Rare giant low surface brightness galaxies (gLSBGs) act as a stress test for the current galaxy formation paradigm. To answer the question 'How rare are they?', we estimate their volume density in the local Universe. A visual inspection of 120 deg^2^ covered by deep Subaru Hyper Suprime-Cam data was performed independently by four team members. We detected 42 giant disky systems (30 of them isolated) at z 50 kpc, 37 of which (including 25 isolated) had low central surface brightness ({mu}0,g => 22.7 mag arcsec^-2^). This corresponds to volume densities of 4.7010^-5^ Mpc^-3^ for all galaxies with giant extended discs and 4.0410^-5^Mpc^-3^ for gLSBGs, which converts to ~12700 such galaxies in the entire sky out to z < 0.1. These estimates agree well with the result of the Evolution and Assembly of GaLaxies and their Environments (EAGLE) cosmological hydrodynamical simulation. Giant disky galaxies represent the large-sized end of the volume density distribution of normal-sized spirals, suggesting the non-exceptional nature of giant discs. We observe a high active galactic nucleus fraction among the newly found gLSBGs.

Cone search capability for table J/MNRAS/520/L85/table1 (Astrometrics, redshifts, structural properties of the giant disc galaxies discovered in the HSC imaging data by visual inspection)