Steel is stronger and about three times stiffer than aluminum, while aluminum weighs about a third as much for the same volume and resists corrosion without a coating. For most building structure steel is the cheaper, more compact choice; aluminum earns its place where weight, corrosion or portability matter more than size or price.
The two metals trade off in ways that are easy to get wrong, especially the idea that aluminum is “weaker” across the board. This guide compares them on weight, strength, stiffness, corrosion, fabrication and cost, with a worked beam example, so you can see which one a job actually calls for.
Steel vs Aluminum at a Glance
The figures below compare a common mild steel with 6061-T6, the aluminum alloy most structural extrusions are made from. They come from the University of Florida mechanical engineering design lab’s materials and material selection notes (checked October 2026), which list 1018 steel and 6061-T6 aluminum side by side.
| Property | Steel (1018) | Aluminum (6061-T6) | Aluminum vs steel |
|---|---|---|---|
| Density | 0.283 lb/in³ | 0.098 lb/in³ | About 35% of the weight |
| Modulus of elasticity (stiffness) | 29,000 ksi | 10,000 ksi | About a third as stiff |
| Yield strength (listed) | 53.7 ksi | 40 ksi | About 75% |
| Ultimate strength (listed) | 63.8 ksi | 45 ksi | About 70% |
| Elongation | 50% | 17% | Less ductile |
Those strength values are listed test properties for those two materials, not design values. Structural steel and structural aluminum are each designed with specified minimums from their own standards, covered below.
Weight
At 0.098 lb/in³ against 0.283 lb/in³, a piece of aluminum weighs about 35 percent of an identical piece of steel. That is the main reason people reach for it: truck bodies, trailers, portable gantries, ladders, scaffolding, boat structures and anything that gets carried, lifted or moved.
The saving shrinks once you size the part properly, because an aluminum member usually has to be bigger to do the same job (see the stiffness section next). You still save weight in most cases, but rarely the full two thirds that the density figure suggests. To check what a steel member weighs, use the steel beam weight calculator.
Stiffness: The Difference That Catches People Out
Stiffness decides how much a beam bends under load, and it depends on two things: the material’s modulus of elasticity (E) and the shape’s moment of inertia (I). Steel’s E is 29,000 ksi and aluminum’s is about 10,000 ksi. So an aluminum beam with exactly the same cross-section as a steel one deflects roughly three times as much under the same load.
The modulus barely changes between alloys or tempers, so a stronger aluminum alloy does not fix this. Neither does a stronger steel grade: every structural steel has the same E, which is why switching from A36 to a 50 ksi steel does not reduce deflection. The only real fix is more moment of inertia, which means a deeper or heavier shape.
Worked example: 8-inch steel vs 8-inch aluminum I-beam
Take two 8-inch-deep beams. The steel W8x10, the lightest of the W8 beam sizes, comes from the AISC Shapes Database v16.0; the aluminum I 8 x 6.181 is the Aluminum Association standard shape listed on our aluminum I-beam sizes chart.
| Beam | Weight (lb/ft) | Ix (in⁴) | E (ksi) | Bending stiffness E × Ix (kip-in²) |
|---|---|---|---|---|
| Steel W8x10 | 10 | 30.8 | 29,000 | 893,200 |
| Aluminum I 8 x 6.181 | 6.181 | 59.69 | 10,000 | 596,900 |
The aluminum shape has almost twice the moment of inertia, because aluminum beams are proportioned with thicker flanges and webs. Even so, the steel beam is about 1.5 times stiffer, at roughly 1.6 times the weight. Per pound of beam the two come out close, which matches the UF notes’ point that steel and aluminum have nearly identical stiffness-to-density ratios. Aluminum does not give you a stiffer structure for free; it gives you a lighter one that is bigger.
This example compares geometry and material only. It is not a span or load rating. Any beam that carries a floor, roof, hoist or people has to be sized and checked by a licensed structural engineer, whichever metal it is made of.
Strength
Strength is closer than most people expect. 6061-T6 at around 35 to 40 ksi yield is in the same range as A36 steel at 36 ksi. Modern structural steel is stronger, though: wide flange beams are rolled to ASTM A992 with a 50 ksi minimum yield, according to AISC’s Modern Steel Construction Steel Quiz, November 2022 (checked October 2026), and plate is most often A572 Grade 50. Our guide to the ASTM A992 steel standard explains that grade.
