66, from a full kilogram of PLA at 15 g apiece. That is 335.28 m of 1.75 mm filament divided into 5 m pieces, with the remainder rounded down because a partial Benchy is not a Benchy.
From a part-used spool reading 480 g gross on a 160 g cardboard reel — 320 g of filament — you get 21.
Why the round number is always an overestimate
Three things eat into it, and none of them appear in the slicer's per-print figure:
- Purge and prime. Every print starts with a prime line and, on a multi-material machine, a purge. Small on one print, real across sixty-six.
- The last few metres. Filament near the hub is tightly curved and often refuses to feed cleanly, so the practical end of a spool arrives before the arithmetic one.
- Failures. At a 5% scrap rate, sixty-six attempts yield about sixty-three keepers.
Treat 66 as a ceiling and around sixty as what actually comes off the machine.
The same spool in another material
Density does the work here. A kilogram of ABS runs 395.95 m against PLA's 335.28 m, and an ABS Benchy weighs 12.7 g rather than 15 g — so the same nominal kilogram yields meaningfully more boats.
Metal-filled PLA goes the other way, hard: at 38.7 g a hull, a kilogram is closer to twenty-five.
Using this as a planning figure
The useful version of this question is rarely about Benchies. It is "will this spool finish the batch", and the arithmetic is identical: filament remaining divided by grams per print.
Weigh the spool, subtract the reel, divide by your slicer's per-part mass, and round down. That sequence answers the question for any part, and it is what the converter does when you give it a grams-per-print figure.
One refinement is worth making if the batch matters: use the mass you measured on a finished part rather than the mass the slicer predicted. Purge, prime lines and flow error all push real consumption above the estimate, and across dozens of parts that gap is the difference between finishing the batch and stopping one short.
That weighed figure belongs in the per-print box of the length and weight converter. Give it the slicer's prediction instead and the count comes back optimistic every time.