How to open an Excel file without Excel, and without uploading it

Open Sheets, press Import and choose the .xlsx. Every sheet in the workbook becomes a tab, with its values, its formulas, bold, italic, alignment, and money, percentage and date formats. Change what you like, then Export Excel writes it back as an .xlsx that opens in Excel, Numbers and LibreOffice, or Export CSV writes the sheet in view as plain text. The file is unzipped and read in this tab; nothing is uploaded, and the page works with the network switched off.

What an .xlsx is

Rename one to .zip and open it: inside are folders of XML. xl/workbook.xml names the sheets. xl/worksheets/sheet1.xml holds one element per cell, with its position, its value or its formula, and a number pointing at a style. xl/sharedStrings.xml holds every piece of text once, so a column of "Yes" is stored as one string and a thousand pointers. xl/styles.xml holds the fonts, the fills and the number formats. There is no secret in it and nothing that needs Excel to decode; it is a format a browser can read with a zip library and an XML parser, both of which it has.

That is why an online converter does not need your file to do the job, and why handing it over is a choice and not a necessity.

What comes across

• Every sheet, as a tab, with its name.
• Values. Text, numbers, TRUE and FALSE. A date is stored by Excel as a number of days since 1900; where the cell's format says it is a date, it comes across as one, written as 2025-01-01.
• Formulas. The engine here has about fifty functions (SUM, AVERAGE, COUNT, MIN, MAX, ROUND, IF-style SUMIF and COUNTIF, the text functions, AND, OR, TODAY and the rest). A formula built from those stays live and recalculates. A formula it does not have keeps the value Excel saved, so the number on the page is still right; it just will not change until the file goes back to Excel.
• Bold, italic and alignment.
• Money, percentages and dates, as formats, so 0.25 shows as 25% and 1234.5 as $1,234.50.

What does not

• Charts. Sheets draws its own from a block of cells, but Excel's chart parts are not read.
• Pivot tables, macros, conditional formatting, data validation, comments, merged cells, column widths and colours. Values in those cells come across; the machinery does not.
• A password-protected workbook. Its contents are encrypted, and there is no reading them without the password in Excel first.
• .xls, the format from before 2007, which is binary and different. Excel or LibreOffice can save it as .xlsx.

If the workbook is a model with pivot tables and macros, this is a viewer for it, not an editor, and the honest advice is to change it where it was made. If it is a list, a budget, a table of figures, it is an editor.

Saving it back

Export Excel writes every sheet as an .xlsx with the formulas and their computed values, bold, italic, alignment and the number formats, and Save to computer writes that same file back into your folder. What was not read is not written, so a workbook that had a chart comes back without it; keep the original if it matters. Export CSV writes the sheet in view as plain text, which everything reads and which keeps values only.

Just looking

If the question is only what is in the file, the same Import shows it, and what is still in a spreadsheet you thought you had deleted is worth reading before the file goes anywhere: hidden sheets, hidden rows, the author's name and the edit history all travel inside an .xlsx, and SCRUB takes them out.