Volume measures how much space an item fills. Volume measures are used for liquids. Measuring a liquid by volume is usually faster than weighing it, and accuracy is good.
Solid ingredients are usually not measured by volume because they cannot usually be measured accurately by this method. One pint of chopped onions will vary considerably in weight, depending on how large or small the onions are cut and whether the pint measure is filled loosely or packed.
Dry ingredients such as flour or sugar are usually weighed in the bakeshop. However, they are sometimes measured by volume in the kitchen, when speed is more important than accuracy. To measure dry ingredients by volume, fill a dry-volume measure until the ingredient is mounded over the top. Then level it off with a spatula or other straightedge.