W8x18 — Dimensions, Weight & Section Properties
American wide-flange (W) beam per ASTM A6 / AISC Steel Construction Manual
W8x18 is an American wide-flange beam 8.14 inches (206.8 mm) deep with a 5.25 in (133.4 mm) flange, weighing 18 pounds per foot (26.8 kg/m) — that is what the "18" 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 | 8.14 in (206.8 mm) |
| Flange width bf | 5.25 in (133.4 mm) |
| Web thickness tw | 0.23 in (5.8 mm) |
| Flange thickness tf | 0.33 in (8.4 mm) |
Weight & Cross-Section
| Weight per foot | 18 lb/ft (26.8 kg/m) |
| 20 ft bar | 360 lb |
| 40 ft bar | 720 lb |
| Area A | 5.26 in² (33.9 cm²) |
Section Properties (strong / weak axis)
| Moment of inertia Ix | 61.9 in⁴ (2,576 cm⁴) |
| Section modulus Sx | 15.2 in³ (249 cm³) |
| Radius of gyration rx | 3.43 in |
| Moment of inertia Iy | 7.97 in⁴ (332 cm⁴) |
| Section modulus Sy | 3.04 in³ (50 cm³) |
| Radius of gyration ry | 1.23 in |
Compared with the neighbouring sizes
| Shape | d × bf (in) | lb/ft | Ix (in⁴) | Sx (in³) | Δ weight | Δ Ix |
|---|---|---|---|---|---|---|
| W8x15 | 8.11 × 4.02 | 15 | 48 | 11.8 | −17 % | −22 % |
| W8x18 (this page) | 8.14 × 5.25 | 18 | 61.9 | 15.2 | — | — |
| W8x21 | 8.28 × 5.27 | 21 | 75.3 | 18.2 | +17 % | +22 % |
Deflection example
A simply supported W8x18 spanning 13 ft with a 10 kip point load at midspan deflects 0.44 in (E = 29,000 ksi, Ix = 61.9 in⁴). The L/360 serviceability limit for this span is 0.43 in — the shape does NOT pass this quick check.
Calculate with your own span and load →
Frequently asked questions
How much does a W8x18 beam weigh?
W8x18 weighs 18 pounds per foot (26.8 kg/m). The number after the "x" in the designation is always the weight in lb/ft. A 20 ft bar weighs 360 lb and a 40 ft bar 720 lb.
What are the dimensions of a W8x18?
W8x18 is 8.14 in (206.8 mm) deep with a 5.25 in (133.4 mm) wide flange. The web is 0.23 in thick and the flanges are 0.33 in thick. Note the actual depth differs from the nominal W8 label — always design with the actual value.
What are the dimensions of a W8x18 in mm?
W8x18 is 206.8 mm deep with a 133.4 mm wide flange; the web is 5.8 mm and the flange 8.4 mm thick. In metric section properties: Ix = 2,576 cm⁴, Sx = 249 cm³.
How much does a W8x18 weigh in kg?
26.8 kg per meter. A 6 m bar weighs 161 kg and a 12 m bar 321 kg — handy when ordering US shapes from metric suppliers.
What is the moment of inertia of a W8x18?
About the strong axis, Ix = 61.9 in⁴ (2,576 cm⁴) with section modulus Sx = 15.2 in³. About the weak axis, Iy = 7.97 in⁴ and Sy = 3.04 in³. Use the beam deflection calculator to turn these into deflection and stress for your span and load.
W8x18 or W8x21 — what does one size up change?
W8x21 is about 17% heavier (21 vs 18 lb/ft) but about 22% stiffer (Ix 75.3 vs 61.9 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