Units and Document Settings

Overview#

Every Fusion design stores its own units. The Document Settings node in the Browser "controls the units for the assembly", and a numeric value you type without a unit is interpreted in the design's unit. Getting units right at the start prevents the most common beginner problem: a part that is 25.4 times too large or small when printed or machined.

Behind the scenes, the Fusion API documents that internal values are held in SI units, except that lengths are held in centimeters rather than meters (angles are radians). You never need to work in those, but they explain why scripts and some unit conversions behave the way they do.

Where to find it#

• Document units: in the Browser, the default component (the top component in every design) contains Document Settings (units), Named Views, analyses, and joints for the whole assembly.
• Default units for new designs: profile menu, Preferences > Default Units. Autodesk's reference lists separate pages for Design, Manufacture, and Simulation and Generative Design. Precision and display of units is controlled on the Unit and Value Display page.
• Parameters: Design > Solid > Modify > Change Parameters opens the Parameters dialog, which lists User Parameters and Model Parameters with their Unit, Expression, Value, and Comments.
• Mesh units: the Insert Mesh dialog has a Unit Type selector.

Step by step: set and review units#

1. In Preferences > Default Units > Design, choose the unit you want for new designs.
2. Create a new design. It starts with the unit from your preferences.
3. In the Browser, expand the default component and then Document Settings. Read the unit it shows.
4. Create a sketch rectangle and check how dimension fields show the unit.
5. Open Modify > Change Parameters and look at the Unit column for the parameters your sketch created.
6. Define the unit before modeling anything important.

Note: not confirmed in Autodesk's documentation: the exact list of units offered for the document and the exact dialog you use to change an existing document's unit. The Parameters unit types list millimeter, centimeter, meter, inch, and foot among others, and Insert Mesh offers centimeter, millimeter, meter, inch, and foot. Check Document Settings in your build.

How unit entry works#

Fields that accept expressions follow the rules in Autodesk's Parameters reference:

• A bare number uses the design's unit. A number with a unit suffix, such as 25 mm or 1 in, is converted. Units are abbreviations from the unit-type list: mm, cm, m, in, ft, and so on.
• Algebraic operators are +, -, *, /, ^ (power), % (floating-point modulo), and parentheses. A semicolon separates arguments of multi-argument functions; avoid commas so you do not clash with decimal commas in some regions.
• Constants include PI and E (unitless), Gravity, and SpeedOfLight.
• Angle units are deg (degrees), rad (radians), and grad; trig functions such as sin() and cos() expect an angle, for example sin(15 deg).
• Logic uses if(test; value if true; value if false) with comparison operators >, <, >=, <=, ==, <>, and the and, or, not functions.
• Order of operations follows algebra: parentheses, exponentiation, negation, multiplication and division, then addition and subtraction.

Tip: Typing the unit suffix every time is a good habit when collaborating across inch and metric teams: it makes dimensions self-documenting.

Parameters and units#

Parameters are named values. In the Parameters dialog each has a Name, Unit, Expression, Value, and optional Comments.

• User Parameters are created by you (Add User Parameter) and referenced in features and other expressions.
• Model Parameters drive the dimensions of timeline features. They are created automatically when you create features, and you can name them in context (for example Length=Width*2).
• A parameter set to No unit is unitless (ratios, counts).
• Parameter names may contain letters and numbers plus the symbols _, ", $, °, and µ.
• Automatic Compute updates the design in the background as you edit; uncheck it to pause updates and reduce compute time, then use Apply.
• Export Parameters saves User Parameters as CSV and Import Parameters reads them back. When editing the CSV, keep names unique, make sure the expression's resulting unit matches the assigned unit type, and keep the header (Name, Unit, Expression, Value, Comments, Favorite) correct.

Example from the help: a user parameter Parameter_1_name with unit in, expression .75, value .75 in, comment "Plywood thickness".

Note: not confirmed in Autodesk's documentation: exactly how bare-number parameter expressions are re-interpreted if you change a document's unit. Review your parameters after any unit change.

Importing and inserting geometry with units#

After importing, use the Measure tool (default shortcut I) to verify a known dimension.

Exporting with correct units#

• Mesh export (STL, 3MF, OBJ): the Save As Mesh dialog (right-click a component or body in the Browser) offers a Unit Type and a file format. STL and OBJ are unitless; 3MF carries units. Slicers often assume millimeters for STL, so export in the unit your slicer expects.
• Other formats: the mirrored help for this section does not describe unit handling for STEP, DXF, drawings, or CAM post output. Check each export dialog, and verify the result by opening the exported file.

Measuring and verifying#

• Measure reports distances, angles, and areas in the display unit.
• Precision and display: the Unit and Value Display preferences control precision and unit display; this affects what you see, not the stored value.

Options reference#

Common problems#

Tips and pitfalls#

Tip: Give key parameters consistent names and set their units explicitly. Later design changes become safe and readable.

Tip: After any import, measure a known feature to confirm scale before modeling on top of it.

Warning: Do not confuse changing how values are displayed with scaling geometry. Scaling resizes geometry; changing the displayed unit does not.

Warning: Teams mixing inch and metric should agree on a unit per project and write it down, for example in a shared template or the project notes.

Warning: Files from other people may use different units; do not assume.

Practice exercise#

1. In Preferences > Default Units > Design, set your preferred unit, then create a new design.
2. Sketch a rectangle and type 1 in for one dimension and 50 for the other. Observe how the values display.
3. Open Modify > Change Parameters, click Add User Parameter, and create wall with the expression 2 mm and a length unit.
4. Use wall in a dimension (type wall or wall*2 in a dimension field).
5. Insert any small STL with Insert Mesh and observe the Unit Type choice; measure a dimension to verify scale.

Expected result: you can predict how any number you type will be interpreted and you catch unit problems before they reach a print or machine.

Related#

• Preferences
• ViewCube and navigation
• Data Panel basics