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The short answer

For the common case — 1.75mm PLA — one metre weighs about 2.98g, and a 1kg spool holds roughly 335 metres. Multiplying metres by three gets you within one per cent for PLA — but not for everything. ABS is 2.50g per metre, so the same shortcut overstates it by a fifth. The table below gives the figure for each material.

How many prints is that? Divide 1,000g by the grams your slicer gives for one print, then allow for the waste described below: purge, supports and the prints that fail. A 20g model is about 50 from a spool on paper, and fewer in practice.

Where the number comes from

Your slicer knows the exact volume of plastic in the toolpath, because it generated the toolpath. It converts that to length by dividing by the filament's cross-section, and to weight by multiplying by density. Both of those are settings you can get wrong.

The cross-section is fixed by the diameter. A 1.75mm filament is a circle of radius 0.875mm, so its area is 2.4053 mm² — meaning one metre of it is 2.405 cm³ of plastic. For 2.85mm the area is 6.3794 mm², so a metre is 6.379 cm³: two and a half times as much plastic in the same length, so a figure for one diameter can't be used for the other.

Length to weight

Multiply the volume by the material's density. These are typical published figures for 1.75mm filament:

For 2.85mm, multiply the grams-per-metre column by 2.65 — PLA becomes 7.91g per metre and a 1kg spool holds about 126 metres.

For any other material, diameter or density, the filament length and weight calculator converts both ways and works out what is left on a spool.

These densities are typical figures, not exact ones. They vary by brand and by colour, and filled materials — wood, carbon, glow — vary a lot more, because the filler changes the density and the manufacturer rarely publishes the result. The figure on your spool's own datasheet is more accurate than this table.

Why the estimate is always low

The slicer tells you what the model needs. The spool always loses more than that, and not by a rounding error:

• Purge and prime. A prime line before the first layer, and on a multi-material machine a purge at every colour change. The purge is the big one — on a four-colour print it can exceed the weight of the model.
• Skirt, brim and supports. Some slicers include these in the estimate and some report them separately. Check which yours does before trusting the number.
• The attempt that failed. A first layer that did not stick still used filament. A print abandoned at 80% used 80% of the plastic and produced nothing.
• Flow variance. An over-extruding machine puts down more plastic than the toolpath specifies, and the slicer has no idea. This is why two printers running the same file empty a spool at different rates.
• The tail. The last few metres on a reel are usually unusable — too close to the core to feed reliably. Paid for, never printed.

What to do about it

If you are printing for yourself, the slicer's number is fine. If you are selling, it will make your prices too low, because it assumes every print works and nothing is wasted.

A better estimate won't fix this. Measure what actually comes off the spool instead: weigh the reel occasionally, record the failures as well as the successes, and keep waste as its own figure rather than folding it into the cost of the good prints. There is more on the arithmetic in what a 3D print actually costs to make, and on the waste side specifically in where your filament actually goes.

If you would rather not do it by hand, the print cost calculator does the same sum for one print, including the failure rate, without an account.