Units, scale and axes

A 3D file stores positions as plain numbers. Whether "1" means a metre, a centimetre or an inch depends on the tool, and so does which axis points up. Getting units, scale and axes right is the first thing that makes an asset work in an engine.

#What one unit means in each tool

Tool	One unit means	Up axis	Notes
Blender	1 metre (internal unit)	Z	Unit Scale only changes what the interface displays (Blender 2.93 Manual; Cinevva; KatsBits Blender Knowledgebase)
Maya	Working unit defaults to centimetres	Y by default, can be set to Z	(Maya 2024 Help)
3ds Max	System unit defaults to inches	—	Change it only before creating or importing geometry (3ds Max 2016 Help)
Unreal Engine	1 centimetre	Z, left-handed, X forward	(Unreal Engine 5.8 Documentation; Unreal Engine 5.8 Documentation)
Unity	1 metre	Y, left-handed	Physics and lighting expect 1 unit = 1 m (Unity 6.6 Manual; Unity 2022.3 Manual)
Godot	1 metre	Y, right-handed	Physics is tuned for this scale (Godot Engine 4.5 Documentation)

Three of these settings need more explanation.

Blender's Unit Scale is display only. The manual describes it as a scale factor "when converting between internal units and values displayed in the user interface", and notes it does not change how things behave internally; physics simulations, for example, ignore it (Blender 2.93 Manual: Scene Properties). Export guides describe Blender's internal unit as the metre (Cinevva: Blender to Unreal Engine Export Checklist…; Substack: Blender to UE5: the complete export pipeline).

3ds Max separates display units from system units. "Display units only affect how geometry is displayed in the viewports," while "system units determine the actual scale of geometry" (3ds Max 2016 Help: Units Setup Dialog). Autodesk warns: "Do not change the system unit in an existing scene" (3ds Max 2016 Help: Units Setup Dialog).

Unity and Godot both say: work at real scale. Unity's manual states that its "physics and lighting systems expect 1 meter in the game world to be 1 unit in the imported Model file" (Unity 6.6 Manual: Preparing your model files for export). Godot's docs say "Physics and other areas are tuned for this scale", so using a different scale "is usually a bad idea (unless you know what you are doing)" (Godot Engine 4.5 Documentation: Introduction to 3D).

Five panels, one each for Blender, Maya, Unreal, Unity and Godot. Each shows an orange arrow pointing up labelled with the up axis — Z for Blender and Unreal, Y for Maya, Unity and Godot — and below it the handedness where documented and the size of one unit. — Two tools can agree on units and still disagree on which way is up. Blender and Unreal are both Z-up but use different units; Unity and Godot both use metres and Y-up but differ in handedness.Diagram by the handbook author

#Y-up and Z-up

Some tools treat Z as up, others Y. Blender is Z-up and Unity is Y-up, so an exported mesh can arrive "rotated around the wrong axis", for example a floor tile standing on its edge (KatsBits Blender Knowledgebase: FBX rotation fix for Unity). Handedness is the other half: Unreal and Unity are left-handed, Godot is right-handed (Unreal Engine 5.8 Documentation: Coordinate System and Spaces in Unreal Engine; Unity 2022.3 Manual: Rotation and orientation in Unity; Godot Engine 4.5 Documentation: Introduction to 3D). Exporters and importers convert between these systems, but the conversion can show up as an unexpected rotation on the imported object (KatsBits Blender Knowledgebase: FBX rotation fix for Unity; Unity Discussions: "Bake Axis Conversion" import setting).

#FBX export pitfalls

Most scale and axis bugs appear when you export through FBX; see 3D file formats.

#Centimetre and metre scale mismatch

Blender works in metres and Unreal in centimetres, so a mesh can import 100 times too large or too small (Substack: Blender to UE5: the complete export pipeline). One guide points out a subtle symptom: the mesh looks the right size, but its scale in Unreal's Details panel reads 100, 100, 100 instead of 1, 1, 1 (Substack: Blender to UE5: the complete export pipeline). That hidden scale causes trouble later with physics, collision and child objects.

Unity handles FBX units through its importer. Its manual lists a default scale factor of 0.01 for .fbx, .max and .jas files, 0.1 for .3ds, and 1 for formats such as .blend, .ma and .mb (Unity 2019.4 Manual: Creating models outside of Unity; Unity 6.6 Manual: Preparing your model files for export), and offers a Convert Units option that converts "the Model scaling defined in the Model file to Unity's scale" (Unity 2022.3 Manual: Model tab).

