That was the point hundreds of years ago, but its now fairly inaccurate. For the purposes of food labeling, a fluid ounce is defined as 30mL, which makes a flu

That was the point hundreds of years ago, but its now fairly inaccurate. For the purposes of food labeling, a fluid ounce is defined as 30mL, which makes a fluid ounce of water weigh almost 5% more than an ounce. Sodas are typically more dense - Coke is 10% more dense than water, for example, for a total weight of over 13.5 ounces for a 12 fl oz can. Seems like a pretty big difference to me.