Red filament, single spools, rated 3.5 stars and up, ranked by £/kg — on offer now

What the per-kilo number actually tells you

Filament is sold by the spool, advertised by the discount, and used by the gram. Those three facts are why a spool at “40% off” can be poor value and an unremarkable listing can be the cheapest thing on the page. The only number that compares two listings honestly is what a kilogram costs you, so that is what this page ranks on.

A few things follow from that. Multi-spool bundles are counted at their total weight, so a 4 × 1 kg pack competes directly with a single spool rather than looking four times more expensive. Where a listing does not state a weight we can trust, we leave the per-kilo figure blank instead of guessing — a wrong ranking is worse than a missing one.

Watch for the spool itself. A “1 kg” spool means a kilogram of plastic; the reel is extra and does not count. Refill spools (no reel) are usually a little cheaper and need a holder you already own. And a 750 g spool priced like a 1 kg one is a third more expensive per kilo than it looks — this is the single most common way filament pricing misleads people.

What actually drives the price

Mainstream PLA and PETG from the big value brands sits in a fairly narrow band, and the cheapest listing on any given day is usually just whoever is running a promotion. Paying more buys you, roughly in order of how much it matters:

• Tolerance and consistency. A tighter diameter tolerance (±0.02 mm rather than ±0.05 mm) and properly wound spools mean fewer failed prints. This is the difference you actually notice.
• Additives. Silk, matte, carbon-fibre and glow filaments cost more because the pigment or filler costs more. Carbon fibre also eats brass nozzles.
• Brand. Beyond the two points above, a lot of the premium is brand. Bambu Lab filament is good and priced accordingly; it is not several times better than a solid mid-market spool.

What does not justify a premium: colour, packaging, or a vacuum bag you will throw away. Buy the cheapest spool from a brand that winds its filament properly.

Diameter, and the one mistake worth avoiding

Almost every current printer takes 1.75 mm. A small number of older and industrial machines take 2.85 mm (often labelled 3 mm). They are not interchangeable and the wrong one will not feed. Check before you order, particularly on a bundle — a four-spool pack in the wrong diameter is an expensive mistake, and the listing title is not always clear about it.

How much should you buy at once?

Bundles are usually the better per-kilo price, and filament does not go off if it is kept dry. But PLA and especially PETG, TPU and nylon absorb moisture from the air, and wet filament prints badly — stringing, popping, rough surfaces, weak parts. If you print occasionally, buying four spools to save a pound a kilo is a false economy unless you store them properly. See our guides on filament storage and whether you need a dryer.