| Home Page | Recent Changes | Preferences

Package File Format

The Unreal Engine uses a single file format to store all its game-content. You may have seen many different filetypes, like .utx (textures), .unr (maps), .umx (sound) and .u (code), but from a technical standpoint there is no difference between those files; the different file endings are only used to help organize the packages in the directory structure. The following article will describe the basic structure of his fileformat. It omits many details (such as tons of constants, for example), but there’s a good reference available on the net by Antonio Cordero Balcazar (see links).


This is a rather technical article. It requires you to have a basic understanding of object oriented programming as well as the will to use an hex-editor, if needed. This is NOT intended to be a full documention of the fileformat, but only a brief introduction.

The Structure of the File


Every package file can be roughly split into three logical parts. The header, the three index tables (name-table, import-table and export-table) and the data itself. But only the header has a fixed position (at offset 0), all other parts can be found anywhere within the file without irritating the engine.

Most of the time, although, the layout looks like the following:

  • Header
  • Name-Table
  • Import-Table
  • Data
  • Export-Table

It may be useful to read a bit about the concept of serialisation, which allows you to (rather) easily store the state of objects within a file. A brief introduction can be found on the Wiki: [Package File Format/Serialisation]?


This global header can be found at the beginning of the file (offset 0). It is the starting point for every operation.

Type Property Description
DWORD Signature Always: “0x9E2A83C1”; use this to verify that you indeed try to read an Unreal-Package
DWORD Version Version of the file-format; Unreal1 uses mostly 61-63, UT 67-69; see the appendix for more details
DWORD Package Flags Global package flags, i.e. if a package may be downloaded from a game server etc; described in the appendix
DWORD Name Count No. Of entries in name-table
DWORD Name Offset Offset of name-table within the file
DWORD Export Count No. Of entries in export-table
DWORD Export Offset Offset of export-table within the file
DWORD Import Count No. Of entries in import-table
DWORD Import Offset Offset of import-table within the file
After the ImportOffset, the header differs between the versions. The only interesting fact, though, is that for fileformat versions => 68, a GUID has been introduced. It can be found right after the ImportOffset:
16 BYTE GUID Unique identifier; used for package downloading from servers

Index Tables

The Unreal-Engine introduces two new variable-types. The first one is a rather simple string type, called NAME from now on. The second one is a bit more tricky, these CompactIndices, or INDEX later on, compresses ordinary DWORDs downto one to five BYTEs. Both types, as well as the ObjectReference, are described in the following paper: Package File Format/Data Details


The first and most simple one of the three tables is the name-table. The name-table can be considered an index of all unique names used for objects and references within the file. Later on, you’ll often find indexes into this table instead of a string containing the object-name.

Type Property Description
NAME Object Name
DWORD Object Flags Flags for the object; described in the appendix


The export-table is an index for all objects within the package. Every object in the body of the file has a corresponding entry in this table, with information like offset within the file etc.

Type Property Description
INDEX Class Class of the object, i.e. ‘Texture’ or ‘Palette’ etc; stored as a ObjectReference
INDEX Super Object Parent; again a ObjectReference
DWORD Group Internal package/group of the object, i.e. ‘Floor’ for floor-textures; ObjectReference
INDEX Object Name The name of the object; an index into the name-table
DWORD Object Flags Flags for the object; described in the appendix
INDEX Serial Size Total size of the object
INDEX Serial Offset Offset of the object; this field only exists if the SerialSize is larger 0


The third table holds references to objects in external packages. For example, a texture might have a DetailTexture (which makes for the nice structure if have a very close look at a texture). Now, these DetailTextures are all stored in a single package (as they are used by many different textures in different package files). The property of the texture object only needs to store an index into the import-table then as the entry in the import-table already points to the DetailTexture in the other package.

Type Property Description
INDEX Class Package Package file in which the class of the object is defined; an index into the name-table
INDEX Class Name Class of the object, i.e. ‘Texture’ or ‘Palette’ etc; an index into the name-table
DWORD Package Package file where the referenced object resides in (filename without ending)
INDEX Object Name The name of the object; an index into the name-table


Each object consists of a list of properties at the beginning and the actual object itself.

Object Properties

When jumping to the offset of an object, you'll first be confronted with the object properties before the actual object starts. The format is rather straightforward. The first byte is an INDEX-type reference into the Name-Table, giving you the property's name. The second byte does the magic of telling you what kind of data follows; for example 0x02 flags a DWORD sized integer type. Then comes the actual property-data. The procedure repeats itself until the reference into the Name-Table returns 'None' as the name.

That said, there are some bit-tricks to deal with arrays, booleans and such. For more info on these, as well as a full list of info-bytes, read Antonio's package docs.

Sample Objects (Texture Class)

After the properties are finished the object starts. It basically consists of a predefined set of properties. As an example, the texture class (for good old UT) will be explained below. The texture class is a native one, which means that it doesn't have a generic header in addition to its own data. The layout looks like this:

Type Property Description
BYTE MipMapCount Count of MipMaps in object

The next set of variables repeats itself for each MipMap.

Type Property Description
DWORD WidthOffset Offset in file; should be the same as SerialOffset in the Export-Table. Only if PkgVer >= 63
INDEX MipMapSize Size of the image data (in bytes)
n BYTEs MipMapData Image data; one byte per pixel; n = MipMapSize
DWORD Width Texture-width
DWORD Height Texture-height
BYTE BitsWidth Number of bits of Width (e.g. 10 for 1024 pixels)
BYTE BitsHeight Number of bits of Height (e.g. 10 for 1024 pixels)


A. Links

B. Notes

The last part about the object properties and the texture class was written in a hurry. I'm sorry it took so long for me to finish that piece.

The fileformat itself, btw, has not changed between the versions of UT (except the odd new property and such). Many of the objects however have changed a lot or were replaced by enhanced types (such as my beloved texture class...).


Jesco: I will continue after here tomorrow. Now it's time for some sleep :)

Mychaeel: Good start. :-) Have a look at UMOD/File Format too if you haven't already. A common thing like the compact index format could move to a shared page, for instance.

Jesco: Ah, I haven't noticed that, yet. Saves me the hassle to explain the compact index ;) Where should the page for the compact index be put to? I suggest making it either a subpage of UMOD/File Format or Package File Format.

Mychaeel: Putting it on a subpage of Package File Format sounds more obvious to me.

Tarquin: Other pages to grab material from / link to / etc:

Jesco: Ok, I'll work on it later today when I come back from university. Maybe I should also mail Antonio and ask im if I could post a copy of his reference docs for all those thousands of different objects that I don't have a clue of ;)

Jesco: I haven't forgot about this article, it just went down my priority list, unfortunately.

The Unreal Engine Documentation Site

Wiki Community

Topic Categories

Image Uploads

Random Page

Recent Changes

Offline Wiki

Unreal Engine

Console Commands


Mapping Topics

Mapping Lessons

UnrealEd Interface


Scripting Topics

Scripting Lessons

Making Mods

Class Tree


Modeling Topics


Log In