What is the WK1 file format?
A .wk1 file is a binary spreadsheet created by Lotus 1-2-3 Release 2.x (1985), the DOS-era spreadsheet that dominated the personal-computer productivity market before Microsoft Excel overtook it in the mid-1990s. The .wk1 format became so widely deployed that virtually every competing spreadsheet of the late 1980s and early 1990s implemented an importer for it.
The file is structured as a sequential stream of type-length-value binary records, opening with a Beginning-Of-File (BOF) record and closing with an EOF record. The BOF record's 2-byte version word 0x0405 identifies the file as Release 2.x (.wk1), distinguishing it from the earlier .wks (Release 1.x) and the later .wk3 and .wk4 formats. Individual records encode cell values (blank, integer, float, formula, or label), column widths, named ranges, print settings, and graph data.
.wk1 is a single-sheet format limited to 256 columns × 2,048 rows (A1 to IV2048). Multi-sheet support did not arrive until .wk3 (Lotus Release 3, 1989).
The format specification was published by Lotus in 1984-1985 and is well-documented for its era. IBM ended Lotus 1-2-3 sales in 2013 and support in 2014. Modern Excel (2007+) dropped the Lotus import filter; LibreOffice Calc and Gnumeric still read .wk1 files natively.
Security & safety
RISK: LOWWK1 is a DOS-era binary spreadsheet with no modern macro/scripting payload. Opening one in LibreOffice or Gnumeric carries essentially no risk. The main concern is data preservation: WK1 is very old and a few Lotus @functions may not survive conversion perfectly, so validate results after opening. Avoid sketchy "WK1 converter" download sites that could bundle adware; use official LibreOffice or Gnumeric instead.
Format details
in a nutshellPrograms that open WK1 files
Windows3 apps
Gnumeric
Open-source Open the .wk1 - Gnumeric supports the full Lotus WK* family (WKS/WK1/WK3/WK4/.123). Particularly good for tricky legacy files. 
LibreOffice Calc
Open-source File > Open the .wk1; Calc imports it directly. Then Save As XLSX/ODS/CSV. The most reliable free opener for WK1 in 2026. 
Corel Quattro Pro (WordPerfect Office)
Paid Quattro Pro historically imported Lotus WK1. Use only if already owned; LibreOffice Calc is the free alternative. 
macOS1 app
Linux2 apps
LibreOffice Calc
Open-source Open the .wk1 - bundled with most Linux desktops; includes the Lotus WK importer. Technical details
deep spec| Container | Sequential binary record stream (type-length-value records, BOF to EOF) |
| BOF version word | 0x0405 at bytes 4-5 identifies Release 2.x (.wk1) |
| Record structure | 2-byte record type + 2-byte length field + variable-length payload |
| Grid limit | 256 columns × 2,048 rows (A1 to IV2048); single worksheet only |
| Cell types | Blank, 16-bit integer, IEEE 754 double, tokenized formula, label (string) |
| Byte order | Little-endian throughout |
| Password protection | Optional; extremely weak scheme, trivially bypassed |
| Typical file size | A few KB to ~1 MB (single-sheet DOS-era data) |
| MIME type | application/vnd.lotus-1-2-3 |
| Modern support | LibreOffice Calc and Gnumeric read .wk1 natively; Microsoft Excel 2007+ dropped the filter |
| Successor formats | .wk3 (Release 3, 1989 - 3D multi-sheet), .wk4 (Release 4, 1993), .123 (Lotus Millennium) |
| Format documentation | Published by Lotus Development Corporation, 1984-1985; widely used by third-party importers |
| Operating system | DOS (native); Windows, macOS, Linux via importers |
| Released | 1985 (Lotus 1-2-3 Release 2.0 for DOS) |
| Latest version | WK1 corresponds to Lotus 1-2-3 Release 2.x; superseded by WK3 (Release 3) and WK4 (Release 4) |
| Specification | fileformats.archiveteam.org |
WK1 conversions
Community Q&A
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