A standard red Solo cup holds about 2 cups of liquid when filled to the very top.

Most classic red Solo party cups are 16-ounce cups, and since 1 cup is 8 fluid ounces, 16÷8=216\div 8=216÷8=2 cups. Some newer or larger versions can be 18 ounces, which is a bit more than 2 cups (2.25 cups), but the widely used “regular” red Solo cup is the 16-ounce size.