# What is Checksum?

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.

_Source: https://sharelargefilesfree.com/what-is-checksum · Updated 2026-10-13_

**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.