W8x40 — Dimensions, Weight & Section Properties
American wide-flange (W) beam per ASTM A6 / AISC Steel Construction Manual
W8x40 is an American wide-flange beam 8.25 inches (209.6 mm) deep with a 8.07 in (205 mm) flange, weighing 40 pounds per foot (59.5 kg/m) — that is what the "40" 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 | 8.25 in (209.6 mm) |
| Flange width bf | 8.07 in (205 mm) |
| Web thickness tw | 0.36 in (9.1 mm) |
| Flange thickness tf | 0.56 in (14.2 mm) |
Weight & Cross-Section
| Weight per foot | 40 lb/ft (59.5 kg/m) |
| 20 ft bar | 800 lb |
| 40 ft bar | 1,600 lb |
| Area A | 11.7 in² (75.5 cm²) |
Section Properties (strong / weak axis)
| Moment of inertia Ix | 146 in⁴ (6,077 cm⁴) |
| Section modulus Sx | 35.5 in³ (582 cm³) |
| Radius of gyration rx | 3.53 in |
| Moment of inertia Iy | 49.1 in⁴ (2,044 cm⁴) |
| Section modulus Sy | 12.2 in³ (200 cm³) |
| Radius of gyration ry | 2.04 in |
Compared with the neighbouring sizes
| Shape | d × bf (in) | lb/ft | Ix (in⁴) | Sx (in³) | Δ weight | Δ Ix |
|---|---|---|---|---|---|---|
| W8x35 | 8.12 × 8.02 | 35 | 127 | 31.2 | −12 % | −13 % |
| W8x40 (this page) | 8.25 × 8.07 | 40 | 146 | 35.5 | — | — |
| W8x48 | 8.5 × 8.11 | 48 | 184 | 43.2 | +20 % | +26 % |
Deflection example
A simply supported W8x40 spanning 13 ft with a 10 kip point load at midspan deflects 0.19 in (E = 29,000 ksi, Ix = 146 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 W8x40 beam weigh?
W8x40 weighs 40 pounds per foot (59.5 kg/m). The number after the "x" in the designation is always the weight in lb/ft. A 20 ft bar weighs 800 lb and a 40 ft bar 1,600 lb.
What are the dimensions of a W8x40?
W8x40 is 8.25 in (209.6 mm) deep with a 8.07 in (205 mm) wide flange. The web is 0.36 in thick and the flanges are 0.56 in thick. Note the actual depth differs from the nominal W8 label — always design with the actual value.
What are the dimensions of a W8x40 in mm?
W8x40 is 209.6 mm deep with a 205 mm wide flange; the web is 9.1 mm and the flange 14.2 mm thick. In metric section properties: Ix = 6,077 cm⁴, Sx = 582 cm³.
How much does a W8x40 weigh in kg?
59.5 kg per meter. A 6 m bar weighs 357 kg and a 12 m bar 714 kg — handy when ordering US shapes from metric suppliers.
What is the moment of inertia of a W8x40?
About the strong axis, Ix = 146 in⁴ (6,077 cm⁴) with section modulus Sx = 35.5 in³. About the weak axis, Iy = 49.1 in⁴ and Sy = 12.2 in³. Use the beam deflection calculator to turn these into deflection and stress for your span and load.
W8x40 or W8x48 — what does one size up change?
W8x48 is about 20% heavier (48 vs 40 lb/ft) but about 26% stiffer (Ix 184 vs 146 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