This is the easiest way to find a downlight to fit an existing hole.
Simply measure the cutout size that you already have then find out which downlights will fit.
Downlights that fit a cutout diameter of less than 44mm
Downlights that fit a cutout diameter of 44mm - 55mm
Downlights that fit a cutout diameter of 56mm - 65mm
Downlights that fit a cutout diameter of 66mm - 75mm
Downlights that fit a cutout diameter of 76mm - 85mm
Downlights that fit a cutout diameter of 86mm - 99mm
Downlights that fit a cutout diameter of 100mm - 124mm
Downlights that fit a cutout diameter of 125mm - 164mm
Downlights that fit a cutout diameter of 165mm - 199mm
Downlights that fit a cutout diameter of more than 199mm