There is no single answer to how much rebar goes in a cubic metre of concrete, because it depends on the element and the design. The useful conversion is this: every 1% of steel by cross-sectional area equals about 78.5 kg of rebar per cubic metre of concrete, since steel weighs about 7,850 kg/m3. A lightly reinforced slab may carry a few tens of kilograms per cubic metre; a heavily reinforced column can carry several hundred.
This post shows how to turn a reinforcement ratio or a drawing callout into kg/m3, what CSA A23.3's minimums look like in those units, and why the ratio is a check, not a way to order steel.
Why kg/m3 varies so much
Rebar quantity is driven by the bending, shear and axial loads an element carries, not by the volume of concrete around it. Two members with the same volume can need very different amounts of steel:
- A slab on grade under Part 9 of the Ontario Building Code needs no reinforcement at all, so it can be zero.
- A structural slab carries bending and needs at least a minimum amount each way, often more over supports.
- A footing that just spreads load can be lightly reinforced; a pile cap or transfer footing can be packed with steel.
- A column carries axial load and has a code minimum of 1% steel, which is already more than most slabs.
- Beams, transfer girders and shear walls in seismic or heavily loaded buildings can go far higher.
Published rules of thumb for kg/m3 by element type circulate widely, but they vary between sources and assume building types that may not match yours, so we don't quote them here. The ratio you can trust is the one you calculate from your own drawings.
Converting a steel ratio to kg/m3
Designers describe reinforcement as a ratio: steel area divided by gross concrete area. Because rebar has a density of about 7,850 kg/m3, the conversion is direct:
kg of rebar per m3 of concrete ≈ steel ratio × 7,850
That density is built into the CSA G30.18 nominal bar masses. A 10M bar has a nominal area of 100 mm² and a mass of 0.785 kg/m; a 25M bar has 500 mm² and 3.925 kg/m. In both cases the mass per metre is the area times 7,850 kg/m3.
| Steel ratio | Rebar per m3 of concrete |
|---|---|
| 0.15% (0.0015) | about 11.8 kg |
| 0.2% (0.002) | about 15.7 kg |
| 0.5% | about 39 kg |
| 1% | about 78.5 kg |
| 2% | about 157 kg |
| 4% | about 314 kg |
The ratio only counts bars running one way through the section. A slab reinforced in two directions has two ratios, and the totals add. Ties, stirrups, laps, hooks and chairs add more on top.
CSA A23.3 minimums in kg/m3
The minimum steel ratios in CSA A23.3 give a floor for structural members. These are from the 2004 edition, which is the one we checked; confirm the edition your designer is using.
| Element | A23.3 minimum (2004) | As kg/m3 (straight bars, before laps) |
|---|---|---|
| Structural slab | 0.002 Ag in each direction (Clause 7.8.1) | about 15.7 per direction, about 31 for both |
| Wall, vertical bars | 0.0015 Ag (Clause 14.1.8.5) | about 11.8 |
| Wall, horizontal bars | 0.002 Ag (Clause 14.1.8.6) | about 15.7 |
| Column, longitudinal bars | 0.01 Ag minimum (Clause 10.9.1) | about 78.5 |
| Column, maximum including lap regions | 0.08 Ag (Clause 10.9.2) | about 628 |
Two cautions. First, these are minimums for designed members. Real slabs, walls and columns usually carry more where the loads demand it. Second, A23.3's own note to the column clause warns that more than 4% steel outside lap regions causes serious practical difficulty placing bars and concrete, so columns near the top of that range are rare.
Worked examples from a drawing
The reliable way to get kg/m3 is to work it out from the actual bar callout. Three examples, using CSA G30.18 bar masses from our rebar weight chart:
Example 1: 150 mm slab, 10M @ 300 mm each way
- Bars per metre width: 1,000 ÷ 300 = 3.33
- Steel per metre width, one direction: 3.33 × 0.785 kg/m = 2.62 kg per square metre of slab
- Both directions: about 5.2 kg per square metre
- Concrete per square metre: 0.15 m3
- Ratio: 5.2 ÷ 0.15 = about 35 kg/m3, before laps and edge bars
Example 2: strip footing 600 mm wide x 250 mm deep, three 15M continuous
- Steel per metre of footing: 3 × 1.570 kg/m = 4.71 kg
- Concrete per metre: 0.6 × 0.25 = 0.15 m3
- Ratio: 4.71 ÷ 0.15 = about 31 kg/m3, before laps and corner bars
Example 3: 400 x 400 mm column with eight 25M bars
- Steel per metre of height: 8 × 3.925 kg/m = 31.4 kg
- Concrete per metre: 0.4 × 0.4 = 0.16 m3
- Ratio: 31.4 ÷ 0.16 = about 196 kg/m3 in vertical bars alone (2.5% steel), before ties and laps
The bar sizes and spacings here are illustrations of the method, not design recommendations. Your structural drawings set the real ones.
What a kg/m3 figure leaves out
Ratios calculated from the main bars always come in light against a real bar list. The difference comes from:
- Laps. Every splice adds a lap length of extra bar. On long slabs and walls, laps are a significant share of the tonnage. See our lap splice chart.
- Hooks, bends and development. Bars that hook into supports or extend past a joint add length.
- Ties and stirrups. In beams and columns these can add a large amount on top of the main bars.
- Extra bars. Trim bars at openings, corner bars, top bars over supports, edge bars, dowels and support bars.
- Cut-off waste. Stock lengths rarely divide evenly into bar lengths.
That is why a kg/m3 figure, even one calculated carefully, is a cross-check and an early budget input, not an order quantity. For the full method of counting bars, adding laps and converting to weight, see our rebar calculator and estimating guide and how to estimate rebar quantity.
Where kg/m3 is useful
Used properly, the ratio does three jobs:
- Early budgeting before drawings are complete, when an estimator uses ratios from similar past projects. The ratios should come from that estimator's own completed jobs, not a generic table.
- Checking a take-off. If a slab take-off comes out at several hundred kg/m3, or a column at a few dozen, something has been missed or double counted.
- Comparing designs. Two structural schemes for the same building can be compared on steel intensity.
It is not a way to order bar. Rebar is ordered by bar mark, size, length and shape from a bar list, and priced by weight once that list exists. Our post on how rebar is priced explains what goes into that number.
Getting an accurate tonnage
The fastest way to an accurate quantity is to send the structural drawings to someone who does take-offs every day. We prepare free take-offs and quotes from your drawings, then detail, cut and bend the bar from 10M to 55M, tag it by bar mark and deliver across the GTA and Ontario.
Send your drawings for a take-off and quote, or see the rebar products we supply.
Frequently asked questions
How many kg of rebar are in 1 m3 of concrete?
It depends entirely on the element. Every 1% of steel by cross-sectional area adds about 78.5 kg per cubic metre. A lightly reinforced slab may carry a few tens of kg/m3, while a heavily reinforced column can carry several hundred.
What is the unit weight of rebar in kg/m3?
Steel reinforcing bar has a density of about 7,850 kg per cubic metre. That is the figure behind the CSA G30.18 nominal masses: a 10M bar with a 100 mm² area weighs 0.785 kg per metre.
What is the minimum rebar per m3 for a slab?
CSA A23.3 requires at least 0.002 times the gross area of a structural slab in each direction. That works out to about 15.7 kg/m3 per direction, or about 31 kg/m3 for both directions before laps. Slabs on grade under Part 9 have no such minimum.
Can I estimate rebar from concrete volume?
Only as a rough early check. Ratios vary too much between slabs, walls, footings and columns to price or order from. A take-off from the drawings is the only reliable way to get a tonnage.
