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Document format

XLS converter

An .xls file is a legacy Microsoft Excel workbook in the binary BIFF format, Excel's default from its earliest releases through Excel 2003. Each sheet is a stream of typed binary records — cells, formulas, formats — rather than anything textual.

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Drop XLS files here

Batch files can each use a different output. Nothing uploads for local conversions.

Working inputs include camera RAW, browser-local audio/video, PDF, CBZ/CBR comics, office documents, ebooks, markup, 3D models, structured text, images, and archives.

Where XLS comes from

BIFF (Binary Interchange File Format) shipped with Excel's 1987 releases and marched through eight major revisions; BIFF8, introduced with Excel 97, wrapped the streams in an OLE2 Compound File and added Unicode text. Microsoft published the MS-XLS specification in 2008, long after .xlsx had taken over.

How XLS works

  • Composed of typed binary records (id, length, payload) — LABELSST for shared-string cells, FORMULA for calculations, and hundreds more.
  • BIFF8 hosts the workbook stream inside an OLE2 Compound File alongside macro and metadata streams.
  • Hard limits of 65,536 rows and 256 columns per sheet — data beyond that simply cannot be stored.
  • A shared string table (SST) deduplicates repeated text across the workbook.
  • Formulas are stored as parsed RPN token streams, not text, so extraction requires a full decoder.

When to use XLS

  • Reading exports from ERP, banking, and lab systems that still emit .xls.
  • Opening spreadsheets created before Office 2007.
  • Feeding legacy line-of-business software that accepts nothing newer.

Strengths and limitations

Strengths

  • Readable by essentially every spreadsheet application of the last 25 years.
  • Open-source decoders are mature — SheetJS implements BIFF in JavaScript, which is why this site can genuinely convert .xls in the browser.
  • Compact binary records load quickly even on old hardware.

Limitations

  • The 65,536-row ceiling truncates modern datasets.
  • Binary structure is opaque to casual inspection, unlike XML-era formats.
  • Macro viruses were historically rampant in .xls files.

Compatibility

Excel, LibreOffice Calc, Google Sheets, and Apple Numbers all still open .xls. Thanks to SheetJS, it is one of the few legacy binary office formats that converts faithfully in a browser without any server involvement.

SummaryInformation streams in the compound file record author, company, and creation and modification timestamps.

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