How to Calculate Metres per Reel of Single-Wound Polythene Film
When two film reels have different weights or thicknesses, comparing the price per reel can be misleading. A simple calculation gives packaging buyers an estimate of the metres of single-wound polythene sheeting on each reel—and a more useful basis for comparing quotations.
The estimate is a starting point, not a guaranteed running length. It uses nominal dimensions and an assumed density. Confirm the actual net film weight, material and tolerances with your supplier.
What information do you need?
For a single flat layer of film, collect the width in millimetres, nominal thickness in microns, net film weight in kilograms and material density in grams per cubic centimetre. Net means film only: exclude the core, wrapping, pallet and other packaging.
Density varies with the grade and blend. For the worked examples below we use 0.92 g/cm³ as an illustration. Ask for the applicable density for a specific grade; for example, polymer manufacturer Chevron Phillips Chemical lists 0.922 g/cm³ for one LDPE grade. Recycled-content blends and additives may change the figure.
The single-wound film calculation
Approximate length in metres = net film weight in kg × 1,000,000 ÷ (width in mm × thickness in microns × density in g/cm³). This formula is for one flat sheet. It should not be applied unchanged to lay-flat tubing, centre-fold or gusseted film, where more than one layer or additional film width is present.
Worked example: a 25 kg reel
For a single-wound reel 500 mm wide and 50 microns thick, with 25 kg net film and an assumed density of 0.92 g/cm³: 25 × 1,000,000 ÷ (500 × 50 × 0.92) = approximately 1,087 metres.
If the stated 25 kg were gross weight and the core weighed 1 kg, net film weight would be 24 kg. On the same assumptions, the estimate would be approximately 1,043 metres. Check whether a quoted weight is net before comparing reels.
What happens when thickness changes?
At the same 500 mm width, 25 kg net film and assumed density, a 40-micron single-wound sheet gives approximately 1,359 metres—25% more theoretical length than the 50-micron example. This follows from the arithmetic; it does not establish that the thinner film will protect the product or run acceptably on the machine.
Before considering a gauge reduction, agree the performance required for sealing, handling and distribution, and check the proposed material under representative production conditions. Different film blends can behave differently even at the same nominal thickness.
Compare cost per metre and accepted pack
If the 25 kg film costs £2.50 per kg, the film costs £62.50 per reel. Divide that by the estimated 1,087 metres: about 5.75 pence per metre for the 50-micron example. At the same price per kg, the 40-micron example is approximately 4.60 pence per metre. These figures exclude core charges, delivery, VAT and other costs, and are illustrations rather than quotes.
A lower theoretical cost per metre is only useful if the film works. For a fuller comparison, include delivered cost, start-up material, rejects, changeovers and usable output. Where pack length is consistent, divide film and other relevant costs by accepted packs, using the same accounting basis for both options.
Why the actual length may differ
Film thickness and width have tolerances, and the density of the supplied blend may differ from the assumed value. Labels may use nominal rather than measured weight. The end of a reel and start-up material may not become usable packs. Printed or multilayer films need their actual construction considered. Ask the supplier for the specified net weight and length or the density used in their calculation.
Free reel length and diameter calculator
If you know the reel diameter but do not have its net film weight, our free Excel calculator provides another way to estimate the metres remaining. It can also estimate reel outside diameter from a known film length.
Enter the film thickness in microns, core internal diameter and core wall thickness in millimetres. Then enter the reel outside diameter to estimate length, or a known length to estimate diameter. Edit the yellow cells; results update automatically.
For example, a single-layer 35-micron film on a core with a 76 mm internal diameter and 14 mm wall thickness gives approximately 1,914 metres at a reel outside diameter of 310 mm.
This is a theoretical winding estimate. Air gaps, winding tension and film thickness variation affect the result. For lay-flat tubing or folded film, account for every wound layer; gusseted constructions need a separate assessment. The calculator uses diameter rather than weight and does not calculate cost per metre.
Need help comparing film reels?
Stretchband Packaging supplies bespoke single-wound sheeting for automated packaging and industrial applications. Send us your current reel label, film width, thickness, net weight and application; we can help check the estimate and discuss what a fair comparison should include.
