  The luminosity determination for the ATLAS detector at the LHC during Run 2
is presented, with $pp$ collisions at $\sqrt{s}=13$ TeV. The absolute
luminosity scale is determined using van der Meer beam separation scans during
dedicated running periods in each year, and extrapolated to the physics
data-taking regime using complementary measurements from several
luminosity-sensitive detectors. The total uncertainties in the integrated
luminosities for each individual year of data-taking range from 0.9% to 1.1%,
and are partially correlated between years. After standard data-quality
selections, the full Run 2 $pp$ data sample corresponds to an integrated
luminosity of $140.1\pm 1.2$ fb$^{-1}$, i.e. an uncertainty of 0.83%. A
dedicated sample of low-pileup data recorded in 2017-18 for precision Standard
Model physics measurements is analysed separately, and has an integrated
luminosity of $338.1\pm 3.1$ pb$^{-1}$.
