Minecraft wiki entry for Hopper, associated text: A hopper does not output any redstone signals by itself, but its fullness can be read using a redstone comparator, which needs to be placed next to it and facing away from it. An empty hopper outputs a signal strength of 0 and a completely full hopper outputs a signal strength of 15. Notably, a single stackable item (16 or 64) outputs a signal strength of 1 and a single non-stackable item outputs a signal strength of 3.