XLS vs XLSX: Key Differences Between Excel Formats
Published on March 15, 2026
XLSX replaced XLS as the default Excel format in Office 2007. The biggest changes: XLSX files are smaller, support over one million rows (XLS maxes out at 65,536), and use an open XML-based structure instead of proprietary binary encoding. If you are using any version of Excel from the last 18 years, you are already working with XLSX.
File Structure
XLS uses the Binary Interchange File Format (BIFF), a proprietary Microsoft format that stores data as a binary stream. Only software that implements the BIFF specification can read it reliably. XLSX is a ZIP archive containing XML files for worksheets, styles, shared strings, and metadata. Like DOC vs DOCX, the newer format is transparent and easier for third-party tools to process. Libraries like openpyxl (Python), SheetJS (JavaScript), and Apache POI (Java) all work with XLSX natively.
Row and Column Limits
This is the most practical difference. XLS supports a maximum of 65,536 rows and 256 columns per worksheet. XLSX supports 1,048,576 rows and 16,384 columns. If your dataset has more than 65K rows, XLS literally cannot hold it. Many data exports from CRMs, databases, and analytics tools easily exceed this limit, which makes XLS unusable for modern data work.
File Size
XLSX files are typically 30-50% smaller than equivalent XLS files because the XML content is ZIP-compressed. A spreadsheet with 50,000 rows of data might be 15 MB as XLS and 5 MB as XLSX. The size difference matters for email attachments, cloud storage, and loading speed. Large XLS files also take longer to open and save because Excel has to parse the entire binary structure.
Macros and Security
XLS files can contain VBA macros embedded directly in the file, which made them a common vector for malware in the 2000s. XLSX files cannot contain macros by design. If you need macros, Excel uses a separate extension: XLSM. This separation means you can trust that a .xlsx file will not execute code when opened, which is why many email servers and firewalls block .xls attachments but allow .xlsx.
When XLS Still Shows Up
Legacy enterprise systems, old financial reports, government archives, and exports from outdated software still produce XLS files. Every modern spreadsheet application (Excel, Google Sheets, LibreOffice Calc) opens XLS files without issues. There is no reason to save new files as XLS unless a specific legacy system requires it.
Need to share a spreadsheet as a fixed document? Our Excel to PDF converter handles both XLS and XLSX files in your browser.
Related reading: CSV vs XLSX compares spreadsheet data formats, and PPT vs PPTX covers the same format transition for presentations, and Numbers vs Excel compares Apple Numbers with Microsoft Excel.