Junior Beams (M-Beams): AISC Sizes, Weights & Uses
Hot-rolled junior beams from M3×2.9 to M12.5×12.4, the light AISC M designations, in ASTM A36 or A572 Grade 50 for light framing, equipment frames and racks.
At a glance
- Sizes
- 16 AISC M designations
- Weights
- 2.9 to 18.9 lb/ft
- Grades
- A36, A572 Gr. 50
- Flanges
- Parallel, except M5×18.9
Junior beams, also sold as M-Beams or junior I-beams, are light hot-rolled I-sections from the AISC M series. AISC uses M for I-sections that ASTM A6 does not class as W, S or HP sections. The AISC table lists 16 of them, from M3×2.9 to M12.5×12.4, weighing 2.9 to 18.9 lb/ft.
Mona Steel supplies junior beams to fabricators, OEMs and contractors. Buy them in random lengths or cut and drilled to your drawings, with mill test reports on every order.
Junior beam sizes and weights
All 16 AISC M designations we supply and quote, with depth, flange width, web and flange thickness and nominal weight. Every row has a “Quote” link; tick several rows to send them in one request.
| Size | Weight (lb/ft) | Depth d (in.) | Flange width bf (in.) | Web tw (in.) | Flange tf (in.) | Quote |
|---|---|---|---|---|---|---|
| M3×2.9 | 2.9 | 3.00 | 2.25 | 0.090 | 0.130 | Quote |
| M4×4.08 | 4.08 | 4.00 | 2.25 | 0.115 | 0.170 | Quote |
| M4×6 | 6 | 3.80 | 3.80 | 0.130 | 0.160 | Quote |
| M5×18.9 | 18.9 | 5.00 | 5.00 | 0.316 | 0.416 | Quote |
| M6×3.7 | 3.7 | 5.92 | 2.00 | 0.098 | 0.129 | Quote |
| M6×4.4 | 4.4 | 6.00 | 1.84 | 0.114 | 0.171 | Quote |
| M8×6.2 | 6.2 | 8.00 | 2.28 | 0.129 | 0.177 | Quote |
| M8×6.5 | 6.5 | 8.00 | 2.28 | 0.135 | 0.189 | Quote |
| M10×7.5 | 7.5 | 9.99 | 2.69 | 0.130 | 0.173 | Quote |
| M10×8 | 8 | 9.95 | 2.69 | 0.141 | 0.182 | Quote |
| M10×9 | 9 | 10.0 | 2.69 | 0.157 | 0.206 | Quote |
| M12×10 | 10 | 12.0 | 3.25 | 0.149 | 0.180 | Quote |
| M12×10.8 | 10.8 | 12.0 | 3.07 | 0.160 | 0.210 | Quote |
| M12×11.8 | 11.8 | 12.0 | 3.07 | 0.177 | 0.225 | Quote |
| M12.5×11.6 | 11.6 | 12.5 | 3.50 | 0.155 | 0.211 | Quote |
| M12.5×12.4 | 12.4 | 12.5 | 3.75 | 0.155 | 0.228 | Quote |
Source: AISC Shapes Database v16.0, with nominal weights, reference dimensions and tolerances per ASTM A6. Check each section with your engineer; each quote confirms availability, lengths and lead time.
Junior beams vs. S-Beams and W-Beams
Fifteen of the 16 junior beams have thin webs, 0.09 to 0.177 in., and parallel flanges. From M6 to M12.5, the flanges are 1.84 to 3.75 in. wide, under half the depth. M5×18.9 is the exception: 5 in. deep with a 5 in. flange, a 0.316 in. web and sloped inner flange faces, at 18.9 lb/ft.
Junior beams and S-Beams are separate AISC series, so order by designation, not by name. An S-Beam (American Standard beam) has sloped inner flange faces and, at most depths, a thicker web and more weight: S6×12.5 has a 0.232 in. web at 12.5 lb/ft, against 0.114 in. at 4.4 lb/ft for M6×4.4.
