What is the SLK file format?
An .slk file is a spreadsheet saved in the SYLK format - short for Symbolic Link, a name it shares with OS filesystem symlinks purely by coincidence. SYLK is a plain-text, line-oriented data interchange format developed by Microsoft for Multiplan, their pre-Excel spreadsheet, in the early 1980s.
Every SYLK file begins with the literal characters ID; on its first line - an identifier record that signals the file type. Subsequent lines carry single-letter record codes: B for table bounds, F for cell formatting, C for individual cell values and formulas (addressed by X;Y coordinates), and E to mark the end. Fields within each record are separated by semicolons.
Unlike CSV, SYLK can store basic cell formatting and formulas alongside values, which is why some legacy accounting, banking and point-of-sale systems still export .slk files today. It cannot carry charts, images or multiple sheets.
The format is ASCII-only; characters outside the ASCII range must be escaped, and there is no Unicode support. SYLK has no versioning - the syntax has been frozen since the Multiplan era.
Microsoft Excel, LibreOffice Calc and Gnumeric can all open and export .slk files. A well-known quirk: any file whose first cell content is the text ID causes Excel to attempt SYLK parsing, often producing the error "SYLK: File format is not valid" even for an ordinary .xlsx or CSV.
Security & safety
RISK: HIGHSYLK is a known malware-delivery vector and must be treated with caution. Because a .slk is parsed as a spreadsheet, it can carry formulas - and historically Excel 4.0 (XLM) macro content - that execute on open; SYLK notably bypassed some of Excel's macro-security and Protected View defaults, so attackers used .slk attachments in phishing to run code (DDE/command execution, macro droppers). Do NOT open .slk files from untrusted or unexpected sources. If you must inspect one, open it first in a plain text editor (it is human-readable) to see its records, or open it in LibreOffice with macros disabled, rather than double-clicking it into Excel. The separate, harmless 'SYLK: File format is not valid' error just means Excel mis-detected a file starting with 'ID' as SYLK.
Format details
in a nutshellPrograms that open SLK files
Windows4 apps
Microsoft Excel
Paid File > Open and choose the .slk (set file type to 'All Files' if needed); Excel imports it into a worksheet. Save As .xlsx to keep it. 
Gnumeric
Open-source Lightweight spreadsheet that opens SYLK; good for quickly viewing/exporting legacy .slk data. 
LibreOffice Calc
Open-source Open the .slk directly - Calc reads SYLK natively. Free alternative to Excel; export to XLSX/CSV/ODS. 
SoftMaker PlanMaker (Office / FreeOffice)
Freemium Open the .slk in PlanMaker; FreeOffice's PlanMaker is the free edition. Imports SYLK into a grid. 
macOS2 apps
Linux2 apps
Google Sheets
Free Upload the .slk to Google Drive and open with Sheets; it imports the SYLK data into an online spreadsheet. Technical details
deep spec| File start marker | Always begins with "ID;" on the first line (the SYLK identifier record) |
| Encoding | ASCII only; no Unicode - non-ASCII characters must be escaped |
| Container | Plain text, one record per line |
| MIME type | text/spreadsheet (also application/x-sylk) |
| Record types | ID (header), B (table bounds), F (formatting), C (cell value/formula), E (end-of-file) |
| Field separator | Semicolon (;) within each record line |
| Cell addressing | 1-based X;Y coordinates where X = column, Y = row |
| Formula support | Yes - formulas stored as text alongside cell values (unlike CSV) |
| Formatting support | Basic cell formatting (number format, alignment) encoded in F records |
| Typical file size | 1 KB - a few MB (scales with populated cell count) |
| Multi-sheet support | None - single worksheet only |
| Known quirk | Any file beginning with "ID" triggers Excel's SYLK auto-detection, often causing a false "File format is not valid" error |
| Released | 1980s (Microsoft Multiplan; widely used by early Excel for data interchange) |
| Latest version | SYLK format is frozen (no versioning); still importable/exportable by current Excel and LibreOffice |
| Open standard | Yes · royalty-free |
SLK conversions
Community Q&A
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