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Roll Diameter from Length Calculator

Estimate final roll outside diameter from winding length, film thickness and core outside diameter. Use it for an initial check against reel-size limits on a machine or packing specification.

Use a saved product specification (optional)
01 / INPUTS

Enter your values

First time using this calculator?

Replace the example values with figures from your supplier sheet or measurements. Choose the unit beside each value. Results update as you type; fix any highlighted field before using the result.

GSM means grams per square metre. Net weight excludes the core and other packaging. MOQ means the minimum order quantity. An optional field can stay blank unless the result you need depends on it.

Prices keep the selected currency; changing its label does not convert an exchange rate. Save or export the result after checking your inputs.

Help with film thickness

Use the measured or specified material thickness. One micron (µm) is one thousandth of a millimetre.

Measure the outside of the core, not its bore.

Results update as you edit. Nothing is saved until you choose to.
02 / RESULTExample result — replace the inputs

Estimated roll outer diameter

331.60 mm

Geometric estimate: assumes uniform thickness, a tightly wound roll and no entrained air. Actual running length may differ.

Core OUTER diameter is used; a nominal core bore is not a substitute.

THE CALCULATION

Roll OD (mm) = √(core OD² + 4 × thickness (µm) × length (m) ÷ π)

√(90² + 4 × 80 × 1,000 ÷ π) = 331.6009101 mm
Example result — replace the sample valuesThe formula works, but the values are illustrative.

Saved calculations stay in this browser. No account or cloud backup is created.

Save to a product project (optional)Choose or create a saved product project ↗

This is an illustrative example. Saved or transferred values must be checked against your job.

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Calculated from your inputs. Confirm specifications with your supplier.

How sensitive is this result to your assumptions?

Vary one input and inspect estimated roll outer diameter

All other entered inputs stay fixed. Ranges are your scenarios, not statistical confidence or supplier performance predictions. This uses the same validated calculator engine.

UNDERSTAND THE CALCULATION

The formula, made clear.

Roll OD = √(Core OD² + 4 × Thickness (mm) × Length (mm) ÷ π)

A film strip of known thickness and length occupies a fixed cross-sectional area when wound without gaps. Add that film area to the core cross-section and solve for the outer circle diameter. The relationship is not linear: increasing length does not increase diameter by the same percentage. The core diameter matters most near the start of the winding, while the accumulating annulus carries the additional film.

How to use this calculator

  1. 01

    Enter planned winding length and total film thickness.

  2. 02

    Enter the core outside diameter, not its bore or shaft size.

  3. 03

    Review estimated finished diameter against the permitted reel diameter with an appropriate operational margin.

WORKED EXAMPLE

A calculation you can check

The example predicts the outside diameter of a planned winding on the stated core using an ideal packed-film geometry.

Roll length
1000 m
Film thickness
80 µm
Core OUTER diameter
90 mm
√(90² + 4 × 80 × 1,000 ÷ π) = 331.6009101 mm
Estimated roll outer diameter331.60 mm
HOW TO VERIFY THIS RESULT

Check the displayed formula and units against your quotation. Use measured GSM, net weight and actual layout dimensions where available. Default densities are planning assumptions, not certified material properties. Calculated cost does not certify strength, compatibility or safe loading.

Method, units and material assumptions ↗