Length of Tape on A Reel

Short of having someone sit down and individually unspooling and counting the individual devices on the tape, then no there isn't an accurate way of accounting for how much there is on a reel. There will some discrepancy in using the weight/counting method as there is the weight of the empty tape "tail" which will skew the numbers. Most surface mount components are the size and weight of roach poop.

The most efficient method is to keep track of the usage of the components on a given reel. Then reducing that number by 5% and writing that quantity on the reel. Good component loading robot operators do this as a matter of course. the percentage reduction is to account for components "lost" in the process of loading the board.

For most components like resistors and capacitors the money value is not sufficient to warrant more than a reasonable guess as to how much is left on the reel.

For most audit valuing purposes near enough is good enough.

There are two ways to do this PROPERLY depending on your need for accuracy.

Almost every reel of SMT components is marked on the spokes with a series of numbers that represent the percentage of a full reel that remains. This is "easy" but not precise.

The other is to us the fact that components are in EXACT correlation with the sproket holes in the tape. That is, length of tape is a precise means to measure the number of components.

At previous employer we used "reel counters" that ran the components "reel to reel" and counted the sproket holes from first part to last part. These machines are relatively cheap to hire. Alternatively, carefully unspool the tape and measure the length. The pitch will be very accurate and you can calculate the count accurately.

In practice using 5% as your Fudge factor is plenty. Remember that it does become accumulative (or compounded) over time/usage.

As Just an Engineer (GA to you) suggested you can use the remaining tape length divided by the real estate the component occupies on the tape to divine a component count.

With My previous employer we weren't so concerned with accuracy of the resistor/capacitors(unless they were really really special) quantity count but certainly with the High value components we kept a close usage count.

On the identifying labels on the reels, there will be the original supplied quantity count, failing that you can (via google) dig up the data sheets on the various components. There at the end of the data sheets you will find the "packaging" information as how the components are shipped and in what nominal quantities.

Of course it begs the question what did your firm do before you came along?

Sorry, but experience says weight doesn't work in this case. Spools are often made from recycled plastic and their material density is not uniform. The difference in weight is significant. Even the weight of the adhesive stickers used for inventory identification represent a significnat variation. The plastics of the spools and some tapes absorb humidity and others don't. Some carrier tapes are cardboard, some are plastic and their width (and thus weight) are not critically controlled.

The components are linked precisely to the pitch of the sproket holes in the feeder tape. Tape length or "reel counters" are the only reliable method.

A "large" component would be 0.08" long and 0.05" wide around 0.01" thick. These are supplied in cardboard tape that is 0.3" wide and 0.04" thick with a peelable transparent tape covering the components in small punched pockets. Our small bits were 0.04" long by 0.02" wide, also supplied in 0.3" wide tape.

We did twice yearly stocktake with over 500 open spools. The reel counters were simple to use, relatively fast and didn't strip the cover tape off components during the process. The sproket wheels on the counter are connected directly to a rotary encoder with 1:1 reading.

With partial reels, there is also a section of "empty" tape at the leading edge that varies in length depending on the opinion of the technician that unloaded the feeder. Part weight is insignificant relative to the variation in all the other materials involved.