The steel beam you need depends on four things: the span between supports, how much floor and roof the beam carries, what sits on top of it, and how much depth you can give up. For a straightforward basement beam carrying floor joists, the 2024 Ontario Building Code answers the question in Table 9.23.4.3: a W200x27 carrying one storey with a 3.6 m supported joist length can span up to 6.6 m. For most load-bearing wall removals, roofs and point loads, the size comes from an engineer, not a table.
This guide explains how sizing works so you can follow the conversation with your designer, sanity-check a proposal and know what to send when you ask for a quote. It isn't a design tool, and the numbers here only apply within the code's stated conditions.
The four inputs that size a steel beam
Every beam sizing, whether from a table or a full calculation, starts with the same questions:
- Span. The distance between the supports: posts, columns or walls. Longer spans need bigger beams, and the requirement grows much faster than the span. For a uniform load, doubling the span quadruples the bending in the beam and multiplies its sag by about sixteen.
- Supported length. How much floor the beam picks up. In the code tables this is the supported joist length: half the sum of the joist spans on both sides of the beam. A beam in the middle of a house with 4.2 m joists on each side carries far more than one near an outside wall.
- What's above. One storey or two, a roof, a bearing wall, a bathtub, a stone island, a post from the floor above. Uniform floor loads are table material. Concentrated loads, roof loads and walls bearing directly on the beam are not.
- Depth. A flush beam hidden in the floor has to fit within the joist depth; a dropped beam takes headroom. Depth limits often decide which of several adequate sections you choose.
How to read the Ontario span table
Table 9.23.4.3 of the 2024 OBC (O. Reg. 163/24, Division B) lists the maximum span of common W-shapes supporting floors in dwelling units. You choose the row for your section, the block for one or two storeys supported, and the column for your supported joist length. Here is the 3.6 m column, with spans in metres:
| Section | One storey supported | Two storeys supported |
|---|---|---|
| W150x22 | 4.9 | 4.1 |
| W200x21 | 5.9 | 4.6 |
| W200x27 | 6.6 | 5.6 |
| W200x31 | 7.1 | 6.2 |
| W250x24 | 7.3 | 5.6 |
| W250x39 | 9.0 | 7.8 |
| W310x31 | 9.4 | 7.2 |
| W310x39 | 10.2 | 8.5 |
Source: 2024 Ontario Building Code, Division B, Table 9.23.4.3. The table also has columns for supported joist lengths from 2.4 m to 6.0 m, and straight-line interpolation is allowed between them.
Worked example. A basement beam supports joists spanning 4.2 m on one side and 3.0 m on the other. The supported joist length is half of 7.2 m, which is 3.6 m. The posts are 5.5 m apart.
- Bungalow (one storey supported): a W150x22 reaches only 4.9 m, so it's too small. A W200x21 reaches 5.9 m and is the lightest section in the table that works.
- Two-storey house: the W200x21 drops to 4.6 m. A W200x27 or W250x24 reaches 5.6 m and covers the span.
The table rests on stated conditions, and the beam only qualifies if they all hold:
- The top flange is laterally supported. Under Sentence 9.23.4.3.(3), that means joists bearing on the top flange at no more than 600 mm along its full length, the load coming through those joists, and 19 x 38 mm wood strips nailed to the joists on both sides of the flange.
- The load is a normal floor in a dwelling. The explanatory note lists the assumptions: simple spans, 350 MPa yield, a live load of 1.9 kPa, a dead load of 1.5 kPa and a deflection limit of span/360.
- The steel meets at least Grade 350W under CSA G40.21, as Sentence 9.23.4.3.(2) requires.
For more on these tables and the roof-and-floor tables that sit beside them, see our steel beam span tables guide. For the main basement girder specifically, see basement steel beam size.
Why rules of thumb and calculators get it wrong
Two shortcuts circulate online: a depth-to-span rule of thumb, and free beam calculators. Both have the same problem: they skip the inputs that actually decide the size.
