What is the FL file format?
An .fl file is a simulation model saved by FEMLAB or early versions of COMSOL Multiphysics, a finite-element analysis (FEA) and multiphysics simulation platform developed by COMSOL AB in Sweden. The format stores the complete model definition - geometry, mesh, material properties, boundary conditions, solver configuration, and optionally solution results - in a proprietary binary container.
FEMLAB was introduced in 1998 and renamed COMSOL Multiphysics in 2005. The .fl format was used through COMSOL 3.5a. With the release of COMSOL 4.0 in 2010, the format was replaced by .MPH, an HDF5-based container that offers better scalability and cross-version compatibility. Modern COMSOL versions can still import .fl files saved in COMSOL 3.5a format, providing a migration path for legacy models.
File size varies widely - from under a megabyte for simple geometry-only models to hundreds of megabytes when full solution data (nodal values across a fine mesh) is stored alongside the model definition.
A common source of confusion: .fl is sometimes incorrectly listed as an Adobe Flash project file. Adobe Flash project files use the .fla extension, not .fl. The FEMLAB/COMSOL model file is the correct association for the .fl extension in scientific computing contexts.
Security & safety
RISK: LOWFEMLAB/COMSOL .fl files contain simulation model data and no executable code. Safe to handle; risk is only loss of work if improperly opened in an incompatible COMSOL version.
Format details
in a nutshell- Freelancer Saved Game - Microsoft Freelancer (2003 space combat game) uses .fl for savegame files; binary, proprietary, not related to FEMLAB.
- FLUID GUI Design File - FLUID (Fast Light User Interface Designer, part of FLTK toolkit) uses .fl for GUI layout source files; text-based C++-like format.
- Flag/Font Library (various) - Various obscure applications use .fl generically; no single dominant secondary format.
Programs that open FL files
Windows1 app
COMSOL Multiphysics
Paid File → Open → select the .fl file. COMSOL can import legacy .fl files from COMSOL 3.5a. Note: requires a valid COMSOL license. 
macOS1 app
Linux1 app
Technical details
deep spec| Format type | Proprietary binary finite-element / multiphysics simulation model |
| Encoding | Binary, little-endian (Windows-native origin) |
| Container | Proprietary FEMLAB/COMSOL binary container; internal structure not publicly documented |
| Typical file size | 100 KB - several hundred MB (scales with mesh density and volume of stored solution data) |
| Stores | Geometry, finite-element mesh, material properties, boundary conditions, solver settings, and optionally full solution result data |
| Cross-version compatibility | COMSOL 3.5a files can be imported by COMSOL 4.0 and later; newer format versions cannot be opened by older tools |
| Version span | Used from FEMLAB 1.0 through COMSOL 3.5a; replaced by `.mph` in COMSOL 4.0 |
| Superseded by | `.mph` (HDF5-based COMSOL model format introduced with COMSOL 4.0 in 2010) |
| Migration path | COMSOL 4.0 and later can import `.fl` files saved in COMSOL 3.5a format |
| Product history | FEMLAB renamed to COMSOL Multiphysics in 2005; the `.fl` format was unchanged by the rename |
| Associated OS | Windows, Linux, macOS (COMSOL Multiphysics is cross-platform) |
| Developer | COMSOL AB, Sweden |
| Extension conflict | `.fl` is incorrectly catalogued as Adobe Flash project in some databases; Adobe Flash uses `.fla`, not `.fl` |
| Application domain | Multiphysics simulation - structural mechanics, heat transfer, electromagnetics, fluid dynamics, chemical reactions |
| Released | 1998 (FEMLAB 1.0) |
| Latest version | COMSOL 3.5a (last version using .fl; replaced by .mph in COMSOL 4.0, 2010) |
FL conversions
Community Q&A
asked by usersNo questions yet - be the first to ask about FL files.
