Can't really tell you how much I use. Kind of an eyeball thing. I do know if you use too much and let it sit til the water gets cold you can have crystals form on the inside of your carboy that are a bugger to get out. I tend to leave the oxyclean in the carboy for a day or two, depends when I get back to the basement. It doesn't take a whole lot of the oxyclean to clean a carboy, maybe a tbs. or so. If you rinse it while it is still warm you can use more of the oxyclean without worrying about the crystals forming. Arne.