Exporting Blender models with correct units and dimensions

What scene units actually control

Blender's Scene panel offers a unit system: None, Metric, or Imperial. According to the Blender 4.5 manual, None uses units with no relation to the real world, which in practice behaves like Metric without unit names. The Length setting then picks the display unit for lengths, either Adaptive or a fixed unit such as meters or centimeters. The manual's metric table lists the millimeter as one thousandth of a meter.

Unit Scale is a conversion factor between internal units and values shown in the interface. The manual says this setting alone does not change internal behavior, such as physics simulations. For STL export, Blender 4.5.0 source adds a specific case: when Scene Unit is enabled and the unit system is not None, export applies the scene’s scale_length to the vertices. Distinguish display alone from the explicit use of Scene Unit.

Changing the display unit does not by itself resize internal geometry. That alone cannot predict export size: Blender 4.5.0 STL code applies a scene-unit factor only when Scene Unit is enabled and the unit system is not None. A reference part can help check the settings for your scene and software version. The Blender for 3D printing guide covers getting started; this page focuses on the export chain.

The STL export options that change size

The Blender 4.5 manual says Scene Unit applies the scene unit to exported data. The v4.5.0 source code specifies the calculation: transformed vertex coordinates are multiplied by Scale (global_scale) and the scene factor. That second factor is 1 unless Scene Unit is enabled and the unit system is not None; then it uses the scene’s unit scale. The French Scale description under Export refers to imported objects, but the export UI definition and code confirm its role here.

Scale and Scene Unit are separate settings that combine in the 4.5.0 export: the applied factor is Scale × scene factor. The UI Scale value defaults to 1.0 and Scene Unit is off by default; the scene factor remains 1 if the unit system is None or the box is unchecked. When enabled with Metric or Imperial, the code uses the scene unit scale. Record these settings before comparing dimensions in the slicer.

There is no universal multiplier to copy. The right setting depends on how your scene was built and what your slicer expects. That is exactly why a reference part, described below, beats a memorized number. If you receive files from other people instead, the case of a downloaded unitless STL file is covered on its own page.

Forward and Up: orienting the part for the target software

An STL only contains triangles; it carries no information about which way is up. According to the Blender 4.5 manual, many applications use a different axis for up, and the Forward and Up parameters handle that axis conversion. By mapping them to different axes, you convert rotations between the default forward and up axes of different applications.

Blender uses Y forward and Z up, since the front view looks along the +Y direction. The manual gives an example: it is common for applications to use Y as the up axis, and in that case -Z forward and Y up is needed. That example does not mean you should always force Y up. If your slicer treats Z as up, Blender's default export already matches, and changing the axes would tip the part over instead.

Set Forward and Up for the receiving software’s convention, then check the orientation of the loaded file. If the part appears on its side, review these axes among the first settings; that observation alone does not establish a single cause. Record the pair used with the file so you can reproduce it or change it for another destination.

Modifiers, selection, and batch export

Three export options decide what actually gets written to the file. The first is Apply Modifiers. According to the manual, it exports objects using the evaluated mesh, meaning the mesh that results after all modifiers have been computed. Without it, an object with an unapplied mirror, solidify, or subdivision modifier can export without those effects, so with different geometry and dimensions than what you see in the viewport.

The second is Selection Only. When checked, only selected objects are exported. The manual adds a useful detail: instanced objects, such as instanced collections in the scene, count as selected when their instancer is selected. If an unexpected instance appears in your STL, check whether its instancer was part of the selection.

The third is Batch, which exports each object as a separate STL file. That is handy for a design made of several pieces you print separately. The manual only describes the split into files; it does not promise that those files will reposition themselves relative to each other in another program. If assembly alignment matters, the project format compared in STL vs 3MF may suit you better. Finally, Format ASCII writes a text STL instead of binary, with no effect on geometry.

Checking dimensions in the slicer with a reference part

One proposed check is to create a simple, asymmetric reference object, record its three dimensions and a reference face, then export it with the settings you want to examine. This procedure does not replace checking an existing model and was not performed for this article.

Open the exported file in the intended slicer and compare its displayed dimensions with the values recorded in Blender. A match on this object shows these settings produced the expected dimensions for this check; it does not automatically validate other scenes or files. If a difference appears, review document units, Scene Unit, Scale, and import settings one at a time, without inferring a cause before comparison.

Also check the reference face to observe orientation. If it has changed, review the Forward/Up pair against the receiving software’s convention. Keep the settings recorded for this file and repeat the check if the scene, options, or software change.

Round trips, import, and the limits of STL

Blender 4.5's STL importer has matching settings: Scale, Scene Unit, and Forward and Up Axis. If you reimport an exported file to check it, those import options come into play in turn. A part that comes back at the wrong size may therefore be an import problem rather than an export problem. Compare dimensions in the slicer, which is the final destination, rather than through a Blender-only round trip.

The importer's Validate Mesh option checks the imported mesh for corrupted data and fixes it if needed. The manual recommends it because data errors are not always obvious, at the cost of a slower import. It does not certify that a mesh is watertight, printable, or dimensionally correct. It is a file integrity check, not a dimension check.

STL describes triangulated geometry, not the CAD model’s parametric history. Compare the exported mesh, its interpretation in the slicer, and the printed part separately when investigating a dimensional difference. These checks can show where the difference appears; they do not by themselves establish its cause. See the dimensional accuracy guide for checks on the printed part.

Sources and limits

• Blender 4.5 — STL manual, French edition
• Blender 4.5 — Scene properties, French edition
• Blender 4.5.0 — stl_export.cc
• Blender 4.5.0 — io_stl_ops.cc

We organize manufacturer and community documentation; we have not measured these products ourselves. The manual for your exact model takes precedence over general guidance.