3DD File

What are 3DD files and how to open them

Are you having problems opening a 3DD file or are you simply curious about its contents? We're here to explain the properties of these files and provide you with software that can open or handle your 3DD files.

What is a 3DD file?

3DD files have multiple uses, and 3D-Calc Spreadsheet is one of them. Read more about the other uses further down the page.

3D-Calc Spreadsheet

A 3D-Calc Spreadsheet file holds data for 3D-Calc, a spreadsheet program originally written for the Atari ST computer by Frank Schoonjans. This spreadsheet is three-dimensional: it stores multiple "pages" (planes), each with rows and columns, allowing formulas and references across pages.

The program writes .3DD files to save cell contents (text, numbers, formulas), cell formatting attributes like precision, hidden or protected status, and spreadsheet-wide settings. Users edit data in the spreadsheet, instructing 3D-Calc to write a .3DD when saving, and reopening that file causes 3D-Calc to read the stored cells and their attributes back into spreadsheet form.

Internally, a .3DD file begins with a two-byte identification integer 5912. After this identifier, records follow per cell: page number, row, column, then the length of the cell's content (including a terminating NUL byte), then the ASCII content string, followed by an integer for precision (or -1 to indicate use of global precision), and a single byte of cell attributes (flags for hidden, protected, italic etc.). At the end of all cells there is an end-of-file marker (integer -1).

How to open 3DD files

Important: Different programs may use files with the 3DD file extension for different purposes, so unless you are sure which format your 3DD file is, you may need to try a few different programs.

While we have not verified the apps ourselves yet, our users have suggested four different 3DD openers which you will find listed below.

Last updated: August 16, 2026

All known file formats using extension .3DD

While 3D-Calc Spreadsheet is a popular type of 3DD-file, we know of 2 different uses of the .3DD file extension. Different software may use files with the same extension for different types of data.

Generic 3D Drafting Drawing

Generic 3D Drafting Drawing (.3DD) files are drawings created by the software Generic 3-D Drafting, a three-dimensional computer-aided drafting (3-D CAD) program for DOS developed in the late 1980s by American Imaging Systems. These files store 3-D wireframe geometry, multiple views (such as isometric and orthographic projections), layers of drawing information, and properties for rendering and modifying 3-D objects. These drawings are used by architects, designers, and engineers to model and visualize 3-D forms before converting or printing them.

Generic 3-D Drafting writes .3DD files as its native project file format. Users create 3-D drawings interactively using menus to draw primitives like lines, circles, spheres, and solids, set units, switch among views, and edit existing geometry. A .3DD file may later be exported or converted into other CAD formats or plotter output as needed. The program allows hiding/showing layers, tracking cursor movement in space, working in "single-plane" mode to restrict drawing to one axis at a time, and editing or modifying 3-D objects after their creation.

Files of this format begin with a distinct byte signature: at byte offset 0 the ASCII text "S..3DPLAN" is encoded as the bytes 0x53 00 01 20 33 44 20 50 4C 41 4E 20. This header identifies the file uniquely and confirms it is a Generic 3D Drafting Drawing file.

Various apps that use files with this extension

These apps are known to open certain types of 3DD files. Remember, different programs may use 3DD files for different purposes, so you may need to try out a few of them to be able to open your specific file.

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