#Axis conversion

Blender's FBX exporter defaults to Forward −Z and Up Y, and two independent export guides recommend leaving those defaults alone (Substack: Blender to UE5: the complete export pipeline; Cinevva: Blender to Unreal Engine Export Checklist…). In Unity, the Bake Axis Conversion import option bakes the axis conversion into the asset instead of compensating with a rotation on the root object (Unity 2022.3 Manual: Model tab). Forum reports on it are mixed: some users got clean zero rotations after setting Blender's export axes correctly, while others reported leftover 90° or 180° rotations, especially in hierarchies (Unity Discussions: "Bake Axis Conversion" import setting).

KatsBits recommends fixing orientation at the source instead: rotate the object in Blender, then apply rotation and scale with Ctrl+A so the values reset to zero before export (KatsBits Blender Knowledgebase: FBX rotation fix for Unity).

Different viewsWhich Blender settings should you use for Unreal?

One Blender-to-UE5 guide keeps Unit Scale at 1.0 for static meshes, sets Apply Scalings to "FBX Units Scale", and says to avoid "FBX All". It suggests Unit Scale 0.01 only for skeletal meshes (Substack: Blender to UE5: the complete export pipeline).

Another checklist gives two equally valid routes: Unit Scale 0.01 and model at Unreal size, or Unit Scale 1.0 with Apply Scalings set to "FBX All". It warns that "leaving both at defaults is what puts a scale of 100 on your root bone" (Cinevva: Blender to Unreal Engine Export Checklist…).

In practice: the guides agree on the goal, an asset that imports at the right size with a scale of exactly 1, and disagree on the route, especially for rigged characters. For static environment meshes, pick one route, test it with the 1 m cube below, and write it into your team's export checklist. Exporters change too: Blender 4.5 introduced a new, experimental FBX importer (Blender Developer Documentation: Blender 4.5 LTS: Pipeline & I/O), so re-test after major updates.

ExampleExporting a 1 m cube from Blender

Start with Blender's default cube scaled to 1 m on each side, with scale applied so it reads 1, 1, 1.

Into Unreal Engine. Unreal measures in centimetres (Unreal Engine 5.8 Documentation: Units of Measurement in Unreal Engine), so the cube should measure 100 × 100 × 100 units. Check two things: that it is the right size next to the mannequin, and that its Scale in the Details panel reads 1, 1, 1, not 100 (Substack: Blender to UE5: the complete export pipeline).

Into Unity. Unity expects 1 unit to be 1 m (Unity 6.6 Manual: Preparing your model files for export), so the cube should be 1 × 1 × 1 next to a default Unity cube. If it imports with a −90° or 90° X rotation, that is the Z-up to Y-up conversion. Either enable Bake Axis Conversion (Unity 2022.3 Manual: Model tab) or rotate and apply the transform in Blender before export (KatsBits Blender Knowledgebase: FBX rotation fix for Unity).

If both checks pass, save the export settings as a preset. Use the same cube to test every new tool or engine version.

#Old charts and old conventions

Unit conventions have changed over time. Epic's Unreal Engine 3 documentation says that in the Unreal Tournament games 1 unit equalled 2 cm, that Gears of War used roughly 2 units per inch, and that "most licensees use a scale of 1 Unreal Unit to 1 cm" (Unreal Engine 3 / UDK documentation: Unreal Units). It also warned that changing the scale by more than a factor of 2 could cause "subtle and hard to track down side effects" (Unreal Engine 3 / UDK documentation: Unreal Units). If you find an old metrics chart, check which convention it used.

#Real-world sizes

Real-world measurements are the best starting point for scale and proportion. A few useful references:

Object	Real-world reference	Source
Interior door (US)	80 in tall (≈ 2.03 m), 32 in wide (≈ 0.81 m)	(Bob Vila)
Entry door (US)	80 in tall (≈ 2.03 m), 36 in wide (≈ 0.91 m)	(Bob Vila)
Stair rise	125–200 mm per step	(British Columbia Building Code)
Stair run	at least 210 mm (private) or 230 mm (public), at most 355 mm	(British Columbia Building Code)
Handrail height	800–965 mm	(British Columbia Building Code)
Headroom over stairs	1,950 mm in homes, 2,050 mm elsewhere	(British Columbia Building Code)

Building codes differ by country, so treat these as one example set and look up the codes for the place your game is set.

