What is the H2W file format?
Files with the .h2w extension are test fill files generated by the H2testw and F3 applications. Both programs test USB drives and SD cards for their real capacity and read/write speed. The data stored in .h2w files is intentionally meaningless from the user's perspective - what matters is not the content but the ability to measure actual input/output performance.
H2testw writes a first file of exactly 1 GB, then continues with 1 MB files until the entire capacity of the tested device is filled. F3 takes a different approach: it generates files exclusively in 1 GB increments, with only the last file being smaller to perfectly fill the remaining space.
The algorithms of both tools measure the write and read speed across all generated .h2w files and present averaged results to the user. Writing the files also reveals the real capacity of the storage device - a critical test for counterfeit flash drives that falsely report a larger size than they actually contain. After testing is complete, all .h2w files must be deleted manually to reclaim the storage space. Files are named sequentially (1.h2w, 2.h2w, and so on). H2testw verifies integrity by re-reading the known deterministic binary pattern and reporting any mismatches per file.
Security & safety
RISK: LOWH2W files themselves are harmless binary fill data. The H2testw tool is safe and well-regarded; download only from h2testw.org or heise.de to avoid bundled PUPs. Do NOT confuse with other uses of the .h2w name - the extension is unique to this tool.
Format details
in a nutshellPrograms that open H2W files
Windows1 app
H2testw
Free Run H2testw.exe, select the target drive, click Write+Verify. Delete .h2w files when done. Technical details
deep spec| Format type | Binary fill data written as a deterministic test pattern |
| Container structure | Raw binary - no container headers or metadata encapsulation |
| Data encoding | Uncompressed deterministic binary sequence, known to the writing application |
| Typical file size | 1 GB per file (H2testw first file); subsequent H2testw files in 1 MB increments; F3 uses 1 GB files throughout with last file filling remaining space |
| File naming convention | Sequential integers - `1.h2w`, `2.h2w`, etc., filling the tested storage device |
| Pattern type | Deterministic pseudo-random binary sequence reproducible by H2testw for byte-by-byte verification |
| Integrity verification | H2testw re-reads all written files byte-by-byte and compares against the known pattern; mismatches indicate write failures or fake capacity |
| Primary use case | Detecting counterfeit flash storage (false advertised capacity) and measuring real read/write throughput |
| File lifecycle | Temporary test artifacts - must be deleted manually after testing to reclaim storage space |
| File identification | No magic bytes or file signature; files identified by sequential naming convention only |
| Target media | USB flash drives, SD cards, eMMC and other flash-based storage devices |
| Primary OS | Windows (H2testw); Linux and macOS via F3 cross-platform equivalent |
| Developer | Harald Bögeholz, c't magazine / heise.de |
| Cross-platform alternatives | F3 (Linux/macOS), ValiDrive (Windows GUI alternative) |
| Released | ~2008 (H2testw tool first published on heise.de) |
| Latest version | H2testw 1.4 (last known release) |
| Specification | h2testw.org |
H2W conversions
Community Q&A
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