  Calibration of pixel values is a fundamental step for accurate measurements
in astronomical imaging. In astronomical jargon this is known as estimating
zero point magnitude. Here, we introduce a newly added script in GNU Astronomy
Utilities (Gnuastro) version 0.20 for the zero point magnitude estimation,
named: astscript-zeropoint. The script offers numerous features, such as the
flexibility to use either image(s) or a catalog as the reference dataset.
Additionally, steps are parallelized to enhance efficiency for big data. Thanks
to Gnuastro's minimal dependencies, the script is both flexible and portable.
The figures of this research note are reproducible with Maneage, on the Git
commit c89275e.
