Comparator Signal Calculator - Items to Signal Strength

How a Comparator Reads a Container

A redstone comparator in comparator mode reads how full a container is. An empty container outputs 0; otherwise the output is signal = 1 + floor(14 * fullness), where fullness is the average across every slot of the items in that slot divided by the item's max stack size. So a chest does not need to be full to send signal 1, and it only reaches signal 15 when every slot holds a full stack.

The key idea is that the game measures fullness, not raw item count. Each slot contributes the number of items it holds divided by that item's maximum stack size, the slot fractions are averaged, and the result is scaled to 0 through 15. It is easiest to reason about in full-stack equivalents (FSE): one 64-stackable item counts as 1 FSE, one 16-stackable item (ender pearls, snowballs, eggs, signs) counts as 4 FSE, and one unstackable item (a sword, a bucket, a boat) counts as a whole 64 FSE because it fills an entire slot. That is why a single bucket in a chest reads the same as 64 cobblestone.

In FSE the formula becomes signal = 1 + floor(14 * FSE / (64 * slots)), and the minimum items to reach a target signal K is items = max(K, ceil(slots * 64 * (K - 1) / 14)). A single chest has 27 slots, so it needs 124 items for signal 2 and a full 1,728 items for signal 15. A double chest has 54 slots and doubles every threshold. Other containers just change the slot count: a hopper has 5 slots, a furnace 3, a dropper or dispenser 9.

Counts assume 64-stackable items. Divide by 4 for 16-stackable items and by 64 for unstackable items, rounding up. Because each unstackable item counts as a full stack, in small containers a single one can overshoot straight to a higher signal than you intended.

Blocks That Do Not Use the Fullness Formula

Several blocks ignore the container formula and output a value tied to their block state instead. These are the ones redstone builders rely on most, so the calculator handles each with its own controls rather than an item count.

A cake is worth 2 signal per remaining slice, so it counts down 14, 12, 10, 8, 6, 4, 2 as it is eaten and reads 0 once gone. A cauldron outputs its fill level 0 to 3 for water or powder snow, while lava only ever has one level and so always reads 3. A composter outputs its compost level 0 to 8 directly, which makes it a clean way to detect when a composter is ready to harvest bone meal.

The chiseled bookshelf is the one that trips people up: it does not count books. It reports the last slot a book was placed in or pulled out of, a number from 1 to 6, and reads 0 only until any slot is touched. That turns it into a six-position book switch rather than a fill meter. The tool also covers lecterns, jukeboxes, beehives, respawn anchors, end portal frames, item frames and crafters, each with its own fixed output.