What is the 123 file format?
The .123 file extension is used by spreadsheets created with Lotus 1-2-3 software.
Files with the .123 extension store spreadsheet data - cells containing numbers, labels, and formulas, along with formatting, named ranges, and multiple worksheet tabs. Unlike earlier Lotus formats such as .WK4, the .123 format was introduced with Lotus 1-2-3 97 and carried through to the Millennium Edition (1999-2002). It is a proprietary binary format built from a stream of type-length-value records; there is no plain-text or XML layer.
Lotus 1-2-3 was originally developed in the early 1980s for MS-DOS, where it was extremely popular among business users. The application was later distributed as part of the Lotus SmartSuite office suite for Windows. IBM acquired Lotus Development Corporation in 1995 and eventually discontinued Lotus 1-2-3 - ending sales in 2013 and support in 2014 - as Microsoft Excel had by then replaced it as the dominant spreadsheet.
Modern Excel no longer opens .123 files (Lotus import filters were dropped after Excel 2003/2007). Free alternatives such as LibreOffice Calc and Gnumeric remain reliable openers. To preserve legacy data, exporting to .XLSX, .ODS, or .CSV is recommended.
Security & safety
RISK: LOWA .123 file is plain spreadsheet data without the macro/scripting payload found in modern Office files, so opening one is low risk. The real concern is data preservation: .123 is obsolete and modern Excel can't read it, so important data should be converted to XLSX/ODS while LibreOffice and Gnumeric still ship the Lotus importers. Avoid sketchy "123 converter" download sites; use the official LibreOffice/Gnumeric. Lotus password-"sealed" files used weak protection and aren't a security guarantee.
Format details
in a nutshellPrograms that open 123 files
Windows4 apps
Gnumeric
Open-source Open the .123 - Gnumeric supports the whole Lotus family (WKS/WK1/WK3/WK4/.123). Good for tricky legacy files. 
LibreOffice Calc
Open-source File → Open the .123; Calc imports it directly. Then Save As to XLSX/ODS/CSV. The most reliable free opener. 
Corel Quattro Pro (WordPerfect Office)
Paid Open/import the .123 in Quattro Pro, which historically reads Lotus formats. Paid suite; use only if already owned.
Apache OpenOffice Calc
Open-source Opens legacy Lotus files; LibreOffice Calc is the better-maintained choice today. 
macOS2 apps
Apache OpenOffice Calc
Open-source Legacy opener for Lotus files on macOS; prefer LibreOffice for active support. 
Linux2 apps
Technical details
deep spec| Format type | Proprietary binary spreadsheet; not an XML or text-based format |
| MIME type | application/vnd.lotus-1-2-3 (also application/x-lotus123) |
| Byte order | Little-endian |
| File structure | Flat record stream: each record is a 2-byte type, 2-byte length, then payload; bounded by BOF and EOF records |
| Magic bytes | 0x00 0x00 0x02 0x00 at offset 0 (BOF record; no ASCII signature string) |
| Compression | Uncompressed - binary records are stored in raw form with no compression layer |
| Password protection | Optional; Lotus sealed/password scheme, considered weak by modern standards |
| Grid size | Up to 65,536 rows × 256 columns per sheet |
| Multi-sheet support | Yes - 3D workbook with multiple named worksheet tabs (introduced with WK4, continued in .123) |
| Formula storage | Formulas stored in bytecode with a cached numeric result embedded in the same record |
| Format lineage | Succeeds the WK4 binary workbook format; part of the WKS → WK1 → WK3 → WK4 → .123 evolution |
| Developer | Lotus Development Corporation; acquired and continued by IBM (1995-2014) |
| Platform support | Primarily Windows; earlier Lotus builds also ran on OS/2; modern read-only support via LibreOffice Calc and Gnumeric |
| Modern Excel compatibility | Not supported in Excel 2010 and later; Lotus import filters were removed after Excel 2003/2007 |
| Released | 1997 (Lotus 1-2-3 97 / SmartSuite); .123 used through Millennium Edition (1999-2002) |
| Latest version | Lotus 1-2-3 Millennium Edition Release 9.8 (2002), the last release |
| Specification | fileformats.archiveteam.org |
123 conversions
Community Q&A
asked by usersNo questions yet - be the first to ask about 123 files.
