cmz
Public beta · Windows
cmz
Public beta · Windows
Lossless compression for Windows
Strong compression at a practical speed: seconds or minutes per file, not hours. Photos, music, Office documents, PDFs and zip files are already compressed, so 7-Zip and WinRAR barely touch them. cmz makes JPEG photos 24% smaller and MP3s 13% smaller, often cuts Office files to less than half, and packs astronomy FITS data 17% tighter than 7-Zip. Every file comes back byte for byte.
Measured October 2026 · AMD Ryzen 5 3600
Size after compression as a share of the original, so shorter is better. 7-Zip runs at its strongest setting (Ultra). Every cmz result was unpacked again and compared with the original, byte by byte: 332 files, 0 differences.
Files already packed with a strong method stay about the same size, with cmz and 7-Zip alike. cmz recognises such data and stores it as it is instead of spending minutes on it. In this test: a RAR archive 100.0% (7-Zip 100.0%); an MSI installer 99.0% (7-Zip 99.6%); an EXE installer 99.8% (7-Zip 99.8%).
Most of these files are private, so nobody else can repeat this test exactly. To check cmz on your own machine, use our 27 public test files: each one comes from its publisher, with a checksum, a download script and the cmz result. On the same page, size against time for cmz, 7-Zip, xz, zpaq and the paq family.
Every .cmz file ends with a checksum of the original. Unpacking checks it, so a damaged file is reported instead of quietly handed back wrong.
The .cmz goes next to the source as a new file, and cmz never writes over anything. If a name is taken, it saves as “name (2)”.
No key, no trial, no internet needed to get your files back. Every new version of cmz opens every older .cmz file.
For astrophotographers and observatories
cmz recognises FITS images and tables, raw camera frames and the zip files Siril writes. On 26 astronomy files from Kepler, Planck, Gaia, Chandra, Fermi, SDSS, SDO, 2MASS, WISE, LIGO and other archives, it gave the smallest result on every single file.
| 26 astronomy files · 811 MB | Size | Unpack |
|---|---|---|
| cmz | 40.6% | 19 min 50 s |
| zpaq -m5 | 43.8% | 33 min 15 s |
| xz -9e | 49.0% | 17 s |
| 7-Zip Ultra | 49.2% | 19 s |
| zstd -22 | 54.4% | 3.0 s |
| gzip -9 | 57.7% | 5.0 s |
| 22 camera frames (CCD, CMOS) · 543 MB | Size | Unpack |
|---|---|---|
| cmz | 35.6% | 13 s |
| zpaq -m5 | 35.6% | 20 min 29 s |
| JPEG XL, effort 7 | 38.9% | 24 s |
| 7-Zip Ultra | 39.4% | 10 s |
| XISF + zstd | 39.8% | 1.9 s |
| fpack, Rice | 45.7% | 3.9 s |
Against fpack, the FITS standard's own compressor, on the 15 files that hold images: its best lossless setting leaves 57.5%, cmz 37.0%. Siril's zipped frame sequences (2 files, 842.5 MB) shrink to 71.2%; 7-Zip leaves them at 100.0%. Camera frames take a dedicated fast path: 542.6 MB of frames packed in 43 s and unpacked in 13 s.
Size and speed, side by side
cmz looks at your data much more closely than a zip program does. That is why its files are smaller, and it is also why it takes longer. The time is spent once, when you pack; the space is saved for as long as you keep the file. That suits photo libraries, finished projects, backups and observation archives. For files you send every few minutes, 7-Zip is the quicker tool.
