What is the SUM file format?
.sum files are compact binary sidecar files generated automatically by Garmin software and GPS devices as integrity companions to Garmin device data files. Each .sum file contains exactly 8 bytes: a single 64-bit little-endian unsigned integer representing the arithmetic byte-sum of every byte in the paired file.
Common pairings include map image files (.img), voice prompt packages (.vpm), and junction view images (.jcv). The .sum file shares the same base name as its companion - for example, gmapbmap.img is accompanied by gmapbmap.sum.
The checksum is not a CRC or cryptographic hash but a simple arithmetic sum of all byte values. Garmin Express and related tools generate these files automatically during map installation and device synchronization. If a .sum file is deleted, Garmin software regenerates it on the next sync. If the computed sum does not match the stored value, Garmin software flags the companion file as potentially corrupted.
Users do not need to open .sum files directly. The format is proprietary to Garmin's device file ecosystem and is unrelated to other uses of the .sum extension - such as plain-text checksum manifests generated by tools like md5sum.
Security & safety
RISK: LOW8-byte binary checksum files; no executable code; no risk.
Format details
in a nutshell- MD5/SHA checksum text file - Many tools use .sum for plain-text checksum files (e.g. md5sum output); unrelated to Garmin binary SUM.
- eSignal Market Data Summary - eSignal trading platform reportedly uses .sum for market data summary files - completely separate binary format.
Programs that open SUM files
Windows1 app
Garmin Express
Free Not meant to be opened directly. Garmin Express manages SUM files automatically during map/software updates. 
macOS1 app
Garmin Express
Free Not meant to be opened directly. Garmin Express manages SUM files automatically. Technical details
deep spec| File size | Exactly 8 bytes (always) |
| Data content | Single 64-bit unsigned integer (little-endian) |
| Checksum algorithm | Arithmetic byte sum (sum of all byte values in companion file, not CRC or hash) |
| Byte order | Little-endian |
| Role | Integrity sidecar companion to a paired Garmin device file |
| Paired file types | `.img` (map image), `.vpm` (voice prompts), `.jcv` (junction view) |
| Naming convention | Same base name as companion file (e.g., `gmapbmap.img` → `gmapbmap.sum`) |
| Auto-generation | Created by Garmin Express and device diagnostics; regenerated automatically if deleted |
| User interaction | Not intended to be opened directly by users |
| Platform | Windows, macOS (Garmin Express), Garmin GPS devices (internal) |
| Released | early 2000s (alongside Garmin IMG map format) |
SUM conversions
Community Q&A
asked by usersNo questions yet - be the first to ask about SUM files.
