A 30x40 foot slab reinforced with 10M bars at 400 mm each way takes 24 bars running the 40 foot direction and 32 bars running the 30 foot direction: about 586 m of bar weighing roughly 460 kg, including laps. With 15M at the same spacing, the counts are the same but the weight roughly doubles to about 906 kg. Tighten the spacing to 300 mm and the 10M figure climbs to about 764 m and 599 kg.
Those numbers depend on four choices that come from your drawings, not from this post: the bar size, the spacing, the edge cover and the lap length. Below is the 30x40 worked through step by step, then a table for six other common slab sizes so you can see how the quantities scale. For the general method on any shape, see how much rebar you need for a concrete slab.
Assumptions behind every number
All figures in this post use the same assumptions so they can be compared:
- One layer of bars, each way (a single mat). A slab with top and bottom mats needs double.
- 75 mm edge allowance at every edge, so bars stop 75 mm short of each side. Your drawings set the actual cover.
- Bar count = distance the bars are spread over, divided by the spacing, rounded up, plus one.
- Bar weights: 10M at 0.785 kg/m, 15M at 1.570 kg/m (CSA G30.18 nominal values).
- Stock lengths: 12 m for 10M and 18 m for 15M, the standard mill lengths listed by the Reinforcing Steel Institute of Canada.
- Laps where a bar is longer than stock: 400 mm for 10M and 550 mm for 15M, close to CSA A23.3 Class B laps for uncoated Grade 400 bar in 30 MPa concrete. Use the lap on your drawings.
Slab dimensions are converted from feet at 0.3048 m per foot, so a 30x40 slab is 9.14 m by 12.19 m.
Worked example: a 30x40 slab in 10M at 400 mm
Bars running the 40 foot (12.19 m) direction
- Length: 12.19 m minus 2 × 0.075 m = 12.04 m
- Spread across: 9.14 m minus 0.15 m = 8.99 m
- Spaces: 8.99 ÷ 0.40 = 22.5, round up to 23, so 24 bars
- Laps: 12.04 m is longer than a 12 m stock 10M bar, so each line is two pieces with a 400 mm lap, 12.44 m of bar per line
- Total: 24 × 12.44 = 298.6 m
Bars running the 30 foot (9.14 m) direction
- Length: 9.14 m minus 0.15 m = 8.99 m
- Spread across: 12.04 m
- Spaces: 12.04 ÷ 0.40 = 30.1, round up to 31, so 32 bars
- Total: 32 × 8.99 = 287.8 m
Total: 586.4 m × 0.785 kg/m = about 460 kg of 10M, or roughly 4.1 kg per square metre of slab.
Notice the lap. A 40 foot slab sits just over the length of a 12 m 10M bar, so every long bar becomes two pieces. That adds 24 laps and about 10 m of bar, and it is a common reason a calculator total comes in light.
The same slab in 15M and at 300 mm
15M at 400 mm each way. The bar counts are the same (24 and 32), but 15M comes in 18 m lengths, so the 12.04 m bars need no lap. Total length is about 576.8 m, and at 1.570 kg/m that is about 906 kg.
10M at 300 mm each way. Closer spacing adds bars in both directions: 31 bars running the long way and 42 running the short way. With laps on the long bars, that is about 763.5 m and about 599 kg.
15M at 300 mm each way. Same 31 and 42 bars, no laps, about 751 m and about 1,179 kg.
Which of these your slab needs is a design question. Our comparison of 10M vs 15M for slabs explains how designers choose, and many slabs on grade in Ontario are not required by the code to have rebar at all. See slab-on-grade rebar requirements for what the Ontario Building Code actually says.
Rebar quantities for common slab sizes
The same method applied to other common sizes, all at 400 mm each way. "Long bars" run the long dimension; "short bars" run the short dimension.
