Steel beam sizes follow a simple pattern: a letter prefix tells you the shape, the first number tells you the depth, and the second number tells you the weight per foot. A label like W12×26 means a wide-flange beam that is 12 inches deep and weighs 26 pounds per linear foot. Once you learn the prefix system, you can read any steel beam size at a glance.
The Basic Format: Shape, Depth, Weight
Almost all American steel beam designations use the same structure. A letter (or letters) comes first to identify the cross-sectional shape. Then two numbers follow, separated by a multiplication sign. The first number is the nominal depth in inches, and the second is the weight in pounds per foot. That weight figure is important because beams of the same depth come in different thicknesses and strengths. A heavier beam has thicker steel in its web and flanges, which means it can carry more load.
Here are the most common shape prefixes you’ll encounter:
- W (Wide Flange): The most widely used structural beam in American construction. W-beams have an H-shaped cross section with parallel flanges.
- S (American Standard Beam): An older I-beam shape with tapered flanges that slope inward. Less common today but still specified in certain applications.
- C (Channel): A U-shaped section, flat on one side. Often used for framing or as a secondary support.
- L (Angle): An L-shaped section. Sized by leg lengths and thickness rather than depth and weight.
- HSS (Hollow Structural Section): Square, rectangular, or round steel tubes.
Wide Flange Beams (W-Shapes)
W-shapes dominate structural steel framing, so these are the designations you’ll see most often. Take W10×49 as an example. The “W” identifies it as a wide flange. The “10” is the nominal depth of roughly 10 inches (the actual depth can vary slightly). The “49” means it weighs 49 pounds per linear foot. A 20-foot length of this beam would weigh about 980 pounds.
If you look up W10×49 in a properties table, you’ll find additional dimensions: flange width, flange thickness, web thickness, and cross-sectional area. These details matter for engineering calculations, but the two numbers in the designation give you the quick essentials. When comparing two beams of the same depth, say W10×49 versus W10×22, the heavier one has thicker flanges and web, giving it significantly more load-carrying capacity.
S-Beams and Channels
S-beams follow the same depth-times-weight format. An S12×35 is an American Standard beam about 12 inches deep weighing 35 pounds per foot. The key visual difference from a W-beam is the flanges: S-beams have inner flange surfaces that taper at a slope, while W-beams have parallel flanges. This makes W-beams easier to bolt and weld connections to, which is one reason they’ve largely replaced S-beams in modern construction.
Channels (C-shapes) also use depth and weight. A C10×30 is a channel 10 inches deep at 30 pounds per foot. Because channels are flat on one side, they’re useful where you need to attach steel to a wall or another flat surface.
How HSS Sizes Work Differently
Hollow structural sections break from the depth-and-weight pattern. Instead, they list outside dimensions and wall thickness. According to the American Institute of Steel Construction, rectangular and square HSS are designated by overall outside dimensions in inches followed by wall thickness. HSS8×8×3/8 is a square tube 8 inches on each side with a 3/8-inch wall. HSS5×3×3/8 is a rectangular tube, 5 inches by 3 inches, with the same wall thickness.
Round HSS use the same logic but with decimal numbers: HSS5.563×0.258 is a round tube with a 5.563-inch outside diameter and a 0.258-inch wall. The AISC’s 16th edition manual includes properties for 210 new HSS shapes beyond what earlier editions covered, so the range of available sizes continues to grow.
Angles: Two Legs and a Thickness
Angle shapes (L-shapes) use a three-number format: the two leg lengths and the thickness. An L4×3×1/4 has one leg that is 4 inches long, another that is 3 inches long, and both legs are 1/4 inch thick. When both legs are the same length, like L3×3×3/8, you have an equal-leg angle. Angles are typically used as bracing, lintels, or connecting elements rather than primary beams.
What the Detailed Properties Tables Tell You
Once you’ve identified a beam by its designation, a standard properties table fills in the rest. The columns you’ll typically find include total depth (h), flange width (w), web thickness, flange thickness, and cross-sectional area. Beyond those physical dimensions, tables list values for moment of inertia and section modulus, which describe how resistant the beam is to bending. A higher moment of inertia means the beam deflects less under load. A higher section modulus means it can handle a greater bending force before it fails.
You don’t need to calculate these values yourself. Engineers and designers look them up in published tables (the AISC Steel Construction Manual is the standard reference in the U.S.) and match them to the loads a structure needs to support. But understanding what the columns mean helps you compare options. If you’re choosing between two beams and one has a section modulus twice as large, it can resist roughly twice the bending force.
Reading Beam Span Tables
Span tables translate all those properties into practical answers: how far can a given beam span while supporting a given load? These tables typically list beam sizes down the left column and allowable spans across the top, or vice versa. The two critical limits built into every span table are strength and stiffness.
Strength is straightforward: the beam must not break. Both live loads (people, furniture, snow) and dead loads (the weight of the structure itself) count toward this limit. Stiffness is about how much the beam bends. Building codes set maximum deflection limits expressed as a fraction of the span. A limit of L/360 means the beam can deflect no more than its span length in inches divided by 360. For a 15-foot (180-inch) span, that’s a maximum sag of half an inch. Common deflection limits are L/360 for floors, L/240 for roofs, and L/180 for less critical applications.
When a span table says a W8×18 can span 16 feet under a certain load, it means both the strength and stiffness checks pass at that distance. The span is measured face-to-face between supports, not center-to-center.
Metric and European Designations
Outside the U.S., steel beams follow a different naming system. European sections are defined under EN 10365 and use letter prefixes like IPE, HEA, HEB, and HEM. The number that follows is the beam’s total height in millimeters. An IPE 200 is 200 mm deep. An HEA 300 is 300 mm deep with wider flanges than an IPE of the same height.
The geometric properties are the same concepts as American tables: total height (h), flange width (b), web thickness (tw), flange thickness (tf), and root radius (r). British sections use UB (Universal Beam) and UC (Universal Column) prefixes with dimensions in millimeters and weight in kilograms per meter. A UB 305×127×48 is a universal beam roughly 305 mm deep, 127 mm wide at the flange, weighing 48 kg per meter.
If you’re working with drawings or specs from different countries, the prefix tells you which standard to look up. IPE and HEA shapes are cataloged in the Euronorm system, while UB and UC shapes come from the British standard BS 4-1.