Aluminum vs Steel Weight: How Much Lighter Is Aluminum, Really?
By Lucius Fox
Aluminum is about one third the density of steel: 6061 is 0.0975 pounds per cubic inch against roughly 0.284 for A36 carbon steel, which makes aluminum about 66 percent lighter for the same volume. That is the number everyone quotes and it is correct. It is also misleading as a guide to how much weight you will actually save on a part, because aluminum is roughly one third as stiff as steel, and a design that has to match deflection needs more aluminum section. This post gives you the density comparison, the strength comparison, and the stiffness comparison, which is the one that decides real designs.
In Short
- →By volume, 6061 aluminum is 0.0975 lb per cubic inch against about 0.284 for A36 steel. Aluminum is roughly one third the density, or about 66 percent lighter.
- →By part, the saving is almost always less than 66 percent, because aluminum is about one third as stiff and you usually need more section to match deflection.
- →For equal strength, aluminum does well. 6061-T6 yields around 40 ksi against A36 steel at 36 ksi, at one third the weight.
- →For equal stiffness in bending, aluminum needs roughly 1.44 times the thickness of steel, which lands you near 50 percent weight saving rather than 66.
- →7075-T651 changes the picture. At roughly 73 ksi yield it beats mild steel on strength outright while staying at one third the density.
- →Stainless is essentially the same weight as carbon steel. Titanium is between the two at about 0.160 lb per cubic inch.
Density: The Simple Comparison
Same volume, different materials. This is where the one third figure comes from.
| Material | lb/in3 | g/cm3 | Relative weight | 1 cu ft weighs |
|---|---|---|---|---|
| 6061 aluminum | 0.0975 | 2.70 | 1.00x | 168.6 lbs |
| 7075 aluminum | 0.102 | 2.81 | 1.05x | 175.4 lbs |
| Ti-6Al-4V titanium | 0.160 | 4.43 | 1.64x | 276 lbs |
| A36 carbon steel | 0.284 | 7.85 | 2.91x | 491 lbs |
| 304 stainless steel | 0.289 | 8.00 | 2.96x | 499 lbs |
Strength: Aluminum Does Better Than People Expect
If your design is strength-limited rather than stiffness-limited, aluminum is extremely competitive, and the high-strength alloys beat mild steel outright.
| Material | Typical yield (ksi) | lb/in3 | Strength to weight |
|---|---|---|---|
| 6061-T6 | 40 | 0.0975 | 410 |
| 7075-T651 | 73 | 0.102 | 716 |
| A36 steel | 36 | 0.284 | 127 |
| 304 stainless (annealed) | 30 | 0.289 | 104 |
| Ti-6Al-4V | 128 | 0.160 | 800 |
Stiffness: Where the 66 Percent Disappears
This is the part most weight comparisons skip, and it is the one that governs plates, panels, brackets, and anything where deflection rather than fracture is the limit.
- Aluminum's elastic modulus is about 10 million psi. Steel is about 29 million psi. Aluminum is roughly one third as stiff, which is almost exactly the same ratio as its density.
- For a beam or plate in bending, stiffness scales with the cube of thickness. To match steel's stiffness you need aluminum thicker by the cube root of about 2.9, which is roughly 1.44 times.
- Take that thicker aluminum section at one third the density and you land near half the weight of the steel part, not one third.
- So a realistic rule: strength-limited parts save close to the full two thirds. Stiffness-limited parts save closer to half. Both are worth having.
- Where aluminum wins big is when the extra thickness is free. If there is room in the envelope, the thicker aluminum section costs nothing but material.
A Worked Comparison
A flat plate, 12 by 24 inches, that has to carry a load in bending. Steel baseline against an aluminum design matched for stiffness.
| A36 steel | 6061 aluminum, stiffness matched | |
|---|---|---|
| Thickness | 0.500 in | 0.720 in |
| Volume | 144 cu in | 207 cu in |
| Density | 0.284 lb/in3 | 0.0975 lb/in3 |
| Weight | 40.9 lbs | 20.2 lbs |
| Weight saving | baseline | 51 percent |
The Other Reasons People Switch
Weight is the headline, but it is rarely the only reason a part moves from steel to aluminum.
- Corrosion. Aluminum forms a passive oxide layer and does not rust. On outdoor or marine parts this frequently matters more than weight, and it is why 5052 and 5083 exist.
- Machinability. Aluminum cuts far faster than steel, so machining time and tooling cost per part drop substantially. On a machined part this can offset a higher material price entirely.
- Thermal conductivity. Aluminum conducts heat roughly three times better than steel, which is why heat sinks and cold plates are aluminum.
- Reasons to stay with steel: higher stiffness in a fixed envelope, better wear resistance, weldability with common shop equipment, lower material cost per pound, and better performance at elevated temperature.
Running Your Own Numbers
- Compute the steel part weight: volume times 0.284.
- Decide whether your part is strength-limited or stiffness-limited. This is the fork in the road.
- Strength-limited: keep similar section, compute aluminum weight at 0.0975 or 0.102, expect around two thirds saving.
- Stiffness-limited in bending: multiply thickness by about 1.44, then compute at aluminum density, and expect around half.
- Then price it. Aluminum costs more per pound and less per machined part, so compare finished cost rather than material cost.
Aluminum is one third the density of steel and that part is not in dispute. Whether your part gets one third lighter depends on whether it is limited by strength or by stiffness. Strength-limited designs capture most of the theoretical saving. Stiffness-limited designs typically land near half. Either way it is a substantial saving, and the corrosion and machining advantages often decide the switch before the weight does.
Frequently Asked Questions
How much lighter is aluminum than steel?
By volume, about 66 percent lighter. 6061 aluminum is 0.0975 pounds per cubic inch against roughly 0.284 for A36 carbon steel, so aluminum is about one third the density. An actual part usually saves less, closer to half, if the design is limited by stiffness rather than strength.
Is aluminum stronger than steel?
Per pound, yes, comfortably. Per cubic inch, mild steel and 6061-T6 are broadly comparable at about 36 and 40 ksi yield, and 7075-T651 at about 73 ksi beats mild steel outright. High-strength alloy steels far exceed any aluminum, so the answer depends entirely on which steel.
Why is my aluminum part not 66 percent lighter?
Because aluminum's elastic modulus is about one third of steel's, so matching stiffness requires more section. Bending stiffness scales with the cube of thickness, so you need roughly 1.44 times the thickness, which brings the weight saving down to about half.
Is stainless steel lighter than carbon steel?
No. 304 stainless is about 0.289 pounds per cubic inch against roughly 0.284 for A36 carbon steel, so stainless is very slightly heavier. Stainless is chosen for corrosion resistance, not weight.
How does titanium compare on weight?
Ti-6Al-4V is about 0.160 pounds per cubic inch, roughly 64 percent heavier than aluminum and about 44 percent lighter than steel. Its strength to weight ratio is the best of the three common structural metals, which is why it is used where weight matters enough to justify the cost and the machining difficulty.
Compute both
Weigh your part in either material.
Run the dimensions through the calculator, then price the aluminum version cut to size with certs included.