Check RAR Dictionary Size Using Unrar

The unrar command-line utility can determine the exact dictionary size used when compressing a RAR archive, but standard listing commands will not show it. To view this technical metadata, you must run specific technical listing flags that expose the internal compression parameters. This guide explains how to extract the dictionary size using unrar, which flags to use, and how to interpret the resulting output.

Using the Technical List Command

Standard unrar l archive.rar commands only display basic file attributes, original sizes, packed sizes, and modification dates. To see the dictionary size, use the technical list command:

unrar lt archive.rar

Alternatively, you can use the verbose technical list command:

unrar vt archive.rar

The lt (list technical) and vt (verbosely list technical) parameters instruct the utility to read the internal archive headers and display advanced compression metadata for each file inside the archive.

Identifying the Dictionary Size in the Output

When you run unrar lt, look for the line labeled Compression within the technical details of the archived files.

For modern RAR5 archives, the output typically formats the compression algorithm and dictionary size using standard RAR switches:

Compression: RAR 5.0 -m5 -md=64M

In this output:

  • -m5 represents the compression method (Best).
  • -md=64M specifies the exact sliding dictionary size used during compression (in this example, 64 megabytes).

If the archive was created using an older format (RAR 4.x), the dictionary size is usually capped between 64 KB and 4 MB, and will be displayed accordingly (such as -md=4096k or -md=4M).

Considerations for Solid Archives

In solid archives (where multiple files are compressed as a single continuous data stream), the dictionary size applies across the entire archive or solid block, rather than individual files. The unrar lt command will still list the dictionary parameter under each file header to indicate the window size required by the decompressor to unpack that block.

If unrar is unavailable or produces truncated output on older builds, you can achieve the same result using the 7-Zip command-line tool by running:

7z l -slt archive.rar

This command will output a distinct Characteristics or Method line, such as RAR5:x:64m, explicitly confirming the dictionary size.