What is the XLSB file format?
.xlsb files are binary spreadsheet workbooks generated by Microsoft Excel 2007 and later. They store spreadsheet data organized in columns and rows, where cells can contain text, images, charts, macros, and formulas.
Creation of the XLSB format
.xlsb was introduced alongside Microsoft Excel 2007, which also brought the .xlsx format, both replacing the older .xls format. The binary workbook format is designed to facilitate working with large spreadsheets. It may be opened by older versions of Microsoft Excel, though certain features may not be fully supported in such cases.
XLSB vs XLSX
Both .xlsb and .xlsx are ZIP-based OPC packages - opening either in an archive tool reveals a folder structure of internal parts. The critical difference lies inside: .xlsx stores sheet data as XML files, while .xlsb stores the same data as binary BIFF12 record streams (.bin parts such as xl/worksheets/sheet1.bin). Because XML parsing is skipped entirely, reading and writing .xlsb files takes significantly less time, which is especially important when working with very large workbooks. The binary format also raises the formula length ceiling to 8,192 characters, compared to the 1,024-character limit of the legacy .xls format.
Note on macros and security
.xlsb files can contain macros - scripts executed on the device of whoever opens the file. Because macro support is built into the format itself (it is always macro-enabled, unlike .xlsx), .xlsb attachments are sometimes exploited by malicious actors. Exercise special caution when opening .xlsb files received from unknown sources to avoid exposing your system to potential threats.
Security & safety
RISK: MEDIUMUnlike plain XLSX, an XLSB CAN contain VBA macros (it embeds xl/vbaProject.bin), so treat an unexpected .xlsb like an .xlsm: Excel opens internet/email files in Protected View and may warn that macros are disabled - leave protection on and don't enable macros for files you didn't expect. Malicious macro-bearing workbooks are a real phishing vector. Opening the file in Google Sheets or LibreOffice avoids Excel's VBA engine entirely if you only need to read the data.
Format details
in a nutshellPrograms that open XLSB files
Windows3 apps
Microsoft Excel
Paid Native app. Double-click to open and edit with full feature support; Save As .xlsx to convert. 
LibreOffice Calc
Open-source Free; double-click to open XLSB (read/write support added years ago). Save As to XLSX/ODS/CSV. 
macOS2 apps
LibreOffice Calc
Open-source Free; opens XLSB and exports XLSX/CSV. Apple Numbers cannot open XLSB directly - convert first. 
Linux1 app
Google Sheets
Free Upload to Google Drive and open with Google Sheets; it imports XLSB and lets you download as XLSX/CSV/PDF. 
Excel for the web
Free Upload to OneDrive and open in Excel online (free Microsoft account); can view and re-save. Technical details
deep spec| Container format | ZIP-based OPC package - identical outer structure to .xlsx and .xlsm |
| Cell data encoding | BIFF12 binary record streams stored in .bin parts (e.g., xl/worksheets/sheet1.bin) |
| Macro support | Always macro-enabled; no macro-free variant exists (contrast .xlsx vs .xlsm) |
| MIME type | application/vnd.ms-excel.sheet.binary.macroEnabled.12 |
| Magic bytes | 50 4B 03 04 ("PK" ZIP local-file-header signature at offset 0) |
| Max rows per worksheet | 1,048,576 (2²⁰) |
| Max columns per worksheet | 16,384 (columns A-XFD) |
| Formula length limit | 8,192 characters per formula (vs 1,024 characters in legacy .xls) |
| File size vs XLSX | Typically 40-75% smaller than an equivalent .xlsx for large data-heavy workbooks |
| Load/save performance | Significantly faster than .xlsx on large workbooks; binary records eliminate XML parse overhead |
| Encryption | Supported via Office Open XML encryption (AES-128 or AES-256) |
| Third-party compatibility | Partial - LibreOffice Calc and Google Sheets can open .xlsb but with limited fidelity for complex features |
| Released | 2007 (Excel 2007) |
| Open standard | Yes · royalty-free |
| Specification | learn.microsoft.com |
XLSB conversions
Community Q&A
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