How do I Determine Number of Parts on a Partial Reel?

By Mike Bailey

Updated September 26, 2017

Surface-mount components are commonplace in modern electronic products.
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Modern electronics rely heavily on tiny surface-mounted components, enabling designers to develop ever-smaller, mass-produced convenience products. Comparing the size and weight of early cellphones with the 2011 versions brings the transformation into sharp relief. Many surface-mount devices are now packaged in continuous taped lengths, supplied on reels that fit directly to feeders for assembly machines. Interrupting a production run to change an empty feeder can be a major inconvenience, and operators loading partial reels need a means of checking that there are sufficient components left to complete an order.


Examine the sides of the reel. Note whether there are quantity indicators molded into the spokes of the reel. For example, the numbers one to five may be molded along each spoke from the center of the reel outward. If there are no quantity indicators, skip the remainder of this section.

Ensure the taped components are wound tightly onto the reel. Tighten the tape by hand if it is loose.

Check the side of the reel to see which of the indicators lines up with the outer diameter of the tape. Note the number that is closest and interpolate if necessary. For example, if the outer diameter of the tape lies mid-way between four and five, you should note 4.5.

Divide the number you noted by the outer-most number and multiply the answer by the number of components on a full reel. Note the result, which is an estimate of the number of components on the partial reel.

Check your math. For example, if you noted 4.5 and the outer-most number is five, then your answer is 0.9. If there are 3,000 components on a full reel, the result is 2,700 components.


Weigh an empty reel and note its weight in ounces. Cut a short length of taped components from the partial reel and weigh it, noting its weight in ounces.

Count the exact number of components in the cut length. Weigh the partial reel of components and record its weight in ounces. Subtract the weight of the empty reel from the weight of the partial reel, then divide by the weight of the cut length. Note this intermediate answer.

Multiply your answer by the number of components in the cut length. Record the result, which is the number of components on the partial reel.

Check each step of your work. For example, if the weight of an empty reel, the cut length and the partial reel are 1 ounce, 1/4 ounce and 16 ounces, respectively, then the intermediate answer is 60. If there are 40 components in the cut length, there are 2,400 components on the partial reel.

Items you will need

  • Partial reel of components

  • Calculator

  • Weigh scale

  • Scissors


Use scales designed for weigh-counting to speed up the process.


Do not rely on estimated component counts when planning critical production runs. If possible, either weigh-count the reel or substitute a new full reel of parts for a partial reel.