A steel beam size like W200x27 reads as: W for wide-flange shape, 200 for the nominal depth in millimetres, and 27 for the mass in kilograms per metre. The imperial name W8x18 works the same way in inches and pounds per foot, and it is the exact same beam: about 207 mm (8.14 in) deep, weighing 26.6 kg/m (18 lb/ft). Canadian drawings use the metric name; US drawings, many suppliers and a lot of homeowners use the imperial one.
The three parts of a beam size
Take W200x27 apart:
| Part | Metric (W200x27) | Imperial (W8x18) |
|---|---|---|
| Shape letter | W = wide-flange | W = wide-flange |
| First number | Nominal depth: 200 mm | Nominal depth: 8 in |
| Second number | Mass: 27 kg/m (26.6 actual) | Weight: 18 lb/ft |
The letter tells you the cross-section shape. W is by far the most common for beams. Other letters are covered further down.
The first number is the nominal depth. It is a family name, not a measurement. Every W200 is "about 200 mm", but the actual depths in that family run from 200 mm for the lightest section to 229 mm for the heaviest, and every W8 is "about 8 inches".
The second number is the weight. Within one depth family, more weight means thicker flanges and web, and often wider flanges, so a higher number usually means a stronger beam of about the same depth.
Why the metric number isn't a straight conversion
If you convert W8x18 with a calculator, 8 in is 203 mm, not 200, and 18 lb/ft is 26.8 kg/m, not 27. Two things explain the mismatch:
- Metric depth groups are rounded names. The groups were chosen as tidy metric numbers that line up with the imperial families: W150 for W6, W200 for W8, W250 for W10, W310 for W12, W360 for W14, W410 for W16, W460 for W18, W530 for W21 and W610 for W24.
- The metric mass is rounded, and calculated slightly differently. The Canadian section tables list W200x27 at 26.6 kg/m, worked out from the section's area and the density of steel. The imperial weight, 18 lb/ft, is a rounded nominal figure that converts to about 26.8 kg/m. The difference is under 1 percent and doesn't change which beam you get.
The practical rule: convert by table, not by arithmetic. Find the beam in a published section table that lists both names. Our Canadian W-shape beam chart has both for the common sizes.
Worked examples: metric and imperial pairs
These pairs are the same physical beam. Dimensions are actual, from the CISC section tables.
| Metric | Imperial | Actual depth | Flange width | Mass |
|---|---|---|---|---|
| W150x22 | W6x15 | 152 mm (6.0 in) | 152 mm (6.0 in) | 22.5 kg/m (15 lb/ft) |
| W200x27 | W8x18 | 207 mm (8.14 in) | 133 mm (5.25 in) | 26.6 kg/m (18 lb/ft) |
| W200x46 | W8x31 | 203 mm (8.0 in) | 203 mm (8.0 in) | 46.2 kg/m (31 lb/ft) |
| W250x33 | W10x22 | 258 mm (10.2 in) | 146 mm (5.75 in) | 32.9 kg/m (22 lb/ft) |
| W310x39 | W12x26 | 310 mm (12.2 in) | 165 mm (6.49 in) | 38.8 kg/m (26 lb/ft) |
| W360x45 | W14x30 | 352 mm (13.8 in) | 171 mm (6.73 in) | 44.8 kg/m (30 lb/ft) |
Small differences of a millimetre between tables (for example 258 vs 259 mm for the W10x22) come from rounding the imperial dimensions.
Reading a W10x22 in full
Searchers often want the full dimensions of a W10x22, so here it is as an example of what a section table tells you. A W10x22 (W250x33) is:
- About 10.2 in (259 mm) deep overall.
- 5.75 in (146 mm) across the flange.
- 0.24 in (6.1 mm) thick in the web.
- 0.36 in (9.1 mm) thick in each flange.
- 22 lb/ft, or about 32.7 kg/m by direct conversion.
The size name alone gave you only two of those numbers, and both were rounded. That's why fabricators detail from section tables, not from the name.
