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Randozi
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Cable Reel Length Calculator

Before you order a drum, or to estimate what is left on one, work out how much cable a reel holds. Enter its dimensions and the cable diameter and read the length, the number of layers and the turns.

Cable that fits

282.7 m

928 ft · 10 layers × 30 turns = 300 turns · full reel Ø 400 mm

How it works

  1. Measure the flange diameter, the core (drum) diameter and the usable width between the flanges.
  2. Enter the outer diameter of the cable and choose millimetres, centimetres or inches.
  3. Optionally enter the clearance you want to keep below the rim, and the length you need to wind.
  4. Read the length in metres and feet, the layers, the turns per layer and whether your length fits.

What people use it for

  • Choosing a reel

    Check that a drum is big enough for the run you plan before you buy the cable.

  • Ordering a drum for an install

    Compare two reel sizes for the same cable and pick the smaller one that is enough.

  • Reeling in a long extension lead

    Estimate how much a hose or cable reel can really take, not what the label says.

Questions

What formula is used?
Layers = how many cable diameters fit in the depth between the core and the rim (minus your clearance). Turns per layer = how many cable diameters fit in the width. Each layer is a circle: the average diameter of layer k is the core diameter plus the cable diameter times (2k − 1), and the length is π times the turns times the sum of those diameters, which simplifies to π × turns × (layers × core + cable × layers²). Example: 400 mm flange, 200 mm core, 300 mm wide, 10 mm cable gives 10 layers of 30 turns, 282.7 m.
Why is the real length often a little different?
The formula assumes perfectly tight, regular winding with no gaps and a cable that does not flatten. In practice, winding is looser, the cable is not perfectly round and the first layer rarely lies flat. Treat the result as an upper estimate, and keep a margin: the clearance field is there for that.
What should the clearance be?
It is the space you want to leave between the last layer and the rim of the flanges so the cable cannot slip off. The tool starts at 0. Many reels are rated for a fill that stays below the rim; if the maker states one, enter it.
Can it work out how much is left on a partly used reel?
Not directly. Enter the current outer diameter of the wound cable as the flange diameter and the width as it is, and the result is what is on the reel at that moment.