If you want to find out how much one quart of 6% bleach will raise your chlorine level, put in .25 since one quart is equal to one-fourth of a gallon. If you want to know about half a gallon, put .5, or if you want to know how much a 3 quart jug will raise it (since that is how much is in most of the bottles sold these days), you put .75. Hope this helps.