Roll length calculator
Two independent ways to get the length on a roll, side by side. Fill in either one, or both — filling both is the point, because the gap between them is what tells you whether a stated length holds up.
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From the outside
Measure the roll. Quick, and good enough for the shop floor.
From the weight
Needs a scale and the grammage. This is the one that holds up.
Fill in both panels to compare them.
Arithmetic only, on the numbers you enter. The diameter method assumes a constant wound thickness and no core compression, which is not true of foil-bubble or foil-foam. Nothing here is a specification — the grammage, gauge and length tolerance that apply to a construction are confirmed in writing on the quotation.
length (m) = π × (D² − d²) ÷ (4000 × t)D, d and t all in millimetres. A gauge quoted in microns must be divided by 1000 first — the unit selector above does it for you.length (m) = net weight (g) ÷ ( grammage (g/m²) × width (m) )Note the brackets. Dividing by grammage and then multiplying by width is a different sum, and a common slip.- How to check a roll on arrival — where both formulas come from, and what else to measure
- Mil, micron, gsm and denier — the units, and how to convert between them
- Slitting yield calculator — the width side of the same order
Questions
Which of the two methods should I trust?
The weight method, when you have a reliable grammage and an accurate net weight. The diameter method depends on the wound thickness of one wrap, which includes trapped air and, on embossed goods, the height of the pattern — so it always overstates thickness and understates length unless you calibrate it. Use the diameter method for a quick check on the shop floor and the weight method when a number has to hold up.
Why do the two answers disagree?
Usually because the wound thickness you entered is the material gauge rather than the real wound thickness. A roll is never wound perfectly tight: there is air between wraps, and an emboss pattern holds the layers apart. The calculator reports the effective wound thickness implied by the weight method — if that comes out well above your material gauge, the roll is wound loosely or the material is embossed, and that is normal.
My thickness is in microns. What do I enter?
Switch the unit selector to µm and enter it directly. This is the single commonest mistake in this arithmetic: the classic formula expects millimetres, so a gauge entered in microns without converting gives an answer 1000 times too small. The calculator handles the conversion so you cannot make that error here.
Can I use this to check a supplier’s stated length?
That is what the stated-length field is for. Enter what the label claims, and the calculator shows the gap against both methods. A gap inside a few per cent is ordinary measurement scatter. A large gap is worth raising before you convert anything — once material is slit or laminated, establishing what arrived becomes very hard.
Does this work for foil-bubble and foil-foam?
The weight method does, if you have the grammage for that construction. The diameter method is unreliable on cored insulation because the core compresses under winding tension, so the wound thickness is neither the free thickness nor constant through the roll.