Formats/XLSX
What Is an XLSX File? The Excel Spreadsheet Format Explained
XLSX is the file format Microsoft Excel has saved workbooks in by default since Excel 2007. Like its cousin DOCX, it is part of the Office Open XML standard (ISO/IEC 29500) — an open specification, despite the Microsoft association — and it replaced the older binary .xls. An XLSX file holds far more than a grid of numbers: it stores multiple sheets, formulas, cell formatting, charts, conditional rules and more, all in one document designed to be worked in, not just read.
What is inside an XLSX file
An XLSX file is not a single blob of data — it is actually a ZIP archive full of XML files. Rename one to .zip and open it and you will find separate parts describing each worksheet, the shared list of text strings, the styles, and any charts or images. This open, inspectable structure is exactly why so many programs beyond Excel — Google Sheets, LibreOffice Calc, Apple Numbers and countless web tools — can read and write XLSX reliably.
That structure is also why XLSX preserves things a plain data file cannot: a formula stays a live formula that recalculates, a currency cell keeps its formatting, a chart stays a chart. The whole point of the format is to keep a spreadsheet fully functional, not to flatten it into static values.
XLSX vs the old XLS
The older .xls format was a proprietary binary format that other software had to reverse-engineer to read, which made it fragile and harder to support. XLSX, being open XML wrapped in a ZIP, is smaller, more robust against corruption, and far more widely compatible. It also raised the limits dramatically: an XLSX worksheet can hold over a million rows and 16,000-plus columns, versus the old 65,536-row ceiling of XLS.
You still occasionally receive an .xls from very old systems, and modern spreadsheet software opens it fine, but there is rarely a reason to save in the old format today. XLSX is the sensible default for any new workbook.
XLSX vs CSV: when to use each
CSV is a plain-text list of values with no formulas, formatting, multiple sheets or charts — just the raw data, separated by commas. It is the universal lowest common denominator for moving a simple table between programs. XLSX is the opposite: a rich, structured workbook that keeps everything a spreadsheet needs to stay alive.
Use XLSX when you need formulas that recalculate, formatting, several sheets, or charts — anything you will keep working in. Use CSV when you just need to hand a flat table of data to another system that will import it, and you do not care about (or actively want to strip) the formatting and formulas. A common workflow is to keep the working file as XLSX and export a CSV only when another tool needs the raw data.
How to open and edit XLSX without Excel
You do not need Microsoft Excel to work with an XLSX file. OfficePad's spreadsheet editor opens .xlsx workbooks in the browser, lets you edit cells and formulas, and exports back to .xlsx — or to CSV when another system needs the raw data. Everything stays on your device; nothing is uploaded. Google Sheets, LibreOffice Calc and Apple Numbers open XLSX too, so the format is genuinely portable across the whole spreadsheet world.
Frequently asked questions
Can I open an XLSX file without Excel?
Yes. Google Sheets, LibreOffice Calc, Apple Numbers and browser tools like OfficePad's spreadsheet editor all open and edit XLSX without Excel installed.
What is the difference between XLSX and XLS?
XLS is the older proprietary binary format; XLSX (since Excel 2007) is an open XML-based format that is smaller, more robust, more widely compatible and supports far more rows and columns.
Should I use XLSX or CSV?
Use XLSX when you need formulas, formatting, multiple sheets or charts. Use CSV when you only need to move a flat table of raw data to another program that will import it.
Work with XLSX on OfficePad
See also in the glossary: Metadata, Character encoding, UTF-8.