To use as a sanitizer: 2 oz per gallon of water.

To use as an antioxidant and bactericide: 1/4 teaspoon to 6 gallons of wine.
Dissolve the potassium metabisulphite in water before adding to the wine. When added to wine, it forms sulfur dioxide gas. This prevents most wild microorganisms from growing, and acts as an antioxidant, protecting both the color and delicate flavors of wine.

Since potassium metabisulphite does not add sodium to the diet, it generally preferred over sodium metabisulphite.