Minecraft wiki entry for Redstone_components, associated text: A redstone comparator that is activated by a container outputs a power level in proportion to how full the container is (rounded up, so a single item in a container produces a power level of at least 1). A container's fullness is measured by stacks: for example, a single shovel (a non-stackable item), 16 signs, or 64 sticks are all considered to be equivalent, full stacks.