W-Beams (wide flange beams) also have parallel flanges, but at the same depth they are usually wider and heavier: W8×10 has a 3.94 in. flange, against 2.28 in. on M8×6.2. The lightest section in each series at the same nominal depth, from the AISC tables:
- 6 in.: M6×3.7 at 3.7 lb/ft, W6×8.5 at 8.5 lb/ft, S6×12.5 at 12.5 lb/ft
- 8 in.: M8×6.2 at 6.2 lb/ft, W8×10 at 10 lb/ft, S8×18.4 at 18.4 lb/ft
- 10 in.: M10×7.5 at 7.5 lb/ft, W10×12 at 12 lb/ft, S10×25.4 at 25.4 lb/ft
- 12 in.: M12×10 at 10 lb/ft, W12×14 at 14 lb/ft, S12×31.8 at 31.8 lb/ft
Where junior beams are used
Junior beams go where the drawings call for a light I-section. Your engineer or equipment designer sizes the member and its connections, and we supply the section on the drawing. Typical uses include:
- Light framing and secondary members
- Equipment frames and supports
- Storage and equipment racks
- Trailer frames
Availability and the nearest alternative
Some M designations are rolled infrequently, so the quote confirms availability, lengths and lead time for each size.
If a size is hard to source, we can quote the nearest W-Beam or S-Beam alongside it for your engineer to review. At the same nominal depth the alternative is usually heavier, and its flange width and section properties differ, so it goes to the engineer of record for approval before material is ordered.
Grades, processing and delivery
Order junior beams in ASTM A36 or A572 Grade 50, the grade on your drawings or specification; other grades are quoted on request. Tolerances follow ASTM A6, and the mill test reports carry heat numbers for traceability.
Junior beams can also ship processed: mitered or cut to length, drilled, coped or punched, with holes and slots laser or plasma cut. Hot-dip galvanizing per ASTM A123 and piece-mark tags are available, and if you only have structural drawings, the detailing team can produce shop drawings and cut lists.
Each load is quoted for delivery, whether it goes across California or ships to a job site in Texas, Arizona, Nevada or elsewhere in the country.
Junior Beams FAQ
What is a junior beam?
A junior beam, or M-Beam, is a light hot-rolled I-section from the AISC M series. The table runs from M3×2.9 to M12.5×12.4 in 16 designations. All but M5×18.9 weigh 12.4 lb/ft or less, with thin webs and parallel flanges.
Junior beam vs. S-Beam vs. W-Beam: what is the difference?
They are three separate AISC series. W-Beams (wide flange beams) and every junior beam except M5×18.9 have parallel flanges, while S-Beams (American Standard beams) have sloped inner flange faces. At the same nominal depth, a junior beam is usually the lightest of the three. At 10 in., the lightest of each is M10×7.5 at 7.5 lb/ft, W10×12 at 12 lb/ft and S10×25.4 at 25.4 lb/ft.
What does M6×4.4 mean?
M6×4.4 is a junior beam with a 6 in. nominal depth and a nominal weight of 4.4 lb/ft. It is 6 in. deep, with a 1.84 in. flange width, a 0.114 in. web and a 0.171 in. flange thickness. Actual depth can differ from nominal: M4×6 is 3.8 in. deep and M6×3.7 is 5.92 in. deep.
What are junior beams used for?
Light framing and secondary members, equipment frames, racks and trailer frames, where the drawings call for a light I-section. Your engineer sets the member size, connections and load rating, and we supply the section specified.
Are all M-Beam sizes in the AISC table easy to get?
No. Some M designations are rolled infrequently, so each size is confirmed on the quote. When one is hard to source, the quote can also show the nearest W-Beam or S-Beam for your engineer to review.
Request a quote
Request pricing on junior beams
Tick the sizes you need in the table, or send your cut list or takeoff. We come back with an itemized quote, mill test reports included with the order.
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