The moment it fails
Word says the document is corrupt and offers to recover its contents, or Excel says it found a problem with the workbook. Often the file came from an email attachment or a download that stopped, or from a USB stick pulled out too soon.
Why Word and Excel refuse it
A .docx or .xlsx file is a ZIP archive: small XML parts for the text, the styles and each sheet, packed together. At the very end sits the central directory, the list of every part and where it starts. A file cut short loses that list first, and Word or Excel will not look any further.
What happens to the file
The tool reads each part from its own header, keeps the ones that unpack whole and writes a new central directory for them. A broken part is left out rather than passed on; if that part held the text or a sheet, that piece cannot come back.
What you come away with
A .docx or .xlsx that opens, with the text and the cells that arrived. An image, a style or a sheet whose part did not arrive whole may be missing. Nothing is retyped or recalculated: what the new file holds is what was in the old one.
Where it works, from our tests
Excel workbooks cut short came back in every case we tested, and so did those with shifted data; most with a damaged zone did too. Word documents with a damaged zone or shifted data came back every time, and most of those cut short. A Word document whose start was lost did not. Scattered errors left about half of each unrecovered. The file types guide has the full table.
Before you start
Keep the original. Try Open and Repair in Word or Excel first: it costs nothing and is sometimes enough. If the file came by email or from a shared folder, a fresh copy from there gives you everything.
Check the new file
Read the document through, or open every sheet of the workbook. Check the figures that matter most against another source, and save the file under a new name before you edit it.
Common questions
Are the formulas in Excel kept?
Where a sheet's part arrived whole, yes: its formulas and values are copied as they were.
Does this apply to old .doc and .xls files?
No. This guide covers .docx and .xlsx, the formats Office has saved by default since 2007. The older .doc and .xls are built differently.
Is the file uploaded anywhere?
No. It is read in your browser, on your device, and never leaves it.
Sources
Documents consulted for this guide.