Internet Checksum Calculator

How it is calculated

Paste your data as hexadecimal digits; spaces and upper or lower case are ignored, but anything that is not 0-9 or a-f (including a 0x prefix) makes the tool return nothing. The digits are cut into 16-bit words of four hex digits. They are added together, and every carry out of bit 16 is folded back into the low 16 bits: sum = (sum & 0xFFFF) + (sum >> 16), repeated until nothing is left over. The checksum is the bitwise complement of that sum, ~sum & 0xFFFF, printed as four hex digits with a 0x prefix. If the data ends in an incomplete word it is padded with zeros on the right, so the single byte ab is treated as ab00. To generate a header checksum, set the checksum field to 0000 first.

Common mistakes

• Forgetting to zero the checksum field before generating. With the old value left in, the sum is wrong and you get a different number (a header that already contains a valid checksum yields 0x0000).
• Including a 0x prefix or separators like colons or commas. Only hex digits and whitespace are accepted.
• Entering decimal or ASCII text. The tool reads every character as a hex digit, so "1500" means bytes 15 00, not the number fifteen hundred.
• Dropping a leading zero in a byte. "4 5 00" collapses to 4500 after whitespace is removed, but "500" is padded on the right to 5000, a different value.
• Expecting a hash. The Internet checksum only catches accidental corruption; it is trivially forged and is not a security check.