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2.3 Block size

Some GNU programs (at least df, du, and ls) display sizes in “blocks”. You can adjust the block size and method of display to make sizes easier to read. The block size used for display is independent of any file system block size. Fractional block counts are rounded up to the nearest integer.

The default block size is chosen by examining the following environment variables in turn; the first one that is set determines the block size.


This specifies the default block size for the df command. Similarly, DU_BLOCK_SIZE specifies the default for du and LS_BLOCK_SIZE for ls.


This specifies the default block size for all three commands, if the above command-specific environment variables are not set.


This specifies the default block size for all values that are normally printed as blocks, if neither BLOCK_SIZE nor the above command-specific environment variables are set. Unlike the other environment variables, BLOCKSIZE does not affect values that are normally printed as byte counts, e.g., the file sizes contained in ls -l output.


If neither command_BLOCK_SIZE, nor BLOCK_SIZE, nor BLOCKSIZE is set, but this variable is set, the block size defaults to 512.

If none of the above environment variables are set, the block size currently defaults to 1024 bytes in most contexts, but this number may change in the future. For ls file sizes, the block size defaults to 1 byte.

A block size specification can be a positive integer specifying the number of bytes per block, or it can be human-readable or si to select a human-readable format. Integers may be followed by suffixes that are upward compatible with the SI prefixes for decimal multiples and with the ISO/IEC 80000-13 (formerly IEC 60027-2) prefixes for binary multiples.

With human-readable formats, output sizes are followed by a size letter such as ‘M’ for megabytes. BLOCK_SIZE=human-readable uses powers of 1024; ‘M’ stands for 1,048,576 bytes. BLOCK_SIZE=si is similar, but uses powers of 1000 and appends ‘B’; ‘MB’ stands for 1,000,000 bytes.

A block size specification preceded by ‘'’ causes output sizes to be displayed with thousands separators. The LC_NUMERIC locale specifies the thousands separator and grouping. For example, in an American English locale, ‘--block-size="'1kB"’ would cause a size of 1234000 bytes to be displayed as ‘1,234’. In the default C locale, there is no thousands separator so a leading ‘'’ has no effect.

An integer block size can be followed by a suffix to specify a multiple of that size. A bare size letter, or one followed by ‘iB’, specifies a multiple using powers of 1024. A size letter followed by ‘B’ specifies powers of 1000 instead. For example, ‘1M’ and ‘1MiB’ are equivalent to ‘1048576’, whereas ‘1MB’ is equivalent to ‘1000000’.

A plain suffix without a preceding integer acts as if ‘1’ were prepended, except that it causes a size indication to be appended to the output. For example, ‘--block-size="kB"’ displays 3000 as ‘3kB’.

The following suffixes are defined. Large sizes like 1Y may be rejected by your computer due to limitations of its arithmetic.


kilobyte: 10^3 = 1000.


kibibyte: 2^{10} = 1024. ‘K’ is special: the SI prefix is ‘k’ and the ISO/IEC 80000-13 prefix is ‘Ki’, but tradition and POSIX use ‘k’ to mean ‘KiB’.


megabyte: 10^6 = 1,000,000.


mebibyte: 2^{20} = 1,048,576.


gigabyte: 10^9 = 1,000,000,000.


gibibyte: 2^{30} = 1,073,741,824.


terabyte: 10^{12} = 1,000,000,000,000.


tebibyte: 2^{40} = 1,099,511,627,776.


petabyte: 10^{15} = 1,000,000,000,000,000.


pebibyte: 2^{50} = 1,125,899,906,842,624.


exabyte: 10^{18} = 1,000,000,000,000,000,000.


exbibyte: 2^{60} = 1,152,921,504,606,846,976.


zettabyte: 10^{21} = 1,000,000,000,000,000,000,000


2^{70} = 1,180,591,620,717,411,303,424.


yottabyte: 10^{24} = 1,000,000,000,000,000,000,000,000.


2^{80} = 1,208,925,819,614,629,174,706,176.

Block size defaults can be overridden by an explicit --block-size=size option. The -k option is equivalent to --block-size=1K, which is the default unless the POSIXLY_CORRECT environment variable is set. The -h or --human-readable option is equivalent to --block-size=human-readable. The --si option is equivalent to --block-size=si. Note for ls the -k option does not control the display of the apparent file sizes, whereas the --block-size option does.

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