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 and formulas, its fonts, fills, borders, alignment and number formats, its merged cells, colour rules, notes and data validation, and its charts and PivotTables. 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 167 functions, each listed on the functions page with what it does differently from Excel, if anything: SUM and AVERAGE, SUMIFS and COUNTIFS, VLOOKUP, XLOOKUP and INDEX with MATCH, IF, IFS and IFERROR, the text and date functions, PMT and the rest. A formula built from those stays live and recalculates. A formula it does not have (FILTER, SORT and UNIQUE, for three) 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.
• How it looks. Fonts, bold and italic, text and fill colours, borders, alignment, and the number formats, so 0.25 shows as 25% and 1234.5 as $1,234.50. Merged cells, column widths and row heights, hidden rows, columns and sheets, frozen panes and tab colours.
• Colour rules and data validation. Conditional formatting stays live, and a drop-down list or a rule on what a cell may hold still applies.
• Notes, the filter, the print area and the page setup.
• Charts. A bar, column, line or pie chart comes across as the same kind over the same cells. An area or scatter chart is drawn as a line, a doughnut as a pie, and a chart drawn from another sheet's cells is left out rather than drawn from the wrong ones.
• PivotTables, with their rows, values and filters, worked out again from the table they read. Only the first column field is kept.

What does not

• Macros. An .xlsx cannot hold them, and Sheets does not open the .xlsm files that do.
• Formulas it does not have, which keep their saved values, as above; and a chart of a kind it does not draw, which is left out.
• 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 that runs on macros, or on functions the engine does not have, 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, with its formatting, its charts and its PivotTables, it is an editor.

Saving it back

Export Excel writes every sheet as an .xlsx with the formulas and their computed values, the formatting, the colour rules, notes and data validation, and the charts and PivotTables as Excel's own, which Excel can change; and Save to computer writes that same file back into your folder. What was not read is not written, so a chart of a kind Sheets does not draw 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.