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STEP versus STL: why the format decides the quality of your dimensions

Updated on September 17, 2026 · The Tracista team

An STL contains no circles. It contains triangles that look like a circle. That is the whole explanation for why a 10 mm hole can end up dimensioned as 9.8, and for why we recommend STEP whenever there is a choice.

Two ways of describing the same part

When you model a cylinder in your CAD, the program stores something like "cylinder of radius 5 and height 20". That is an analytic description: exact, compact and with the intent inside it. If you change the radius to 6 tomorrow, it is still a cylinder.

When you export to STEP, that description travels as it is. The file says there is a cylinder, with its axis and its radius.

When you export to STL, the program tessellates: it replaces the curved surface with a pile of flat triangles that approximate it. The cylinder becomes a prism of, say, 64 faces. There is no radius in the file any more; there are 64 faces and their vertices.

Where the 0.2 mm error comes from

When tessellating, something has to decide how far a triangle may stray from the real surface. That parameter is usually called chord deviation or tolerance, and many programs leave it at a value that is comfortable for 3D printing, not for metrology.

The geometry is simple: if you approximate a circle of radius r with an inscribed polygon of n sides, the midpoints of the sides fall inside it. With few sides, the "diameter" a tool measures between two opposite faces is smaller than the real one.

With a 10 mm hole and a coarse mesh, losing one or two tenths is normal. It is not a failure of the program doing the dimensioning: the information was simply no longer in the file.

What cannot be reconstructed

A good algorithm can look at 64 faces arranged in a ring and deduce "this was a cylinder". We try to. And it works reasonably well on clean, perpendicular holes.

What does not come back is:

  • The intent. A 0.5 mm chamfer and a 0.5 mm mesh error look very much alike when seen as triangles.
  • Gentle curves. A large fillet radius and a slightly domed surface are indistinguishable when the mesh is coarse.
  • Symmetry axes on organic parts, which are exactly what anchors the dimensions.

What to do in practice

If you control the export, use STEP. That is the normal case when the model is yours. AP214 or AP242, either works.

If all you have is STL, demand quality from it. Take the chord deviation below 0.01 mm. The file gets bigger and that does not matter: it is uploaded once.

If the STL comes from a third party, measure before trusting it. Open the file and check a couple of dimensions you know. If a hole you know is 10 comes out as 9.8, you now know the margin you are working with and can decide whether it is good enough.

And if the drawing is for manufacturing, have a person review it. This holds for any automatic drawing, from us or from anywhere else. The tool removes the mechanical work; it does not remove the signature.

Summary

STEP carries the geometry; STL carries an approximation of the geometry. Dimensioning an approximation gives approximate dimensions, and no tool can give back information the file no longer holds.