You calculate the thickness of aluminum foil by weighing a piece of known area and dividing that mass by the alloy’s density and by the area.
Thickness = mass ÷ (density × area)
The standards print the formula in the units a mill actually works in.
E = M / (10 × A × D)
E is the gauge in micrometres, M the mass in milligrams, A the area in square decimetres and D the density in grams per cubic centimetre.
D is where most calculations go wrong. 2.70 g/cm³ is pure aluminium, while container stock is 3003 or 8011 and EN 546-3 Table 2 assigns each designation its own value: 2.71 g/cm³ for EN AW-8011A, 2.73 g/cm³ for EN AW-3003.
Cut a coupon of about 1 dm² off the coil, measure its sides, degrease it, weigh it to 1 mg and divide.
A 1 dm² coupon cut from a nominal 60 µm EN AW-8011A coil should weigh 1,626 mg.
E = 1,626 / (10 × 1.000 × 2.71) = 60.0 µm
Weigh that same coupon at 1,550 mg and the gauge is 57.2 µm.
The balance sets how fine a call you can make. On 1 dm², one milligram of weighing error moves the answer 0.037 µm; a scale resolving 0.1 g moves it 3.7 µm, more than 6 % of a 60 µm coil.
Every conversion between millimetres, micrometres, mils and grams per square metre is fixed. The calculation gives micrometres, the trade quotes micrometres or millimetres, North American suppliers quote mils, and the mill controls the same foil by substance in grams per square metre.
| Unit | Millimetres | Micrometres | Inches |
|---|---|---|---|
| 1 cm | 10 | 10,000 | 0.3937 |
| 1 mm | 1 | 1,000 | 0.03937 |
| 1 µm | 0.001 | 1 | 0.00003937 |
| 1 mil | 0.0254 | 25.4 | 0.001 |
| 1 inch | 25.4 | 25,400 | 1 |
Substance converts through the same density: gauge in micrometres × density in g/cm³ gives grams per square metre, because one square metre of one-micron foil is exactly one cubic centimetre. A 60 µm 8011A coil is therefore 162.6 g/m². ISO 7271 prints the reciprocal, covering area in square metres per kilogram — 6.15 m²/kg for that same coil — so a certificate may quote any of the three.
The packing list alone gives an average gauge without cutting anything.
Em = P × 10⁶ / (L × W × D)
P is net mass in kilograms, L length in metres, W width in millimetres, D density in g/cm³, and Em the average gauge in micrometres. Run the same three quantities the other way and you get the foil a production run consumes.
Compare the measured gauge with the nominal on the certificate and with the tolerance that belongs to it. EN 546-3 allows ±6 % on a single measurement of single-rolled foil, the class container stock falls in, so a nominal 60 µm coil is accepted anywhere from 56.4 to 63.6 µm on one reading.
The average across a lot is held tighter, and tighter still as the lot grows.
| Lot size | Tolerance on average gauge |
|---|---|
| ≤ 3,000 kg | ±6 % |
| > 3,000 kg to ≤ 10,000 kg | ±5 % |
| > 10,000 kg | ±4 % |
With no certificate to compare against, check the result for plausibility first. EN 546-4 puts container foil, single rolled, at 35 µm to 200 µm and consumer foil at 10 µm to 24 µm; EAFA gives 50–150 µm for a menu tray. A figure outside those bands is usually an arithmetic error rather than a coil.
Treat the 200 µm end loosely: ISO 7271 notes that the top of its own thickness range moves with national convention.
Two things sit outside the number on the certificate. The EN 546 series does not apply to lacquered, painted, embossed or laminated product, so the nominal is bare rolled metal. And ASTM B479 stops below 0.006 in. (0.15 mm), which puts a 150–200 µm smoothwall coil under EN 546-3 or ISO 7271 instead.
Outside tolerance, hold the coil: cut a second coupon from another part of the width, run the reel calculation against the packing list, then take it to the mill certificate before you call the supplier. A coil outside the band will still form containers, but the clearance in the container mould was cut for the nominal gauge.
A micrometer gives the gauge at a point, and that is the routine incoming check.
Zero the instrument, lay the foil flat and clean on the anvil, and close the spindle with the ratchet so the jaws seat without compressing the foil. Read several points across the coil width, both edges and the centre, and record the spread as well as the average. No standard fixes the number of points; that count is agreed with the supplier. A caliper is not an instrument for foil.
When the micrometer and the coupon disagree, the standard treats the weighing result as correct.
With no micrometer at hand, fold a counted number of layers, measure the stack and divide by the count. That multiplies the instrument’s error along with the reading, so treat it as a screen and not as an acceptance test.
The result is only as good as its weakest input, and on a plant floor that is usually the sample, the density or a coating.
