Getting a length from a scale reading.

Start with a gross weight from a certified scale, not a bathroom scale and not a crane readout. Subtract the drum, which is often stencilled on the flange and otherwise has to be estimated from the drum's size and construction. What is left is the cable, still carrying water, dirt and whatever is wrapped around it.

Multiply that remainder by the published length per unit of weight for the exact construction, size and stranding. Tables usually give weight per thousand feet, so the calculation is a division rather than a guess. Run it twice with a low and a high figure from the table rather than once with a single number.

Four things push a weight estimate off, and the drum tare is the largest. A steel reel that has been repainted, or a timber lagging that has taken water, can be far heavier than its stencil suggests. Water inside a jacket, on a wet shield or in a flooded tray adds weight that is not conductor. Banding, lagging and retained wraps add a little more.

Scale accuracy is the quiet one. A yard scale that has drifted, or a reading taken while the truck was still rocking, moves the total by more than most sellers expect. Use one certified scale, write the number down at the time, and weigh the same reel twice when the first result looks wrong.

Counting wraps works well on a full reel with an even, tight, machine-wound layer. It fails on a reel that has been re-reeled, on one with a lumpy outer layer, and on any reel where the last few hundred feet were pulled by hand and wound back badly. On a part-used reel, weight and a thickness measurement are usually more honest.

Weight also wins when the reel is large and a count would take an hour, or when the outer wraps are damaged and not worth counting. A count is better when the reel is small, dry and clearly machine-wound. Where both are available, report both and say which one you trust and why.

A weight estimate is a range, not a measurement. Present it with the method attached and the range will hold up at the scale; present a bare figure and someone will re-weigh the reel and start again.

Write down the scale reading, the drum tare you used and where it came from, the construction you matched and the table figure. A buyer who can see the working will accept a wide range. A buyer handed a bare number will re-weigh the reel and arrive at their own figure, which is usually the one that gets used.

Photograph the scale display next to the reel if the scale is on site. It sounds excessive until the first time a disputed figure has to be settled by looking at a picture taken on the day, rather than by two people remembering a number that nobody wrote down at the time.