.SKP File Format

Introduction

These are the models saved by SketchUp from version 3 to SketchUp 2016, under @Last Software, Google and Trimble. The file is an MFC archive: SketchUp for Windows was built on Microsoft’s MFC library, and the model is written with MFC’s CArchive, which stores strings, numbers and objects in a compact binary form.

The files examined for this site were saved by SketchUp 8 (8.0.3117), 2015 (15.3) and 2016 (16.1). That SketchUp 3 to 7, 2013 and 2014 write the same layout is an inference: SketchUp 3 is the oldest version that the SketchUp SDK can still write, and Fab Lab Iceland’s 2008 notes on a SketchUp 6 file describe the same header, version map and thumbnail. Each release changed the versions of some classes, which may change their layouts, but the parts described here are the same in every file seen.

A file dropped on any page of this site opens in the explorer, which shows the header, the thumbnail, every class in the version map, the option groups, and the names of the materials and layers, found by searching the object stream.

Structure

All numbers are little-endian. The file is one CArchive stream from start to end, but the parts before the version map are plain values rather than objects.

Every string is an MFC CString, and every object is written with MFC’s class tags. Both are described on the Header and Object stream pages. The version map is what makes the file work across releases: it tells the reader which version of each class’s layout follows, so a newer SketchUp can read an older file. The objects after the options are not decoded here.

Telling it apart

The explorer’s parser for this format checks, in order:

• the 32 bytes of “SketchUp Model” at the start, which every SKP file has;
• a version string in braces, such as {16.1.1449};
• no VFF straight after the version, which would mean the ZIP container of SketchUp 2021 and later;
• after the GUID, path and save time, the class record of the version map, FF FF 00 00 0B 00 and CVersionMap;
• in the version map, a version below 5 for CEntity (3 in every file seen);
• after the map, three 32-bit model fields followed by the CDib class record of the thumbnail.

A file that fails a check is turned down, and the explorer tries the parsers for the other SKP formats. A file with CEntity at version 5 or more is a SketchUp 2017 to 2020 model. A version of 21 or more with this layout is accepted but reported, since SketchUp 2021 and later write the ZIP container.

Variants

• SketchUp 3 to 2016 – the classic MFC CArchive file: a version map, a PNG thumbnail and the object stream
• SketchUp 2017 to 2020 – the classic file with persistent IDs on every entity and two more model fields
• SketchUp 2021 and later – the “versionless” format: a short header, then a ZIP archive

Links