W8x21 — Dimensions, Weight & Section Properties
American wide-flange (W) beam per ASTM A6 / AISC Steel Construction Manual
W8x21 is an American wide-flange beam 8.28 inches (210.3 mm) deep with a 5.27 in (133.9 mm) flange, weighing 21 pounds per foot (31.3 kg/m) — that is what the "21" in the designation means. All values below are the standard AISC published section properties used in US structural design, with metric equivalents alongside.
Typical uses: floor and roof framing, columns, headers, and general steel construction. For a quick stiffness check, see the deflection example below or open the beam calculator with your own span and load.
Dimensions
| Depth d | 8.28 in (210.3 mm) |
| Flange width bf | 5.27 in (133.9 mm) |
| Web thickness tw | 0.25 in (6.4 mm) |
| Flange thickness tf | 0.4 in (10.2 mm) |
Weight & Cross-Section
| Weight per foot | 21 lb/ft (31.3 kg/m) |
| 20 ft bar | 420 lb |
| 40 ft bar | 840 lb |
| Area A | 6.16 in² (39.7 cm²) |
Section Properties (strong / weak axis)
| Moment of inertia Ix | 75.3 in⁴ (3,134 cm⁴) |
| Section modulus Sx | 18.2 in³ (298 cm³) |
| Radius of gyration rx | 3.49 in |
| Moment of inertia Iy | 9.77 in⁴ (407 cm⁴) |
| Section modulus Sy | 3.71 in³ (61 cm³) |
| Radius of gyration ry | 1.26 in |
Compared with the neighbouring sizes
| Shape | d × bf (in) | lb/ft | Ix (in⁴) | Sx (in³) | Δ weight | Δ Ix |
|---|---|---|---|---|---|---|
| W8x18 | 8.14 × 5.25 | 18 | 61.9 | 15.2 | −14 % | −18 % |
| W8x21 (this page) | 8.28 × 5.27 | 21 | 75.3 | 18.2 | — | — |
| W8x24 | 7.93 × 6.5 | 24 | 82.7 | 20.9 | +14 % | +10 % |
Deflection example
A simply supported W8x21 spanning 13 ft with a 10 kip point load at midspan deflects 0.36 in (E = 29,000 ksi, Ix = 75.3 in⁴). The L/360 serviceability limit for this span is 0.43 in — the shape passes this quick check.
Calculate with your own span and load →
Frequently asked questions
How much does a W8x21 beam weigh?
W8x21 weighs 21 pounds per foot (31.3 kg/m). The number after the "x" in the designation is always the weight in lb/ft. A 20 ft bar weighs 420 lb and a 40 ft bar 840 lb.
What are the dimensions of a W8x21?
W8x21 is 8.28 in (210.3 mm) deep with a 5.27 in (133.9 mm) wide flange. The web is 0.25 in thick and the flanges are 0.4 in thick. Note the actual depth differs from the nominal W8 label — always design with the actual value.
What are the dimensions of a W8x21 in mm?
W8x21 is 210.3 mm deep with a 133.9 mm wide flange; the web is 6.4 mm and the flange 10.2 mm thick. In metric section properties: Ix = 3,134 cm⁴, Sx = 298 cm³.
How much does a W8x21 weigh in kg?
31.3 kg per meter. A 6 m bar weighs 188 kg and a 12 m bar 375 kg — handy when ordering US shapes from metric suppliers.
What is the moment of inertia of a W8x21?
About the strong axis, Ix = 75.3 in⁴ (3,134 cm⁴) with section modulus Sx = 18.2 in³. About the weak axis, Iy = 9.77 in⁴ and Sy = 3.71 in³. Use the beam deflection calculator to turn these into deflection and stress for your span and load.
W8x21 or W8x24 — what does one size up change?
W8x24 is about 14% heavier (24 vs 21 lb/ft) but about 10% stiffer (Ix 82.7 vs 75.3 in⁴). Going one step up within the same depth group is usually the cheapest way to cut deflection.
Values are entered from the governing standard’s published tables and cross-checked programmatically against its mass and section formulas before publication. Spotted an error? Write to advertising@metalabout.net — verified corrections ship within days. About