How much ketchup do you need to make a gallon?

Liquids and other watery substances (such as ketchup, mustard, relish, etc) are typically measured in ounces (oz). It takes approximately 128 ounces to make a gallon, so depending on how large your bottle of ketchup is, you may need to buy more or less accordingly in order to have one gallon (or how many gallons you need) of ketchup.