Choose the Appropriate Tools and Units to Estimate Measurement

Volume measures liquid. Capacity measures how much a container will hold. They are about the same.

The English unit is the cup. A glass of milk is often a cup. 
ounces make 1 cup. 2 cups make a pint. 2 pints make a quart. 4 cups make a quart, too. 
4 quarts make a gallon. Milk comes in gallons.