Two things work against aluminum in practice:
- Welding weakens it. Heat-treated alloys such as 6061-T6 lose a large part of their strength in the heat-affected zone next to a weld, and the design has to use that reduced strength. Steel loses little or nothing at a properly made weld.
- Fatigue. Steel has a fatigue limit below which repeated loading does no damage. Aluminum does not, so parts that vibrate or cycle millions of times need careful detailing.
Design Codes Are Different
Steel buildings in the US are designed to the AISC Specification. Aluminum has its own rules. The Aluminum Association’s 2020 Aluminum Design Manual release (checked October 2026) notes that Chapter 20 of the International Building Code requires compliance with Part I of that manual for aluminum construction. Its specification covers weld-affected strengths and buckling rules that differ from steel’s.
In short, you cannot take a steel beam from a steel beam span chart, swap in an aluminum beam of the same depth and expect it to work. The aluminum member has to be designed from scratch.
Corrosion
Aluminum forms a thin oxide film as soon as it meets air, which protects it in most outdoor settings without paint. Bare carbon steel rusts and needs paint, hot-dip galvanizing or another coating outdoors. That gives aluminum an edge for docks, marine structures, signs and exposed frames where repainting is a chore.
Watch the joints. Where aluminum and steel touch in the presence of moisture, galvanic corrosion attacks the aluminum. Separate the two with non-conductive washers, sleeves or tape, and use stainless or suitably coated fasteners.
Fabrication
Cutting and drilling
Aluminum cuts and drills easily with carbide woodworking-style blades and ordinary bits, which suits small shops and site work. Steel needs abrasive saws, band saws or torches and slower drilling.
Welding
Aluminum welding normally needs TIG or spool-gun MIG equipment, careful cleaning of the oxide layer, and a welder experienced with it. Mild steel is far more forgiving. Our guide to MIG vs TIG welding covers the trade-offs between the two processes.
Shapes
Aluminum is extruded, so custom profiles with built-in slots, ribs and screw channels are practical. Steel beams are hot rolled in standard shapes, with hundreds of sizes available.
Cost
Aluminum costs more per pound than structural steel, and since an aluminum member is usually larger than the steel member it replaces, the gap on a finished structure is often wider than the per-pound price suggests. Aluminum can win back some of that on lifetime cost where it avoids painting, and on transport and installation where weight drives the price. Get current quotes for both, because metal prices move.
Which One Should You Use?
| Job | Usually better choice | Why |
|---|---|---|
| House or garage beams and columns | Steel | Smaller sections, lower cost, standard design tables |
| Portable gantry, ladder, scaffold | Aluminum | Light enough to move by hand |
| Dock, marine or coastal frame | Aluminum (marine alloys) | Corrosion resistance without paint |
| Truck bodies and trailers | Aluminum | Weight saving raises payload |
| Heavily loaded or vibrating machinery | Steel | Stiffer, better fatigue behavior, welds keep strength |
| Tight depth limit with heavy load | Steel | Three times the stiffness in the same space |
Steel vs Aluminum FAQ
Is aluminum stronger than steel?
No, not for structural grades. 6061-T6 aluminum is in the range of A36 steel, but structural beams today are 50 ksi steel, and aluminum is only about a third as stiff. Aluminum is stronger for its weight, not stronger outright.
How much lighter is aluminum than steel?
For the same volume, aluminum weighs about 35 percent as much as steel (0.098 vs 0.283 lb/in³). A properly sized aluminum member is larger, so the real saving is smaller.
Can I replace a steel beam with an aluminum one of the same size?
Not safely without a full redesign. A same-size aluminum beam deflects about three times as much and follows a different design code. A licensed structural engineer has to size it.
Does aluminum rust?
Aluminum does not rust, because rust is iron oxide. It does oxidize, but the oxide film protects it. It can still corrode in salt water, in contact with some chemicals, or where it touches steel in wet conditions.
Is aluminum more expensive than steel?
Yes, per pound and usually per finished member. It can still be the cheaper choice over time where painting, transport or handling costs are high.
Reference information only, not engineering advice. Disclaimer.