| Slab (feet) | Slab (metres) | Long bars | Short bars | 10M @ 400 | 15M @ 400 |
|---|---|---|---|---|---|
| 10x10 | 3.05 x 3.05 | 9 at 2.90 m | 9 at 2.90 m | 52 m, 41 kg | 52 m, 82 kg |
| 10x12 | 3.05 x 3.66 | 9 at 3.51 m | 10 at 2.90 m | 61 m, 48 kg | 61 m, 95 kg |
| 24x24 | 7.32 x 7.32 | 19 at 7.17 m | 19 at 7.17 m | 272 m, 214 kg | 272 m, 427 kg |
| 30x30 | 9.14 x 9.14 | 24 at 8.99 m | 24 at 8.99 m | 432 m, 339 kg | 432 m, 678 kg |
| 20x40 | 6.10 x 12.19 | 16 at 12.04 m | 32 at 5.95 m | 389 m, 306 kg (laps) | 383 m, 601 kg |
| 30x40 | 9.14 x 12.19 | 24 at 12.04 m | 32 at 8.99 m | 586 m, 460 kg (laps) | 577 m, 906 kg |
| 40x60 | 12.19 x 18.29 | 32 at 18.14 m | 47 at 12.04 m | 1,178 m, 925 kg (laps) | 1,164 m, 1,827 kg (laps) |
And with 10M at 300 mm each way:
| Slab (feet) | Long bars | Short bars | Total | Weight |
|---|---|---|---|---|
| 10x10 | 11 | 11 | 64 m | 50 kg |
| 10x12 | 11 | 13 | 76 m | 60 kg |
| 24x24 | 25 | 25 | 358 m | 281 kg |
| 30x30 | 31 | 31 | 558 m | 438 kg |
| 20x40 | 21 | 42 | 511 m (laps) | 401 kg |
| 30x40 | 31 | 42 | 764 m (laps) | 599 kg |
| 40x60 | 42 | 62 | 1,550 m (laps) | 1,217 kg |
A few patterns stand out:
- Weight per square metre stays nearly constant for a given layout: about 4 kg/m² for 10M at 400 mm, 5.3 kg/m² for 10M at 300 mm and 8 kg/m² for 15M at 400 mm. Small slabs run slightly higher because the extra edge bar is a bigger share of the total.
- Laps appear at 40 feet for 10M and at 60 feet for 15M, because of the 12 m and 18 m stock lengths.
- On a 40x60, both directions of 10M need laps. The 47 short bars are 12.04 m long, just over a 12 m stock bar.
What these numbers leave out
These are quantities for a flat rectangle with one mat. Real slabs usually need more:
- Thickened edges and grade beams with their own longitudinal bars and stirrups.
- Extra bars around openings, drains and at re-entrant corners if the drawings show them.
- Dowels into an existing slab or foundation wall, and dowels across construction joints.
- A second mat in structural or heavily loaded slabs.
- Bar supports. RSIC notes slab-on-grade steel is generally placed about 50 mm below the top surface on chairs or concrete blocks. Supports are estimated as a separate item.
- Tie wire. RSIC standard practice is that a tie at every fourth or fifth intersection is enough. On the 30x40 in 10M at 400 mm there are 768 intersections, so that works out to roughly 150 to 200 ties, though many crews tie slabs more often so the mat survives being walked on.
If you cut 12 m stock bars yourself, there is also offcut waste: a 9 m bar from a 12 m stock length leaves 3 m that rarely gets used. Ordering bars cut to length avoids that. Our guide to estimating rebar by hand covers waste, hooks and laps in more detail.
Getting an exact bar list for your slab
The tables above are for comparing options and checking a number. To order, work from the structural drawings, or the approved house drawings, so the bar size, spacing, cover, laps and every extra bar are counted. A bar list from those drawings is the number to buy from.
We prepare slab take-offs from your drawings free of charge, then cut, bend and tag the bars so each line drops into place. Send us your slab drawings for a take-off and quote, or see our rebar products for straight and fabricated bar.
Frequently asked questions
How much rebar do I need for a 30x40 slab?
With 10M bars at 400 mm each way and a 75 mm edge allowance, a 30x40 foot slab takes 24 bars about 12 m long and 32 bars about 9 m long, roughly 586 m and 460 kg including laps. With 15M at the same spacing it is about 577 m and 906 kg.
How much rebar for a 40x60 slab?
Using 10M at 400 mm each way, about 1,178 m and 925 kg including laps on the long bars. With 15M at 400 mm, about 1,164 m and 1,827 kg. Your bar size and spacing come from the drawings, so treat these as examples.
How much rebar for a 24x24 garage slab?
At 10M and 400 mm each way, 19 bars each way about 7.2 m long, or roughly 272 m and 214 kg. Tightening to 300 mm raises that to about 358 m and 281 kg.
Do I need laps on a 40 foot slab?
With 10M bar, usually yes. 10M comes in 12 m standard mill lengths, and a 40 foot (12.19 m) slab needs bars about 12.04 m long after edge cover, so each long bar is made from two pieces lapped together. 15M comes in 18 m lengths and does not need a lap at 40 feet.