Depth alone tells you little. Look at the table again. A W250x24 and a W250x39 are both nominally 250 mm deep, yet one storey up at a 3.6 m supported length they span 7.3 m and 9.0 m. A W310x31 is deeper than the W250x39 but lighter, and its two-storey span is shorter (7.2 m against 7.8 m). Weight per metre, flange size and how the beam is braced all matter. See how to read steel beam sizes for what the numbers in a W-shape name mean.
Calculators assume a tidy beam. Most assume a uniform load, a fully braced top flange and one deflection limit. Real house beams often carry a post from the floor above, a bearing wall, a point load from a girder truss or a stair opening alongside. A calculator that doesn't ask about those can't give a size you can build or submit for a permit.
Load-bearing wall removal: why it's usually engineered
"What size beam do I need to remove a load-bearing wall?" is the most common version of this question, and the honest answer is that it's rarely a table lookup:
- The wall may carry more than a floor. Walls on the main floor often carry the second floor plus a wall, posts or a roof load above that.
- Posts land on the beam. A wall upstairs that sits on the wall being removed becomes a line load or point load on the new beam.
- Flush beams change the bracing. A beam set into the floor with joists hung from its sides doesn't meet the table's lateral support conditions as written.
- The load has to reach the ground. The posts at each end carry concentrated loads, which often need new footings in the basement.
That's why most wall removals end up with a structural engineer or a qualified designer (BCIN), and why the beam size is part of the permit drawings. Our guide to permits for load-bearing wall removal explains who can prepare them, and load-bearing wall removal cost breaks down the rest of the job.
The beam size decides more than the beam
Once a section is chosen, it sets several other things:
- Posts. Each end of the beam carries half its load to a column, which has to be sized for it. The code's minimum steel pipe column is 73 mm in outside diameter with a 4.76 mm wall (Article 9.17.3.1), with other sizes allowed where their capacity is shown.
- Footings. Longer spans put more load on fewer posts, and the code ties column footing area to column spacing (Article 9.15.3.7).
- Bearing. Beams need even, level bearing at least 89 mm long at their end supports under Article 9.23.8.1, and more where the design calls for it.
- Connections. Cap plates, base plates, holes for nailers and beam-to-post connections are all fabricated to the size chosen.
A slightly deeper section can sometimes save a post, and a heavier, shallower one can save headroom. Those trade-offs are worth discussing with your designer before the drawings go in.
Getting your beam sized and made
If you already have a beam schedule from your drawings, we can quote it as is. If you don't, tell us about the project when you ask for a quote: engineering services are available for projects that need them. Useful information to send:
- The span between supports and the joist spans on each side
- How many storeys sit above, and whether a wall, post or roof bears on the beam
- Whether the beam will be flush or dropped, and any depth limit
- Photos of the existing framing and posts
Send us your drawings or project details for a quote on the beam, posts and plates. We supply, fabricate and install steel beams and posts for basements and renovations across the GTA and Ontario. See our steel products for what we make.
Frequently asked questions
How do I calculate what size steel beam I need?
For a simple house beam carrying floor joists, measure the span between supports and the supported joist length (half the sum of the joist spans on both sides), count the storeys above, and look up a section in Table 9.23.4.3 of the 2024 Ontario Building Code. Anything with point loads, roofs or walls bearing on it is calculated by an engineer.
Is there a rule of thumb for steel beam size?
Not one you can rely on. Beams of the same depth can carry very different loads: in the Ontario table, a W250x24 and a W250x39 are both about 250 mm deep, but at the same supported length one storey up they span 7.3 m and 9.0 m.
What size steel beam do I need to remove a load-bearing wall?
That depends on the span, the floors, walls and roof above, and any posts that land on the beam. Most wall removals carry concentrated or roof loads that fall outside the Part 9 tables, so the beam is sized by an engineer or qualified designer as part of the permit drawings.
Are online steel beam calculators accurate?
They can give a starting point, but they usually assume a uniform load, full lateral bracing and a single deflection limit. They rarely account for point loads, the posts and footings below, or the building department's review, so their answer can't go on a permit.
