Pick one unit standard before assets leave 3ds Max

In short

• Consistency is the key. The unit you pick matters less than everyone using the same one, written down and applied the same way, every time.
• The system unit is the scene; the display unit is only how you read it.
• Millimetres suit studios that start from CAD and Revit. Centimetres suit anyone who might take assets into a real-time engine.

Consistency matters more than the unit

Ask five studios whether they work in millimetres or centimetres and you'll get both answers, each with a good reason. Many work in millimetres, because CAD and Revit handovers arrive that way. Neither is wrong.

What goes wrong is the mix. One artist builds in centimetres, another merges a model built in millimetres, and the scene quietly fills with objects at ten times or a tenth of their real size. Nothing errors. It just looks slightly off until a render, a collision or a client measurement shows it. A mediocre standard applied consistently will cost you less than a perfect one applied by half the team.

Display units are a view; system units are the scene

3ds Max has two settings people confuse. The display unit changes how measurements are shown to you. The system unit sets what a unit in the scene actually is, and that's the one that decides the real size of anything you export or merge. If you only change the display unit, the numbers on screen look right while the file stays on the old scale.

Set the system unit before you create or import anything. Changing it in a scene that already has geometry is how people end up with half a scene at the wrong size.

What the mismatch dialog is really asking

Open, merge or XRef a file built in different units and Max may stop you with the units mismatch dialog. You choose between rescaling the incoming objects to your scene, or adopting the file's units. Autodesk's own documentation points out that adopting the file's units can lose texture coordinates on UVW Unwrap objects, and that rescaling is the safer choice in that case. For a shared library, my default is rescale, so your scene's standard never changes underneath you. That is consistency again: the scene keeps one unit, and incoming models adapt to it.

Millimetres if you start from CAD, centimetres if anything real-time is coming

Unreal Engine treats one unit as one centimetre. A model authored in a centimetre system unit therefore lands at the right size with no conversion. A model authored in millimetres needs a scale step on the way in, and a scale step is where mistakes live: done once by one person it's fine, done inconsistently across a team it isn't.

If you never leave 3ds Max and your work starts in Revit or CAD, stay in millimetres; you'd be adding a conversion at the front to save one at the back. If you expect real-time work, even in a year or two, centimetres remove the conversion at the point where it's easiest to get wrong. Reports on the exact export settings differ between versions and workflows, so don't take any guide's word for it, including this one. Test it.

Test with a reference object before you trust a pipeline

Build a box exactly one metre long and a door-height box. Merge both into a clean scene, export them the way you'd export for the next stage, and measure them on the other side. If the metre isn't a metre, find out why now. This takes a few minutes and settles an argument that otherwise costs a project.

Decide at the library door, not in the scene

The cheapest place to enforce a standard is when a model enters your library, not when it lands in a project at the wrong size. Every model from a marketplace has an author with their own habits, so every one needs checking: real-world size, units, pivot, front. SAM's Clean-up does exactly this for models in your library, bringing them to real-world size so the unit question has one answer.

Whatever you use, make "checked on entry" the rule rather than "fixed when someone notices".

Write it down

A standard nobody can find isn't one. A single page is enough:

• System unit and display unit for new scenes
• The unit models must be in to enter the library
• Pivot position and front direction for placed models
• The folder and naming rule for each model
• The reference-object test, and who last ran it

Your next step

Open your scene template and check the System Unit Setup. If the answer isn't the one you'd defend in front of a colleague, that's the first thing to fix, and the second is making sure everyone else's template says the same.

Further reading — 3dsky models never arrive consistent: the fix · Revit to 3ds Max: which format actually works for ArchViz? · How to relink 3ds Max files without opening them

Sources — Autodesk: System Unit Setup dialog