Nothing is uploaded. When you drop or pick files, the tool reads each workbook locally with the browser’s File API, parses it in memory with the bundled ExcelJS library and builds the merged workbook on your device — the cell data never touches a network. There is no server, API, database or tracking in the processing path, so financial models, customer lists or any other sensitive spreadsheet stay on your machine; the tool works offline once cached, and closing the tab discards everything. Even the final .xlsx download is generated in the browser. The only optional network use anywhere on the site is consent-gated advertising, which never sees your data.

Separate sheets is the safe default: every selected worksheet is copied into one output workbook as its own tab, so formatting, formulas and column widths survive and nothing is reshaped — perfect for gathering many reports into a single file. Append rows is a vertical merge that concatenates the data from every sheet into one long table; when the first row is a header you choose whether columns align by header name (so a sheet ordered name, id, amount lines up with one ordered id, name, amount) or strictly by position, and you can add a Source File column and drop duplicate rows. Side-by-side is a horizontal merge that puts each workbook’s columns next to the others, lining rows up by position. A live preview and the output row and tab counts let you confirm the shape before you download.

It depends on the mode. Separate sheets copies each worksheet intact, so number formats, fonts, fills, column widths and in-cell formulas are preserved (you can toggle style preservation in Advanced). Append rows and Side-by-side necessarily rebuild the data into a new layout, so they carry the cell values and their types — numbers stay numbers, dates stay dates, text stays text — but do not attempt to reproduce every source style, and formulas are read as their current values rather than re-pointed at moved cells. If preserving live formulas and exact formatting matters, choose Separate sheets; if you need one flat table to analyse or pivot, choose Append rows.

You can add up to 50 Excel workbooks at once, each up to 100 MB, in the modern .xlsx or macro-enabled .xlsm format (the older binary .xls is not supported — re-save it as .xlsx first). Every worksheet inside each file is listed so you can tick or untick exactly which sheets take part in the merge, and files can be dragged to reorder them because order controls the sheet sequence and the append/side-by-side layout. The whole job runs in browser memory, so very large workbooks are bounded by your device’s RAM rather than any server limit, and the merged result downloads as a single merged.xlsx.