An aluminum coil is sold by weight but specified by dimensions, and the question buyers actually need answered is: given a coil's inner and outer diameter, how much does it weigh, and how many metres of strip are on it? This page gives the aluminum coil weight calculation formulas, the standard core sizes, and the one correction that prevents a planning number from becoming a dispute.
The two formulas
There are two ways into the same number, depending on what you know.
If you know the developed strip length:
Weight (kg) = thickness (mm) × width (m) × length (m) × density (g/cm³)
A 0.5 mm × 1,000 mm coil with 500 m of strip, in 1060 (density 2.71), weighs 0.5 × 1.0 × 500 × 2.71 = 677.5 kg.
If you only know the coil diameters — the common case at the warehouse, where the coil is still wound:
Weight (kg) = π/4 × (OD² − ID²) × width (m) × density × 1000
and the developed length comes out of the same geometry:
Length (m) = π × (OD² − ID²) / (4 × thickness) / 1000
All diameters in millimeters. A coil of 2 mm strip, 1,250 mm wide, on a 508 mm core, wound to 1,400 mm OD, at 2.70 g/cm³ weighs:
π/4 × (1400² − 508²) × 1.25 × 2.70 × 1000 / 10⁹ ≈ 4,511 kg, and carries about 668 m of strip.
The idea is simple: a coil is a ring of wound strip, so the ring cross-section (OD² − ID²) times the width gives the volume, and volume times density gives the weight. The strip length is the same volume divided by thickness.
Standard core and coil sizes
| Parameter | Common value |
|---|---|
| Core ID | 505 / 508 / 610 mm |
| Mill coil OD (max) | 1,800 – 2,000 mm |
| Strip width | 1,000 – 1,500 mm |
| Strip thickness | 0.5 – 8 mm |
| Typical coil weight | 2 – 8 t |
The core ID matters for the buyer because it has to match the decoiler. Tell the supplier the core you need (505, 508 or 610 mm) at enquiry stage — re-coring a slit coil afterwards adds cost and another process step. If your line can only take small coils, say the maximum weight, and the mother coil can be split rather than shipped as one heavy unit.
The two corrections that keep the number honest
Two physical realities make the geometric figure a planning number, not a bill of lading:
- Winding tension. Coil is wound under tension, so the real strip length runs 1–3% under the geometric figure. The weight is unaffected (the same metal is there), but the metres-per-coil you can actually feed are slightly less than the formula says.
- Gauge tolerance. The strip is rolled to a tolerance band, not an exact gauge, so the actual weight drifts a little from the nominal calculation — the same theoretical-versus-actual distinction that applies to sheet.
For quoting, treat the calculated figure as a planning number and let the mill's scale ticket settle the actual tonnage.
How the three variables trade off
A useful way to reason about a coil order: thickness changes the footage, not the weight. A coil of a given OD, ID and width weighs the same whether the strip is 1 mm or 2 mm — the thicker strip simply gives half the metres. So when you compare two coils of the same diameter, the heavier one is not "more material" in the way buyers sometimes assume; it is the same volume at the same density, just thicker.
This is why slit coil is priced per tonne, not per metre. Convert a supplier's per-metre price to a per-tonne price (or the reverse) before comparing two quotes, using the metres-per-tonne figure.
A worked example to check against
Take 1,000 mm wide coil, 1,300 mm OD, 508 mm core:
- Weight: π/4 × (1300² − 508²) × 1.0 × 2.70 × 1000 / 10⁹ ≈ 3,050 kg, regardless of thickness.
- Length at 1 mm thick: the ring area ÷ 1 mm ≈ 1,125 m.
- Length at 0.5 mm thick: double, ≈ 2,250 m.
The diameter determines the weight; the thickness determines how many metres that weight turns into.
The checksum before you commit
When a coil quote lands, check three numbers against each other: the OD and ID give the weight, the weight and gauge give the footage, and the footage and width give the weight again. If all three do not agree to within a couple of percent, one of the inputs is wrong.
For the alloy densities that go into the formula, see the alloy reference pages — the density varies from 2.66 to 2.81 g/cm³ by grade. The weight calculator does coil weight and footage directly if you prefer not to work the formula by hand.