| File | Original | cmz | 7-Zip, Ultra | ||||
|---|---|---|---|---|---|---|---|
| Result | Pack | Unpack | Result | Pack | Unpack | ||
| JPEG photo, 12 megapixels | 4.9 MB | 77.9%3.8 MB | 9.3 s | 4.6 s | 100.0%4.9 MB | 0.3 s | 0.0 s |
| MP3 track | 9.8 MB | 87.8%8.6 MB | 37 s | 11 s | 99.8%9.7 MB | 0.8 s | 0.2 s |
| Excel workbook | 203 KB | 19.5%39 KB | 29 s | 7.8 s | 84.9%172 KB | 0.1 s | 0.0 s |
| Word document | 94 KB | 38.6%36 KB | 41 s | 3.0 s | 89.4%84 KB | 0.0 s | 0.0 s |
| Scanned PDF magazine | 20.7 MB | 64.0%13.2 MB | 2 min 48 s | 53 s | 92.1%19.0 MB | 1.4 s | 0.6 s |
| Source code, .tar.gz | 4.4 MB | 43.6%1.9 MB | 2 min 14 s | 75 s | 96.0%4.2 MB | 0.3 s | 0.0 s |
| Plain text, a novel | 2.0 MB | 22.4%449 KB | 19 s | 19 s | 30.9%620 KB | 0.4 s | 0.0 s |
| Camera frame, FITS (ZWO ASI2600MC) | 52.2 MB | 49.9%26.0 MB | 3.9 s | 1.4 s | 56.4%29.4 MB | 22 s | 1.5 s |
| Siril sequence, zipped | 553.0 MB | 70.6%390.5 MB | 2 min 16 s | 41 s | 100.0%553.0 MB | 109 s | 0.5 s |
| Planck sky map, FITS | 83.9 MB | 63.5%53.3 MB | 4 min 45 s | 2 min 51 s | 75.8%63.6 MB | 15 s | 3.0 s |
Result = size after compression, as a share of the original. Times are wall-clock seconds on one PC (AMD Ryzen 5 3600, 6 cores); cmz uses several cores on large files. The 7-Zip figures for the three astronomy files were measured earlier in October on the same PC. cmz times were measured with beta 2; beta 3 added a calibration step that made it about 3% slower and its files a little smaller. Where beta 4 changed the result (the Word document, the scanned PDF and the .tar.gz), size and times are beta 4's own.
Under the hood
Only the outline; the details stay in the workshop for now.
How it works for you
Right-click files or a folder in Explorer and pick , or drop them on the window.
Each file gets its own .cmz next to it. The list shows the new size, the ratio and the time for every file.
Double-click a .cmz file and the original is back where it was, identical to the last byte.
There is no installer. On first start cmz offers to install itself for your user account: a Start menu entry, the Explorer menu and the .cmz file type. It needs no administrator rights, and Windows Settings → Apps removes it again.
Public beta
The beta is free to use until 31 December 2026. After that, cmz will be sold as a personal and a business licence, with prices announced before the beta ends. Whatever happens later, unpacking stays free and never asks for a key, and every .cmz file made with the beta will open in every later version.
Tell us which files you would like smaller, and what cmz did with them. Write to fmolnar.no1@gmail.com.
Questions
Yes. The unpacked file is identical to the original, bit for bit, and every .cmz carries a checksum to prove it. A JPEG stays the same JPEG, an MP3 the same MP3. (For astronomers there is an optional lossy mode for floating-point images, off by default and clearly marked.)
Videos (MP4, MKV), 7z and RAR archives, most installers and encrypted files are already packed as tightly as they go, so they stay about the same size with any tool. cmz recognises such data and stores it unchanged within seconds (the RAR in 0.2 s, the EXE installer in 9.9 s). In our test a RAR archive 100.0% (7-Zip 100.0%); an MSI installer 99.0% (7-Zip 99.6%); an EXE installer 99.8% (7-Zip 99.8%).
Yes. Each new version keeps the code for every older format. If an older cmz meets a file made by a newer one, it says so and asks you to update. It never guesses.
No. The beta works fully offline, and unpacking never needs a connection.
During the beta the program is not code-signed yet, so SmartScreen warns about a new publisher. Choose “More info”, then “Run anyway”. To check that your copy is the original, compare its SHA-256 checksum (PowerShell: ) with this one:
b3f04c642a53d743b2d0b3caa635ed5c057e72a5f29fc124a9f0f839f4565f62
Not in this version. A folder is packed file by file, and each file gets its own .cmz.
64-bit Windows 10 and 11.
About
Ferenc Molnár is from Hungary. He spent more than thirty years in industrial IT, most of them designing and building production information systems in Delphi and Oracle. Compression has been his hobby since 2015, and cmz is the first of his compressors to leave the workshop.
cmz is a joint work: Ferenc built it together with Claude, an AI model made by Anthropic. Ferenc set the goals and the standards and decided what stays and what goes; Claude wrote much of the code and ran the measurements. Nothing went in on opinion alone: every idea had to win a measurement against the version before it, and every test file had to come back byte for byte.
Questions, results and bug reports are welcome at fmolnar.no1@gmail.com.