The US Geological Survey (USGS) classifies water: - with up to 3.5gpg of hardness minerals as ‘soft’ water - 3.56 thru 7.01gpg as ‘moderately hard’ - 7.06 thru 10.51gpg as ‘hard’ - 10.57gpg or more as ‘very hard’ water. In terms of ppm or mg/l that equates to: - less than 60 for ‘soft’ water - 60-120 for ‘moderately hard’ - 121-180 as ‘hard’ - above 180 as ‘very hard’ water.