Aluminium box section weight calculator

Box section in aluminium alloy: 12 common sizes with the mass per metre, and a calculator for your own. Aluminium weighs under 35 % of the steel of the same section.

Short answer

The mass of an aluminium box section per metre is the cross-section times the density of 2.70 g/cm³, with the cross-section F = 2·t·(a + b − 2t). Examples per metre: 20×20×2 — 0.39 kg, 40×40×3 — 1.20 kg, 60×40×3 — 1.52 kg, 60×60×3 — 1.85 kg, 80×80×4 — 3.28 kg. The same 60×40×3 box in steel weighs 4.43 kg/m, so aluminium is 0.344 of the steel mass. The table of 12 sizes is below.

Common sizes and mass per metre

Where the number comes from

The cross-section works out as F = a·b − (a − 2t)·(b − 2t). For 40×40×3 mm that is 444 mm², which at a density of 2.70 g/cm³ gives 1.20 kg/m.

Aluminium is welded with TIG on alternating current or with a wire feeder and aluminium wire; the oxide film melts at around 2050 °C, far above the metal itself, and has to come off with a stainless brush immediately before welding. The detail is in welding aluminium.

How to work out box section mass by hand

The formula F = a·b − (a − 2t)·(b − 2t) subtracts the hollow centre from the full rectangle. Multiplied out, it simplifies to F = 2·t·(a + b − 2t) — the perimeter measured along the middle of the wall times the wall thickness. The second form is easier to do in your head.

Example: 60×40×3 box, 6 m length.

1. Area: 2 · 3 · (60 + 40 − 6) = 6 · 94 = 564 mm². Check with the first formula: 2400 − 54·34 = 2400 − 1836 = 564 mm².
2. Mass per metre: 564 × 2.70 / 1000 = 1.523 kg/m.
3. A 6 m length: 1.523 × 6 = 9.14 kg. A bundle of 20 lengths is about 183 kg — worth knowing before unloading.

The second form also shows why the wall matters most: an 80×80×4 box has 1216 mm² and weighs 3.283 kg/m, while with a 2 mm wall it would weigh just over half that. Outside dimensions change the mass linearly, and so does the wall — but the wall usually changes in whole-millimetre steps, which on thin sections means tens of per cent.

Aluminium versus steel box: what you really save

The same 60×40×3 box weighs 4.427 kg/m in steel and 1.523 kg/m in aluminium — 0.344 of the steel mass. But the aluminium box is three times as flexible: aluminium’s modulus of elasticity is 70 000 MPa, steel’s 210 000 MPa. There are two ways to make up for it, and they end in very different masses:

• A thicker wall with the same outside size. The stiffness of a thin-walled section rises roughly in proportion to the wall, so you would need three times the wall. The mass would come out almost the same as steel (3 × 0.344 ≈ 1.03) — no saving.
• A bigger outside size with the same wall. Stiffness rises with the cube of the size, so enlarging the section about 1.44 times is enough. The mass is then about half the steel (1.44 × 0.344 ≈ 0.50).

The practical conclusion: when swapping steel for aluminium in a frame, go bigger, not thicker. This is a deflection check; the load capacity of structural members is the designer’s job under EN 1999-1-1. The same relationship for plate is covered in the aluminium sheet calculator.

What alloy box sections are made of

Box sections are extrusions, most often in 6xxx-series alloys. The site’s density table describes 6060 as the alloy for extruded profiles and light structures and 6082 for structural profiles and plate with higher strength; both have a density of 2.70 g/cm³, so the table above applies to them without correction. Only an alloy outside that series changes things: a 60×40×3 box in 7075 (2.81 g/cm³) would weigh 1.585 kg/m, 4.1 % more — but the table marks 7075 as not weldable, so it is no good for welded frames.

Welding box sections: what to watch

• Thin walls and burn-through. 2 mm boxes are usually TIG welded on AC; aluminium draws heat away quickly and then overheats suddenly, so burning through at a corner is easy.
• Filler for a 6xxx alloy. According to the site’s filler table, ER 4043 copes better with hot cracking on 6xxx alloys, ER 5356 gives a stronger weld and keeps its colour after anodising, but is not recommended for service above 65 °C.
• Oxide. A stainless steel brush right before welding, including the inside of the edge if the box is mitred.
• Softening next to the weld. 6xxx alloys lose strength in the heat-affected zone — in load-bearing structures the design accounts for it.

The full technique — current, gas, preparation — is on the welding aluminium page, and the aluminium weight calculator handles every shape at once. Round tubes have their own page: aluminium tube.

Common mistakes with box sections

• Inside size instead of outside. Box sections are described by outside dimensions; 40×40×2 is 40 mm across the outside, not the bore.
• Corners taken as sharp. Real boxes have rounded corners, so their area is a little smaller than the formula gives — the result errs slightly high, which is safe for an estimate.
• Guessing the wall. A 40×40×3 box weighs almost one and a half times a 40×40×2 (444 against 304 mm²); measure with a caliper before you order.
• Steel density left in the calculator. A result 2.91 times too high means the material field is still on steel.

Author: Dmitry Frolov, welder and metal fabricator Updated: 15 October 2026