Aluminum vs Steel Weight: Side-by-Side Comparison
Reviewed by Jerry Croteau, Founder & Editor
Table of Contents
I Grabbed the Wrong Beam Once
So I was on a job site maybe six years ago, helping frame out a mezzanine for a warehouse retrofit, and somebody had ordered aluminum box beams instead of steel. I didn't realize it until I went to lift one and it practically flew off the stack. I remember thinking, "there's no way this is right" — and it wasn't, not for that application. But it got me obsessing over the weight differences between aluminum and steel, because honestly, I'd been treating them as interchangeable in my head for way too long.
That mistake cost about 3,400 in returns and re-orders.
The weight difference between these two metals matters more than most people think, and it shows up everywhere — from structural loads to shipping costs to how many guys you need on a lift. So yeah, I figured it was time to lay it all out side by side.
The Numbers, Plain and Simple
Steel is roughly three times heavier than aluminum. That's the headline. But the actual numbers depend on which alloy you're talking about, because "steel" and "aluminum" are really families of metals, not single materials. Mild steel (the stuff you see on most construction sites) has a density of about 490 lbs per cubic foot. Aluminum — the common 6061 alloy — sits around 169 lbs per cubic foot. Stainless steel is even heavier, pushing 500 lbs per cubic foot depending on the grade.
Here's a comparison table that I keep bookmarked on my phone:
| Material | Density (lbs/ft³) | Density (g/cm³) | Relative Weight vs Mild Steel |
|---|---|---|---|
| Mild Steel (A36) | ~490 | 7.85 | 1.0x (baseline) |
| Stainless Steel (304) | ~500 | 8.00 | ~1.02x |
| Aluminum 6061-T6 | ~169 | 2.71 | ~0.34x |
| Aluminum 5052 | ~173 | 2.77 | ~0.35x |
| Aluminum 7075 | ~175 | 2.81 | ~0.36x |
That 0.34x number is the one that sticks with me. An aluminum piece is about a third the weight of the same-sized steel piece. A third! If you're hauling material up scaffolding or calculating dead loads on a roof structure, that difference is enormous.
Use our
Density of mild steel ≈ 490 lbs/ft³
Density of aluminum 6061 ≈ 169 lbs/ft³
Result is in pounds (or kg if you use metric density)
So say you've got a plate that's 4 ft × 2 ft × 0.25 inches thick. First convert that thickness: 0.25 inches is about 0.0208 ft. Volume = 4 × 2 × 0.0208 = 0.1667 ft³. In steel, that plate weighs roughly 81.7 lbs. In aluminum? About 28.2 lbs. You could carry the aluminum version under one arm. The steel one, you're grabbing a buddy or a dolly.
When Weight Actually Changes Your Decision
Not every project cares about weight. If you're pouring a foundation and bolting down steel columns, the heaviness of steel is basically irrelevant — it's sitting on concrete. But there are real scenarios where the aluminum-vs-steel weight question changes everything:
Roof structures and overhead work. I worked on a canopy project where the engineer spec'd aluminum purlins specifically because the existing building couldn't handle additional dead load. We saved something like 1,800 lbs across the whole roof area. That's not nothing — that's the difference between "this building can support it" and "you need to reinforce the walls first." If you're calculating roof loads, our
Shipping and transport costs. A flatbed has weight limits. If you're moving 40-foot lengths of tube steel versus aluminum tube, you can fit way more aluminum pieces per load before you hit the DOT weight cap. I've seen guys save a full extra truck trip on bigger jobs just by switching to aluminum where the specs allowed it.
Portable or temporary structures. Staging platforms, temporary walkways, event structures — aluminum dominates here because crews are assembling and tearing down by hand. Nobody wants to deadlift steel deck panels all day.
Corrosion environments. This isn't strictly a weight thing, but it's related. Aluminum naturally forms an oxide layer that resists corrosion, so in coastal or high-moisture environments you might choose aluminum and skip the galvanizing or painting you'd need on steel. Less coating means less weight (and less maintenance cost). Our
But here's the thing — aluminum costs more per pound. Roughly 2 to 3 times more, depending on the alloy and market conditions. So you're always doing this mental tradeoff: is the weight savings worth the price premium? For a lot of structural work, the answer is no, steel wins on cost and strength. For specialty applications, aluminum earns its keep.
Strength-to-Weight: The Number People Forget
Raw weight doesn't tell the whole story.
Steel is stronger than aluminum in absolute terms — mild steel has a yield strength around 36,000 psi, while aluminum 6061-T6 is about 40,000 psi. Wait, that sounds like aluminum wins, right? But remember, to get the same stiffness (resistance to bending), you need a thicker aluminum section, which eats into some of that weight advantage. The modulus of elasticity for steel is roughly 29,000,000 psi versus about 10,000,000 psi for aluminum. That means aluminum flexes almost three times as much under the same load, and to compensate you end up using more material.
So the real-world weight savings on a structural member isn't always that clean 3:1 ratio. It's more like 1.5:1 or 2:1 once you've sized the aluminum member to match the steel one's performance. Still significant, but not as dramatic as the raw density numbers suggest.
For non-structural stuff — cladding, trim, lightweight framing, railings — you get closer to that full 3:1 savings because stiffness requirements are lower. If you're doing a
Quick Reference for Common Shapes
I put together this cheat sheet because I got tired of looking up individual weights. These are approximate weights per linear foot for some common profiles:
| Profile | Size | Steel (lbs/ft) | Aluminum (lbs/ft) |
|---|---|---|---|
| Round tube | 2" OD × 0.125" wall | ~2.67 | ~0.91 |
| Square tube | 2" × 2" × 0.125" wall | ~3.05 | ~1.04 |
| Flat bar | 2" × 0.25" | ~1.70 | ~0.58 |
| Angle | 2" × 2" × 0.25" | ~3.19 | ~1.09 |
| Round bar | 1" diameter | ~2.67 | ~0.92 |
See how consistent that roughly 3:1 ratio is across shapes? That's because it's driven by density, and density doesn't care about geometry. The
And if you're estimating material costs alongside weight, the
Can I just substitute aluminum for steel in an existing design?
Almost never without re-engineering. Aluminum has about a third the stiffness of steel, so a direct swap means your members will deflect way more under load. You'd need to upsize the aluminum sections, which changes connection details, bolt patterns, and sometimes the overall geometry. Always get an engineer involved — I learned this the hard way on that mezzanine job I mentioned.
How much lighter is aluminum than steel, exactly?
About 65% lighter by volume. A cubic foot of aluminum weighs roughly 169 lbs versus 490 lbs for mild steel. So for the same-sized piece, aluminum is approximately one-third the weight.
Is aluminum strong enough for structural construction?
It depends on the application. Aluminum 6061-T6 has a yield strength comparable to mild steel (around 40,000 psi vs 36,000 psi), but its lower stiffness means it bends more easily. It's commonly used in lightweight structures, curtain walls, canopies, and pedestrian bridges. For heavy structural work — columns, primary beams, moment frames — steel is still the standard. The exceptions tend to be situations where weight is the driving constraint, like rooftop additions on older buildings.
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