The total capacity or amount of something measured in gallons, or a charge based on the number of gallons.
From 'gallon' plus '-age' (a suffix creating nouns from measurement units). The word allows for abstract discussion of volume in commercial and industrial contexts.
Gallonage became essential business vocabulary when oil, milk, and water companies needed to bill customers by volume—it's the reason your water bill shows gallonage instead of pounds.
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