Measuring filament diameter incorrectly can sometimes give the impression that the filament spool is out of the specified diameter. Knowing the average diameter of a specific spool can also be helpful in fine tuning your print settings. Here's a quick guide that will help you more accurately asses the average spool diameter and the tolerance accuracy of your 3D printing filament.
Important: When measuring your filament, please first and foremost ensure that you are using a high quality caliper with a reading error of 0.01mm or better.
You will need to measure several different points along the spool, so we recommend unrolling at least a 6 meter section of filament for this purpose. (The longer section and more points you measure the more accurate your final result will be, but 6 meters will give you a pretty good idea of the average diameter and tolerance for your spool)
Measure your filament every 2 meters. For a 6 meter section, you will measure at 4 different points within equal distance along the section: 0m, 2m, 4m, 6m
At each of the 4 points, you will need to take 2 measurements at a 90° angle from each other as illustrated below (Measure using the tip of your caliper):
Record your results. You should end up with a set of measurements, see example table below:
| Measuring Point | Measurement #1 | Measurement #2 |
| 0 meters | 1.73 mm | 1.69 mm |
| 2 meters | 1.70 mm | 1.74 mm |
| 4 meters | 1.75 mm | 1.79 mm |
| 6 meters | 1.77 mm | 1.75 mm |
Next, you will need to calculate averages for your measurements as shown in the example table below:
(Measurement #1 + Measurement #2) / 2 = Average point diameter
(Average point 1 + Average point 2 + Average point 3 + Average point 4) / 4 = Average spool diameter
| Measuring Point | Measurement #1 | Measurement #2 | Average point diameter: |
| 0 meters | 1.73 mm | 1.69 mm | 1.71 mm |
| 2 meters | 1.70 mm | 1.74 mm | 1.72 mm |
| 4 meters | 1.75 mm | 1.79 mm | 1.77 mm |
| 6 meters | 1.77 mm | 1.75 mm | 1.76 mm |
| Average spool diameter: 1.74 mm |
Finally, now that you know the average diameter of your spool, you can easily find out the tolerance accuracy of the spool as well:
1.75 mm - Spool average diameter = tolerance accuracy
In the case of the example above, this would be 1.75 mm - 1.74 mm = 0.01 mm tolerance accuracy.
Please keep in mind that although the results you get can certainly be helpful, they are still only based on sampling and of course, cannot be 100% accurate. Most state-of-the-art filament extrusion lines actually measure the filament along the entire length of the spool, which is the only way to produce a true representation of the accuracy and tolerance of the filament spool.