Aluminium round bar weight calculator
Round bar in aluminium alloy: 11 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 per metre of aluminium round bar is roughly d² · 0.00212 kg (d in millimetres): Ø10 weighs 0.212 kg/m, Ø20 0.848 kg/m, Ø30 1.909 kg/m and Ø40 3.393 kg/m. It follows from the circle area π/4 · d² and a density of 2.70 g/cm³. Mass grows with the square of the diameter, so Ø20 is four times heavier than Ø10. A 3 m length of Ø30 weighs 5.73 kg and a 6 m bar 11.45 kg. The common sizes and mass per metre are below.
Common sizes and mass per metre
| Size, mm | Cross-section, mm² | kg/m |
|---|---|---|
| 6 | 28 | 0.08 |
| 8 | 50 | 0.14 |
| 10 | 79 | 0.21 |
| 12 | 113 | 0.31 |
| 14 | 154 | 0.42 |
| 16 | 201 | 0.54 |
| 20 | 314 | 0.85 |
| 25 | 491 | 1.33 |
| 30 | 707 | 1.91 |
| 40 | 1 257 | 3.39 |
| 50 | 1 963 | 5.30 |
Where the number comes from
The cross-section works out as F = π/4 · d². For 16 mm that is 201 mm², which at a density of 2.70 g/cm³ gives 0.54 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 round bar mass by hand
The area of a circle is F = π/4 · d², and multiplying by the 2.70 g/cm³ density leaves a simple rule: mass per metre ≈ d² × 0.00212 kg with the diameter in millimetres. The 0.00212 factor is just π/4 × 2.70 / 1000.
Example: Ø30 bar, 3 m piece.
- Area: π/4 · 30² = π/4 · 900 ≈ 707 mm².
- Mass per metre: 707 × 2.70 / 1000 = 1.909 kg/m (or 900 × 0.00212 = 1.908 — the same figure to rounding).
- A 3 m piece: 5.73 kg; a 6 m length 11.45 kg.
The key property of round bar: mass rises with the square of the diameter. Ø10 weighs 0.212 kg/m, Ø20 four times as much at 0.848 kg/m, Ø40 sixteen times as much at 3.393 kg/m. “A bit thicker” quickly becomes “a lot heavier”: Ø50 is already 5.301 kg/m.
Turning blanks: how much to buy and how much ends up as swarf
Aluminium round bar is mostly bought for turned and milled parts. Then two masses matter: the blank (what you buy) and the finished part (what you sell or fit). The difference is swarf.
- Ø50 × 20 mm disc: π/4 · 2500 · 20 ≈ 39 270 mm³ → 0.106 kg.
- Turning Ø50 → Ø40 over 100 mm: π/4 · (2500 − 1600) · 100 ≈ 70 690 mm³ → about 0.19 kg of swarf.
- Length allowance: add the cut width and face-off to every part; for short parts from a long bar, the bar cutting plan does the sum.
Aluminium swarf has scrap value, so on large batches its mass goes into the costing as a recovery, not just a loss.
Aluminium versus steel round bar
A Ø30 bar weighs 5.549 kg/m in steel and 1.909 kg/m in aluminium (0.344 of the mass). The bending stiffness of a round bar rises with the fourth power of the diameter, and aluminium’s modulus of elasticity is a third of steel’s (70 000 against 210 000 MPa). For an aluminium bar to deflect like a steel one, the diameter has to grow by the fourth root of 3, about 1.32 times — a Ø30 steel bar corresponds roughly to a Ø40 aluminium one. The mass is then about 0.60 of the steel: a smaller saving than with tube, because a solid section uses the metal less efficiently. Where stiffness at low mass matters, aluminium tube is the better choice.
Round bar alloys
Extruded and drawn round bar is made among others in 6xxx alloys (6060, 6082 — 2.70 g/cm³ each in the site’s density table). Highly stressed parts also use 7075: the table describes it as an aerospace alloy for loaded parts that is not weldable. Its density is 2.81 g/cm³, so a Ø30 bar in 7075 weighs 1.986 kg/m, 4.1 % more than the table above. When ordering 7075 bar for machining, use that density — over a few tens of kilograms the difference shows.
Welding round bar
- 7075 does not weld. If the bar is going to be welded, check the alloy before ordering — 6xxx yes, 7075 no.
- Bar to plate. A fillet all round a bar is π · d long: 94 mm for Ø30. A thick bar draws a lot of heat, so aim the arc at the bar, not at the thin plate.
- Bars end to end. On larger diameters the ends are chamfered to a cone so the weld reaches the middle; a Ø30 bar without preparation will lack penetration in the centre.
- Filler. For 6xxx alloys the site’s filler table names ER 4043, and ER 5356 where higher strength or colour after anodising is needed (not above 65 °C).
Welding technique is on the welding aluminium page, and every shape is handled by the aluminium weight calculator.
Common mistakes with round bar
- Radius instead of diameter. Entering 15 instead of 30 gives a result four times too small.
- d² without π/4. A square instead of a circle overstates the mass by 27 %.
- Hex bar as round. An area of 0.866 · s² is about 10 % more than a circle of diameter s.
- Finished part mass instead of blank. You buy bar, not parts — order by diameter and length before machining.
Frequently asked questions
Does the alloy change the weight?
Barely. The density of aluminium alloys runs between 2.66 and 2.80 g/cm³, and trade calculations use 2.70. Across a whole pallet the difference is smaller than the rolling tolerance, so 2.70 is enough for quoting and haulage.
Why does this differ from the weight at the stockist?
Because the dimensional tolerance works both ways, and sheet and section rarely come out at exactly the nominal thickness. On a thin wall the gap reaches a few per cent — order by the tonne with a margin.
How do I work out the mass of a disc or a turning blank?
Volume times density: π/4 · d² · length · 2.70 g/cm³. A Ø50 disc 20 mm thick is π/4 · 2500 · 20 ≈ 39 270 mm³, about 0.106 kg. Swarf is worked out the same way from the difference in diameters: turning Ø50 down to Ø40 over 100 mm removes about 0.19 kg.
Can I use this calculator for hexagon bar?
Not directly. A hexagon of size s across flats has an area of 0.866 · s², about 10 % more than a circle of diameter s. A 30 mm hex bar is therefore 0.866 · 900 ≈ 779 mm² and about 2.10 kg/m, not 1.909 kg/m like a Ø30 round.
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