What is a checksum?
A checksum is a small value derived from data to detect accidental corruption. Learn how checksums work and compute one free.
A checksum is a compact value computed from a block of data so you can later detect whether the data changed. Download a file, compute its checksum, compare to the published one — if they match, the file arrived intact.
Detecting accidental change
Checksums are designed to catch corruption — flipped bits from a bad disk, a truncated download, a garbled transfer. If even one byte changes, a good checksum changes too, so a mismatch tells you the data is not what was published. Hash functions like SHA-256 are the modern way to compute file checksums.
Checksum vs cryptographic hash
Simple checksums (like CRC32 or Luhn) catch accidental errors cheaply but can be forged deliberately. Cryptographic hashes (SHA-256) also resist an attacker crafting a collision, so they double as integrity checks against tampering, not just accidents.
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