Manual page from samtools-1.12
released on 17 March 2021


samtools flagstat – counts the number of alignments for each FLAG type


samtools flagstat in.sam|in.bam|in.cram


Does a full pass through the input file to calculate and print statistics to stdout.

Provides counts for each of 13 categories based primarily on bit flags in the FLAG field. Information on the meaning of the flags is given in the SAM specification document <>.

Each category in the output is broken down into QC pass and QC fail. In the default output format, these are presented as "#PASS + #FAIL" followed by a description of the category.

The first row of output gives the total number of reads that are QC pass and fail (according to flag bit 0x200). For example:

122 + 28 in total (QC-passed reads + QC-failed reads)

Which would indicate that there are a total of 150 reads in the input file, 122 of which are marked as QC pass and 28 of which are marked as "not passing quality controls"

Following this, additional categories are given for reads which are:


0x100 bit set


0x800 bit set


0x400 bit set


0x4 bit not set

paired in sequencing

0x1 bit set


both 0x1 and 0x40 bits set


both 0x1 and 0x80 bits set

properly paired

both 0x1 and 0x2 bits set and 0x4 bit not set

with itself and mate mapped

0x1 bit set and neither 0x4 nor 0x8 bits set


both 0x1 and 0x8 bits set and bit 0x4 not set

And finally, two rows are given that additionally filter on the reference name (RNAME), mate reference name (MRNM), and mapping quality (MAPQ) fields:

with mate mapped to a different chr

0x1 bit set and neither 0x4 nor 0x8 bits set and MRNM not equal to RNAME

with mate mapped to a different chr (mapQ>=5)

0x1 bit set and neither 0x4 nor 0x8 bits set and MRNM not equal to RNAME and MAPQ >= 5


The -O option can be used to select two alternative formats for the output.

Using -O tsv selects a tab-separated values format that can easily be imported into spreadsheet software. In this format the first column contains the values for QC-passed reads, the second column has the values for QC-failed reads and the third contains the category names.

Using -O json generates an ECMA-404 JSON data interchange format object <>. The top-level object contains two named objects QC-passed reads and QC-failed reads. These contain the various categories listed above as names and the corresponding count as value.

For the default format, the mapped, properly paired, and singletons show the count as a percentage of the total number of QC-passed or QC-failed reads after the category name. For example:

32 + 0 mapped (94.12% : N/A)

In the tsv and json formats, these percentages are listed in separate categories mapped %, properly paired %, and singletons %. If the percentage cannot be calculated (because the total is zero) then in the default and tsv formats it will be reported as `N/A'. In the json format, it will be reported as a JSON `null' value.


-@ INT

Set number of additional threads to use when reading the file.


Set the output format. FORMAT can be set to `default', `json' or `tsv' to select the default, JSON or tab-separated values output format. If this option is not used, the default format will be selected.


Written by Heng Li from the Sanger Institute.


samtools (1), samtools-idxstats (1), samtools-stats (1)

Samtools website: <>