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Understanding Excel binary workbook

XLS is the Excel 97 to 2003 binary workbook, with BIFF8 records in a compound file. Read about its content, macros, formulas and bank data risks.

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An XLS file is the binary workbook of Excel 97 to 2003, also called BIFF8. It is a compound file, a small file system inside one file, with a Workbook stream of binary records: workbook globals first, then one substream per sheet. Microsoft specifies it as MS-XLS. It can hold formulas and macros, so cells may not be plain values. Klarbon support is planned, not available yet.

What is in an XLS file?

Part Holds
Compound file storages and streams, like folders and files
Workbook stream all workbook records, in BIFF8
Globals substream fonts, formats, shared strings and the sheet list, always first
Sheet substreams worksheets, chart sheets, macro sheets or dialog sheets, with cells, values and formulas
Property Value
Encoding BIFF8 binary records
Container Compound File Binary (MS-CFB)
Specification Microsoft MS-XLS
Excel versions 97, 2000, 2002 and 2003 as the default format

Microsoft publishes MS-XLS, and the Library of Congress describes the structure and history of the format (MS-XLS, Library of Congress).

XLS, XLSB or XLSX?

XLS XLSB XLSX
Container compound file ZIP ZIP
Content binary records binary records XML
Macros yes, macro sheets yes no, XLSM instead
Open standard no, Microsoft specification no, Microsoft specification ECMA-376, ISO/IEC 29500

What does Klarbon do with XLS files?

Klarbon does not open XLS files today; reading bank transactions from an XLS sheet, with a column mapping you confirm, is planned, not available yet. Klarbon does not run macros or calculate formulas from a file. Klarbon writes bank statements as XLSX; see supported bank statement files.

Frequently asked questions

What is an XLS file?

The binary workbook format of Excel 97, 2000, 2002 and 2003, also called BIFF8. Excel still opens and saves it, but since Excel 2007 the default is XLSX.

How is an XLS file built?

As a compound file, a small file system inside one file, with a Workbook stream of binary records. The stream starts with the workbook globals and then one substream per sheet.

Can an XLS file contain macros?

Yes. An XLS workbook can hold macro sheets as well as worksheets. Do not enable macros in a file from an unknown sender.

Should a bank still send XLS files?

Use an export that your importing software supports. If your bank offers only XLS, check that dates and amounts are values, not formulas or text, before using the data.

Sources

Updated 5 November 2026

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