Bird seed is sold by weight, but its mass can vary widely depending on the type of seed, packaging, and moisture content. A standard small bag might hold a few hundred grams, while bulk containers can contain several kilograms. Because different seeds—such as sunflower, millet, and nyjer—have different densities, equal volumes do not always equal equal mass. For precise measurements, it is best to check the product label or weigh the seed using a kitchen scale.