W 6X12 vs S 6X12.5
W 6X12 is an W section from the American AISC Shapes Database; S 6X12.5 is the closest equivalent in the American AISC Shapes Database. They are close to interchangeable in bending: 136.01 cm³ against 138.47 cm³ of plastic modulus, a difference of 1.8%. W 6X12 is the lighter of the two at 17.9 kg/m against 18.6 kg/m, a saving of 3.8% on steel weight. Both are about 153 mm deep, so either fits the same construction zone.
| Property | W 6X12 W | S 6X12.5 S | Difference |
|---|---|---|---|
| Mass | 17.9 kg/m | 18.6 kg/m | -3.8% |
| Depth | 153.2 mm | 152.4 mm | +0.5% |
| Width | 101.6 mm | 84.6 mm | +20.1% |
| Web thickness | 5.8 mm | 5.9 mm | -1.7% |
| Flange thickness | 7.1 mm | 9.1 mm | -22.0% |
| Area | 22.9 cm² | 23.6 cm² | -3.0% |
| Second moment, major (Iy) | 919.9 cm⁴ | 915.7 cm⁴ | +0.5% |
| Plastic modulus, major (Wpl,y) | 136.01 cm³ | 138.47 cm³ | -1.8% |
| Elastic modulus, major (Wel,y) | 119.79 cm³ | 120.28 cm³ | -0.4% |
| Second moment, minor (Iz) | 124.5 cm⁴ | 74.92 cm⁴ | +66.2% |
| Plastic modulus, minor (Wpl,z) | 38.018 cm³ | 30.48 cm³ | +24.7% |
| Radius of gyration, minor (iz) | 2.33 cm | 1.78 cm | +30.9% |
| Torsion constant (It) | 3.76 cm⁴ | 6.95 cm⁴ | -45.9% |
W 6X12 to AISC Shapes Database v16.0; S 6X12.5 to AISC Shapes Database v16.0. Difference is expressed relative to S 6X12.5.
How to choose between them
On stiffness, W 6X12 has the larger second moment of area: 919.9 cm⁴ against 915.7 cm⁴, a difference of 0.5%. This matters more often than the strength comparison, because on normal floor spans deflection rather than bending capacity sets the beam size - so if the two are close on modulus but apart on inertia, the stiffer one is usually the better answer even when it is not the stronger one.
Measured as bending capacity per kilogram of steel, W 6X12 is the more efficient section: 7.6 against 7.4 cm³ per kg/m, 2.1% better. Over a floor of repeated beams that difference is real tonnage, though it is often outweighed by whichever section the fabricator can actually source.
The two diverge much more about the minor axis: W 6X12 has 38.018 cm³ of minor-axis plastic modulus against 30.48 cm³, 24.7% more. If the member sees any minor-axis bending, or if it is unrestrained over a long length and lateral-torsional buckling governs, that gap decides the comparison even though the major-axis numbers looked equivalent.
In short, S 6X12.5 is the stronger section and W 6X12 is the lighter one; where those are the same section, it is the clear choice, and where they are not, decide on whether your design is governed by capacity or by tonnage.