How to Check a Big File Arrived Intact (Checksums in 2 Minutes)

You’ve just sent a 70 GB archive. The recipient says it arrived. But did all of it arrive, exactly as it left?

Most of the time you don’t need to worry. Transfer tools check data as it goes, and BoltShare tracks every byte by its position in the file. But for things that really matter (a final video master, a disk image, an archive you’re about to delete the original of), it’s worth spending two minutes to be sure.

The tool for this is a checksum, also called a hash.

What a checksum is

A checksum is a short string of letters and numbers calculated from every single byte of a file. Something like:

3A7BD3E2360A3D29EEA436FCFB7E44C735D117C42D1C1835420B6B9942DD4F1B

Change even one byte of the file, a single letter in a 10 GB video, and the checksum comes out completely different. So if your copy and the recipient’s copy produce the same checksum, the files are identical. If they don’t match, something went wrong.

The most common type today is SHA-256. You might also see MD5 or SHA-1 mentioned. They’re older and fine for spotting accidental damage, but SHA-256 is the safe default.

On Windows

Open PowerShell (search for it in the Start menu) and run:

Get-FileHash "C:\Users\you\Videos\final-cut.mov"

The easiest way to get the path right is to type Get-FileHash (with a space), then drag the file from File Explorer into the PowerShell window. It pastes the full path for you.

You’ll get output like this. The long string in the middle is the SHA-256 checksum:

Algorithm  Hash                                                              Path
---------  ----                                                              ----
SHA256     3A7BD3E2360A3D29EEA436FCFB7E44C735D117C42D1C1835420B6B9942DD4F1B  C:\Users\...

Prefer the old Command Prompt? certutil -hashfile "path\to\file" SHA256 does the same thing.

On a Mac

Open Terminal (Applications → Utilities) and run:

shasum -a 256 ~/Movies/final-cut.mov

Again, you can type shasum -a 256 and drag the file into the window instead of typing the path.

On Linux

sha256sum final-cut.mov

Comparing them

Both of you run the command on your own copy of the file, then compare the results. You don’t need to check every character by eye. The first and last six or so are plenty, since a damaged file won’t match anywhere.

Two things that trip people up:

• Upper vs lower case doesn’t matter. Windows shows capitals, Mac and Linux show lowercase. 3a7bd3 and 3A7BD3 are the same.
• It takes a while on big files. The computer has to read every byte, so a 100 GB file can take several minutes on a fast SSD, and much longer on an external hard drive. That’s normal.

If they don’t match

Don’t panic, and don’t delete the original. Common causes:

• The download didn’t actually finish. Compare the file sizes first, down to the byte (Properties on Windows, Get Info on Mac). If they’re different, the copy is incomplete.
• The file was opened and saved on the receiving end before you checked. Some apps quietly rewrite metadata, especially photo and video tools.
• It’s a genuinely damaged copy. Send it again.

For a ZIP or 7z archive, there’s a shortcut: most archive tools have a Test option (in 7-Zip it’s right-click → 7-Zip → Test archive). It checks every file inside against the checksums stored in the archive itself, so the recipient can check it without needing anything from you.