How to Measure a Filament Refill's Inner Diameter
Updated September 22, 2026
When a refill or spool isn't in SpoolNook's dataset yet, or you want to double-check a documented figure yourself, a set of digital calipers and five minutes will tell you more than a product photo. Here's the method we'd use.
What you need
- Digital calipers (a ruler works in a pinch, but is far less precise for a ~50-55 mm measurement)
- The empty refill core, or the spool's hub, on a flat surface
Measuring the refill core (inner diameter)
- Open the caliper jaws wider than the core opening, then close them onto the inside edge of the cardboard or plastic core, at the widest point.
- Take the reading in millimeters. Rotate the refill 90° and measure again - cores are occasionally slightly out of round, and you want the larger of the two readings.
- Record the width of the refill coil itself (the flat caliper jaws span the two outer faces of the wound filament) in addition to the core diameter.
Measuring the spool hub
- With the spool empty (or the refill removed), measure the outside diameter of the hub the refill would sit on - this should be slightly smaller than the refill's inner core diameter measured above, so the refill can slide on.
- Measure the spool's overall outer diameter and width the same way, jaws set to the outside of the flanges.
Reading the numbers
A refill generally needs its core inner diameter to be a close (not loose) match to the spool hub's outer diameter, and its width to be at or under the spool's internal width, for the locking mechanism (clips, threads, or snap features) to actually engage - a close diameter alone doesn't guarantee the lock catches. If you're within a millimeter or two, that's usually consistent with a documented fit elsewhere in our dataset; if you're off by several millimeters, that's consistent with the kind of generation mismatch described in identifying your spool.
Share what you measure
If you've measured a pairing that isn't in SpoolNook's checker yet, tell us - we review corrections and community measurements before adding anything to the dataset, and we'll always note where a figure came from.
