STL has no units, and it will cost you money

The STL format stores triangles as three floating-point numbers each. It does not store a unit. Not “defaults to millimetres” - it genuinely does not record one, and neither did the CAD system that exported it care.

So when a file arrives containing a part 3.15 units long, you do not know whether that is a 3 mm pin or an 80 mm bracket exported in inches.

Why it is worse than it sounds

Material scales with volume, which scales with the cube of length. Get the unit wrong by 25.4 and you are wrong on material by a factor of about 16 400.

In one direction you quote a bracket at four cents and someone orders a hundred. In the other you quote a small pin at a price that makes the buyer close the tab and never come back. The second is invisible - you will never know it happened.

Why you cannot just guess from the size

You can guess, and a good guess helps. A part whose longest dimension is 0.4 units is probably not in millimetres; a part 300 units long is probably not in inches. But the ambiguous middle is wide, and it contains most real parts: something 60 units long is entirely plausible as 60 mm and entirely plausible as 60 inches if it is a large fixture.

Silently guessing in that range is how you get the four-cent bracket.

What to do instead

Guess, but say so. Make an inference from the size and the format, then show the buyer what you assumed and let them change it in one click. “We read this as millimetres - 80 × 50 × 40 mm. Not right?” is a question a buyer can answer in a second, because they know how big their part is.

Show the dimensions, in their units. The single most effective check is putting “80 × 50 × 40 mm” in front of someone who designed the part. They will notice immediately if it is wrong.

Record where the unit came from. Whether it was guessed, taken from the file format, or stated by the buyer. When something does go wrong, that is the field that tells you whether it was your inference or their answer - and it means a part re-quoted later does not silently re-guess.

Prefer formats that say. 3MF records units. STEP records units. If you can encourage buyers towards them, do - but you will still receive STLs, because everyone does.

The broader pattern

Units are one instance of a general rule about uploaded files: make your assumptions visible and correctable at quoting time. The same applies to orientation, to which process you assumed, to whether a part with a sealed cavity is meant to be sealed. Every one of these is a question that costs a second to answer before the quote and a refund to answer after it.

What the widget does with an uploaded file →