at 400 mm spacing?
15M at 400 mm gives 500 mm² per metre width (200 × 1,000 ÷ 400).
How do I work out spacing from a required steel area?
Divide 1,000 times the bar area by the required area per metre, then round the result down to a practical spacing. For 600 mm²/m with 15M bars: 200,000 ÷ 600 = 333 mm, so 300 mm spacing.