#Game metrics compared with real-world sizes

Games rarely copy real sizes exactly. Cameras, fields of view and collision capsules change how big a space feels.

World of Level Design's starting values for Unreal Engine 5 use a 180 cm character, doors 210–230 units tall and 110–140 units wide, walls 300–400 units high, and steps 15 units high and 30 deep (World of Level Design: UE5: Guide to Scale, Dimensions and…). Compare the door width with the 81 cm real interior door above: the game door is roughly 35–75% wider.

Different viewsReal-world or game-world sizes?

Real-world references give believable proportions. A US interior door is 80 in tall and 32 in wide (Bob Vila: Standard Door Size 101: Important…), and building codes set stair dimensions people are used to (British Columbia Building Code: Division B, Part 9, Section 9.8: Stairs…).

Level designers argue that sizes have to "feel right" to the player. World of Level Design's guide says "everything is relative to the player and how the player sees the world", and that a space built to real measurements should be reworked if it feels too small (80 Level: Guide to Player Scale and World Architecture…). On Polycount, an artist noted that game doorways "are usually a bit bigger" than real ones so characters can move through them under pressure, while another listed Source engine doorways at 96 units, about 2.5 m (Polycount: Way early stages of a project, planning out…).

In practice: start from real measurements so the world reads correctly, then adjust for the camera and the player's collision. Once you settle on a set of metrics, keep them consistent across the whole project. See grids and metrics.

#Systems that depend on scale

Lighting. Light falloff, shadow softness and exposure depend on distance. Unity names lighting alongside physics as a system that expects 1 unit = 1 m (Unity 6.6 Manual: Preparing your model files for export).

Physics. Unity's and Godot's physics are tuned for real-world scale (Unity 6.6 Manual: Preparing your model files for export; Godot Engine 4.5 Documentation: Introduction to 3D), so an object built at the wrong size will move, fall and collide unconvincingly.

Texel density. Texel density is measured in pixels per metre. If assets are at the wrong scale, their textures are too. Artists in the Polycount planning thread chose 1 unit = 1 cm, then argued over 4 or 8 pixels per unit (400–800 px/m) (Polycount: Way early stages of a project, planning out…). Those numbers only work if every asset is built at the agreed scale.

Modular kits. Modular pieces snap together on a grid. Keep your modeling grid in multiples of the engine's grid so pieces line up (Polycount: Way early stages of a project, planning out…).

#Common mistakes

Changing 3ds Max's system unit mid-project. Autodesk says to change it only before creating or importing geometry (3ds Max 2016 Help: Units Setup Dialog).
Expecting Blender's Unit Scale to fix physics. It only changes displayed values (Blender 2.93 Manual: Scene Properties).
Not applying transforms before export. Unapplied rotation and scale travel into the engine (KatsBits Blender Knowledgebase: FBX rotation fix for Unity).
Modeling without a reference figure. Keep a 1.8 m human box or the engine's mannequin in every scene.

#Further reading and watching

Units of Measurement in Unreal Engine

and Coordinate System and Spaces — the official definitions of Unreal's units and axes. Beginner. Free.

DocsBeginnerFree

Preparing your model files for export (Unity)

Unity's checklist for scale, deformers and FBX settings. Beginner. Free.

DocsBeginnerFree

Introduction to 3D (Godot)

Godot's units, axes and coordinate system. Beginner. Free.

DocsBeginnerFree

UE5: Guide to Scale, Dimensions and Proportions (World of Level Design)

starting metrics for characters, doors, walls and stairs. Beginner. Free (paid courses linked).

WebBeginnerFreePaid

Blender to UE5: the complete export pipeline

a detailed 2026 walkthrough of FBX settings for static and skeletal meshes. Intermediate. Free.

WebIntermediateFree

FBX rotation fix for Unity (KatsBits)

a short, illustrated fix for the Z-up to Y-up rotation problem. Beginner. Free.

WebBeginnerFree

Units, scale and axes

#What one unit means in each tool

#Y-up and Z-up

#FBX export pitfalls

#Centimetre and metre scale mismatch

#Axis conversion

#Old charts and old conventions

#Real-world sizes

#Game metrics compared with real-world sizes

#Systems that depend on scale

#Common mistakes

#Further reading and watching

References

Pages that link here