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.
Runs in your browser. Nothing is sent anywhere, no sign-up.
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.About this video
How to use the free roll length calculator at www.leetaiheng.com/tools/roll-length-calculator/. It gives two independent ways to get the length on a roll, side by side, and runs in your browser with no sign-up. Method A, from the outside: enter the outside diameter and core diameter in mm and the wound thickness of one wrap in µm or mm. That is not the material gauge: the wound thickness includes trapped air and emboss height. Length (m) = π × (D² − d²) ÷ (4000 × t), with D, d and t in millimetres; a gauge quoted in microns is divided by 1000 first, and the unit selector does it for you. Method B, from the weight, is the one that holds up: enter the net weight of the roll only (deduct the core and the packaging), the grammage from the data sheet or a weighed cut square, and the roll width. 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. Worked example with the page’s own example values: 300 mm outside diameter, 76 mm core and 120 µm wound thickness give about 551 m; 42 kg net weight, 180 g/m² and 1,400 mm width give about 167 m. Filling in both is the point. Under 3% apart the two agree; 3–12% is normal if the roll is wound loosely or the material is embossed; 12% or more is worth checking; a factor of 2 or more means one of the inputs is wrong, almost always a unit. The example values are 3.3× apart, and the weight method implies an effective wound thickness of 397 µm. Enter a supplier’s stated length to see the gap: within 2% is normal scatter, and a large gap is worth raising before you convert anything. The calculator is arithmetic only. Example values are not LTH product specifications; the grammage, gauge and length tolerance that apply to a construction are confirmed in writing on the quotation.
- 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.