W8×18 vs W8×21
A side-by-side comparison of two AISC wide-flange beams, 16th Edition. Detailing dimensions to the nearest 1/16 in.
W8×21 is the heavier section at 21 lb per foot, versus 18 for W8×18 (3 lb/ft more).
Both are nominally 8 in. deep.
W8×21 has the greater bending strength: section modulus Sx of 18.2 in³ versus 15.2, about 20% more.
| Property | W8×18 | W8×21 |
|---|---|---|
| Weight | 18 lb/ft | 21 lb/ft |
| Depth (d) | 8 1/8" | 8 1/4" |
| Flange width (bf) | 5 1/4" | 5 1/4" |
| Web thickness (tw) | 1/4" | 1/4" |
| Flange thickness (tf) | 5/16" | 3/8" |
| Area (A) | 5.26 in² | 6.16 in² |
| Moment of inertia (Ix) | 61.9 in⁴ | 75.3 in⁴ |
| Section modulus (Sx) | 15.2 in³ | 18.2 in³ |
| Plastic modulus (Zx) | 17 in³ | 20.4 in³ |
Open W8×18 in the interactive size chart →
Full detail: W8×18 · W8×21. Verify critical dimensions against the current AISC Steel Construction Manual before fabrication.
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