Quick answer: A checksum, or hash, is a short fingerprint calculated from a file's contents, used to confirm that a copy is exactly identical to the original.
Checksum explained
A checksum is a short fingerprint calculated from everything inside a file. Algorithms such as SHA-256, SHA-1 and MD5 turn a file into a fixed-length string of letters and numbers. Change a single byte and the result is completely different.
After a big transfer, you and the recipient can each calculate the checksum and compare them. If they match, the file arrived intact. On Windows, PowerShell's Get-FileHash does this, and on a Mac the Terminal command shasum -a 256 does.
Use SHA-256 when you can. MD5 is fine for spotting accidental corruption, but it is no longer trusted against deliberate tampering.
MD5 vs SHA-1 vs SHA-256
| MD5 | SHA-1 | SHA-256 | |
|---|---|---|---|
| Length | 32 characters | 40 characters | 64 characters |
| Catches accidental damage | Yes | Yes | Yes |
| Safe against deliberate tampering | No | No longer recommended | Yes |
For example, the SHA-256 of a completely empty file is always e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855. Any file with even one byte in it gives a totally different string.
How to check a file
- On this site: each file on a download page shows its SHA-256 checksum. After downloading, press Check it, add your file, and the file hash checker tells you whether it matches.
- Windows: open PowerShell in the file's folder and type Get-FileHash followed by the file name. It uses SHA-256 by default.
- Mac: open Terminal and type shasum -a 256, a space, then drag the file into the window and press Return.
- Linux: run sha256sum followed by the file name.
When a checksum is worth it
- Very large downloads, where a connection drop could leave a file slightly short without any error.
- Footage, archives and backups that will be deleted from the original device after sending.
- Software or disk images, where you want to be sure nobody swapped the file.
For a handful of holiday photos, simply opening them is check enough. If checksums do not match, do not try to repair the file: download it again, ideally on a stable connection, and check once more.
Frequently asked questions
Why would a checksum not match?
The file was changed, for example compressed by an app, cut short by a failed download, or it is simply a different version.
Is MD5 safe to use?
For checking accidental damage, yes. For security, use SHA-256, because MD5 collisions can be created on purpose.
Does a checksum keep my file private?
No. A checksum only proves a file is unchanged. To keep a file private, encrypt it, for example in a password-protected ZIP.
Do checksums depend on the file name?
No. Only the contents count, so renaming a file does not change its checksum.
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