Units and precision

Metric only

Every length in a design is millimetres. Every angle is degrees. There is no alternative unit, no per-field unit suffix and no conversion layer.

units: mm

The units key accepts only mm. It exists so that a document written in inches is rejected with a clear message rather than compiled into a piece of furniture forty times too large.

This is not a preference. Mixed units are one of the classic ways a parametric model goes quietly wrong, and the cost of supporting them — every expression, every catalogue entry and every finding carrying a unit — buys nothing that a converted number at the point of authoring does not.

If you think in inches, convert once, when you write the file: 3/4″ is 19 mm, 1½″ is 38 mm, 4′ × 8′ is 1220 × 2440. The catalogue’s milled sizes are metric actuals, so 19 mm and 25 mm are real thicknesses you can buy.

Degrees rather than radians for angles, because a human writes this file.

Exact inside, rounded only to show you

Internally every length is a floating-point millimetre and stays that way. Nothing is rounded in transit — not between the resolver and the assembler, not between the assembler and the validator, not on the way into an export.

Rounding happens at presentation only: 0.1 mm on the cut list, in the inspector and on a drawing’s dimensions. A part 1636.0000001 mm long reads as 1636 mm, and the validator still saw the number it actually was.

Two lengths closer than 10⁻⁶ mm are the same length as far as validation is concerned. That tolerance sits well below the display precision on purpose: anything you can see in the UI is also distinguishable to the validator.

The rule that matters

A value rounded for a cut list must never silently violate a clearance that passed on the exact value.

Consider a clearance that requires at least 6 mm and comes out at 5.96 mm. It fails, and you are told. Now consider one that comes out at 5.9999 mm against a 6 mm minimum: it fails on the exact number, but a cut list printed at 0.1 mm would say 6.0, and a person reading that sheet would have no way of knowing the design did not pass.

The compiler detects exactly that case — where the exact value is on one side of a limit and the rounded value is on the other — and does not let the rounded figure stand alone. What you read on a sheet and what the validator checked cannot disagree about which side of a limit a number is on.

What this means when you author

Do not pre-round. Write the expression, not the answer:

derived: mattress.width + 2 * mattress_clearance     # yes
value: 1536                                          # no

The second form is correct until something upstream moves, and then it is wrong with nothing to say so.

Do not chase the last 0.1 mm. If a finding says a shelf sags 4.6 mm against a 4.5 mm limit, the fix is a thicker shelf or a shorter span, not a slightly different number. The suggestion in the finding is already computed — it will tell you which.

Trust the material, not the nominal size. The catalogue’s thicknesses are what the board actually measures, not what it is sold as. A “1 inch” board is 19 mm or 25 mm depending on the family, and the resolver will reject a thickness that nothing is milled at rather than let you design around a board that does not exist.

Display, in practice

Where	Precision
Cut list, BOM	0.1 mm
Inspector, part properties	0.1 mm
Drawing dimensions	0.1 mm
Exports (STEP, GLB, STL)	Full precision
Validation findings	0.1 mm, computed from the exact value

Angles display as whole degrees unless they are not whole.