Common mistakes when reading beam sizes
These show up regularly when people order or substitute beams:
- Assuming the first number is exact depth. A W250x45 is 266 mm deep while a W250x49 is 247 mm. If the beam has to sit flush with joists or fit a pocket, check actual depth.
- Assuming a heavier beam is deeper. In the W310 family, the W310x60 (303 mm) is shallower than the W310x39 (310 mm).
- Ignoring flange width. A W250x28 has a 102 mm flange and a W250x33 a 146 mm flange. That changes bearing on a post, room for bolts and how a wood nailer sits on top.
- Mixing up metric and imperial names. "A 10 by 22" could mean a W10x22 (imperial) or someone may misread W250x22, which is a lighter beam (a W10x15). Always write the full designation with the system it belongs to.
- Swapping sizes without approval. Two beams with similar weight can have very different strength and stiffness. Any substitution needs the designer's or engineer's sign-off.
Other shape letters you'll see
Beam size notation uses the same "letter, depth, weight" pattern across most shapes:
- S (standard I-beam): for example S310x47 (S12x31.8). Narrower flanges with sloped inside faces. Common in older buildings and under hoist trolleys.
- HP (bearing pile): for example HP310x79 (HP12x53). An H-shaped pile section with flange and web of the same thickness.
- C and MC (channels): for example C200x17 (C8x11.5). See our C-channel size chart.
- L (angle): written by leg lengths and thickness, for example L102x102x9.5 (4 x 4 x 3/8 in). Angles are the usual shape for masonry lintels.
- HSS (hollow structural section): written by outside dimensions and wall, for example HSS 102x102x6.4. Note that HSS names use wall thickness, not mass. See our HSS size chart.
- WWF (welded wide-flange): a Canadian welded beam built up from plate, written by depth and mass like a W-shape. Mostly found on drawings for older buildings.
"I-beam" and "H-beam" are everyday names, not designations. People say I-beam for most wide-flange beams, and H-beam for the wide-flange sections whose flanges are about as wide as they are deep (W200x46 is a good example at 203 by 203 mm). On a drawing, they are all W-shapes.
Reading beam sizes on drawings and shop drawings
On structural drawings, beam sizes appear on the framing plan next to each member, sometimes with a camber or elevation note. On shop drawings, each piece gets a piece mark, and the size appears in the bill of materials with its length and grade. When you call a supplier or fabricator, have this ready:
- The full designation exactly as drawn, for example W250x33, not "a 10-inch beam".
- The grade, if the drawings give one.
- Length, and whether it's cut to length or needs connections.
- Any notes such as camber, holes, plates or finish.
If your drawings are imperial and you're buying in Canada (or the reverse), that's fine. Fabricators work in both, and any good one will confirm the equivalent before cutting.
Need help reading your drawings?
If you're not sure what a size on your drawings means, or whether a substitute is acceptable, we can help you read it and then put the right steel together. Final substitutions still go back to your designer or engineer. Send us your drawings for a take-off and quote, and see our steel products for the beams, posts, channels and plates we supply and fabricate.
Frequently asked questions
What does W8x18 mean?
W means wide-flange shape, 8 is the nominal depth in inches and 18 is the weight in pounds per foot. A W8x18 is actually 8.14 in (207 mm) deep with a 5.25 in (133 mm) flange.
Is W200x27 the same as W8x18?
Yes. They are the same rolled section with a metric name and an imperial name. W200x27 is about 207 mm deep and 26.6 kg/m; W8x18 is about 8.14 in deep and 18 lb/ft.
What are the dimensions of a W10x22 beam?
A W10x22 (metric W250x33) is about 10.2 in (259 mm) deep, with a 5.75 in (146 mm) flange, a 0.24 in (6.1 mm) web and 0.36 in (9.1 mm) flanges, and weighs 22 lb/ft (about 32.7 kg/m).
Does the first number tell me the exact beam depth?
No. It is a nominal depth that names a family of beams. Actual depths in the same family can differ by 20 mm or more, so check a section table when exact depth matters.
