intel hex checksum calculator

What this Intel HEX checksum calculator does

This page helps you calculate and verify checksums for Intel HEX records used in firmware flashing, microcontroller programming, bootloaders, and memory image pipelines. If you have ever seen a programmer reject a file with a checksum error, this tool gives you a quick way to diagnose and fix it.

• Generate a checksum from record fields (byte count, address, record type, and data).
• Build the full HEX line including the leading colon and checksum byte.
• Validate an existing record and show whether its checksum is correct.
• Parse a valid line back into fields so you can inspect or edit it quickly.

Intel HEX record format refresher

Each record line is structured as:

:LLAAAATTDD...DDCC

• : Start code.
• LL = data byte count.
• AAAA = 16-bit address.
• TT = record type (00 data, 01 EOF, etc.).
• DD...DD = data bytes (optional depending on type).
• CC = checksum byte.

Checksum formula

The checksum is the two’s complement of the least significant byte of the sum of all bytes in the record except the start colon and checksum itself.

checksum = (0x100 - (sum & 0xFF)) & 0xFF

Another way to think about it: when you add every byte in the record including CC, the low 8 bits should equal 0x00.

Worked example

For the record :10010000214601360121470136007EFE09D2190140, the tool will parse LL=10, AAAA=0100, TT=00, and 16 data bytes. It computes the checksum as 40, which matches the line, so the record is valid.

How to use this calculator effectively

• Paste data bytes with or without spaces (both formats work).
• Leave Byte Count blank if you want it calculated automatically.
• Use Validate Record when debugging generated HEX output.
• Make sure address and record type are in hexadecimal.

Common checksum mismatch causes

• Data length does not match LL.
• Dropped or extra byte while copying lines manually.
• Wrong record type selected for non-data records.
• Hidden characters, line breaks, or whitespace inserted in tooling.
• Editing payload bytes without recomputing checksum.

Practical firmware workflow tip

If you post-process HEX files (for version stamping, serial IDs, or calibration constants), always recompute checksums immediately before output. A final validation pass over every line can prevent failed updates in production programming stations.