Two different numbers, both called the size
A filament listing usually carries two figures that people conflate. The diameter, normally 1.75mm or 2.85mm, describes the strand and must match your printer’s feed path and hot end. The weight, normally 500g, 750g or 1kg, describes how much filament is on the spool.
Neither tells you whether the spool fits your machine. That is a separate question about the physical reel, and it is the one that catches people out.
Net filament weight is not the package weight
A 1kg spool means one kilogram of filament, not a one kilogram package. The empty reel adds anywhere from around 150g to over 250g depending on whether it is cardboard, moulded plastic or a heavier reusable design.
This matters when comparing prices. Cost per kilogram of actual filament is the meaningful comparison, and a low headline price is not a saving if the material is unsuitable or produces failed prints.
The four measurements that decide whether it fits
Physical compatibility comes down to four dimensions, and a product page does not always publish them. Measure the space your printer actually provides rather than assuming a standard, because spool geometry varies between manufacturers and sometimes between product lines from the same manufacturer.
| Measurement | Why it matters |
|---|---|
| Outer diameter | A large reel can foul the frame, the enclosure lid or an adjacent spool |
| Spool width | Too wide and it will not sit in the holder or the material bay |
| Hub or bore diameter | The centre hole must match the holder’s spindle |
| Total loaded weight | Heavier reels can drag on lighter holders and cause under-extrusion |
Automatic material systems are a separate check
A printer supporting a material through an external spool holder does not mean its automatic feeder supports the same material. Automatic systems impose their own constraints on spool geometry, on filament stiffness and sometimes on the material itself.
Flexible filament is the usual example. A machine may print TPU perfectly well fed directly from an external holder while excluding it from the automatic changer entirely. Check the two paths separately, because the manufacturer documents them separately.
Refills need a spool you already own
Refill or coil products ship without a reel, at a lower price, and require the appropriate reusable spool. They are economical if you already have the matching hardware and useless if you do not.
Check which reusable spool a refill is designed for. They are not universally interchangeable.
Sample sizes are worth using
Where a manufacturer offers a small sample coil, typically around 50g, it is a cheap way to test a formulation against your printer before committing to a full spool.
This is particularly worthwhile when moving to a new material family, an unfamiliar brand or a composite grade, where the differences from your usual profile are largest.