How many calories are in half a pint of skim milk?

About 90 calories.

Traditionally, skimmed milk contains 0.5% butterfat. Some dairies sell fat-free skimmed milk, which would contain less.

Most of calories in fat-free skimmed milk come from lactose (milk sugar) and such proteins as casein. Some dairies add non-fat milk solids to milk to improve the flavor (and the nutritional value.) The guy down the road when I was growing up once had a minnow show up when his milk was strained; obviously, his milk would be lower in calories and nutrition.

As an agricultural product, milk varies in composition when they are on good pasture or poor, or eating hay of varying quality. If the cows are stressed by a pack of barking dogs, or a milkmaid who is rough because she just had a fight with her boyfriend, or even if her friend is sold to another farmer. To you, it's just a tasty drink, but cows care.