How to Use a Redstone Comparator in Minecraft

Guide Java Bedrock

Quick answer: A redstone comparator has two modes. In compare mode (default) it passes its back input through only if that signal is at least as strong as any side input. Click the front torch to switch to subtract mode, which outputs the back signal minus the strongest side signal. Placed against a container, it instead reads how full the container is.

What you need

Redstone comparator (3 stone + 3 redstone torches + 1 nether quartz)
Chest or hopper to practice container reading
Redstone dust and a lamp to see the output

Step by step

Craft it. A comparator takes 3 stone, 3 redstone torches and 1 nether quartz - the quartz means one Nether trip first.
Learn the layout. The two short prongs face the input (back), the single torch is the output (front), and the two bare sides accept comparison signals. Unlike a repeater, a comparator keeps the signal strength as-is instead of refreshing it to 15.
Compare mode (front torch off, the default): the output equals the back input only if the back is stronger than or equal to both side inputs; otherwise the output is off. Use it to require two conditions at once.
Subtract mode (click the comparator so the front torch lights up): output = back signal minus the strongest side signal. Feed a 15-strength input and a 3-strength side and you get 12 out - the basis of analog counters.
Read containers. Against a chest, hopper, furnace, barrel, shulker box, cake, item frame, lectern, jukebox or command block, the comparator ignores signals and outputs 0-15 based on fullness or state. A full double chest gives 15; each cake slice left changes the level.
Build an item filter. Put exact filler stacks in a hopper so the comparator reads strength 2-3, run that through a torch and back into the hopper below - the classic design only unlocks when enough of the target item arrives. This is the heart of every storage system.
Mind the delay. A comparator always adds 1 game tick (0.05s) of delay, half a repeater tick, which matters in fast clocks.

Java vs Bedrock

Compare, subtract and container reading work the same in Java and Bedrock. One Java quirk: comparators can read some block states (like a cake or item frame rotation) through a solid block, which Bedrock does not always replicate - test filters on your edition.

Versions: Comparators arrived in Java 1.5, the Redstone Update (January 2013), and container measuring was added in the same snapshot cycle. The recipe and both modes have been unchanged since.

Frequently asked questions

What is the difference between a repeater and a comparator?

A repeater refreshes the signal to full 15 and only passes one way. A comparator keeps the exact strength, can subtract signals, and reads container fullness - but it does not boost a fading signal.

How do I make a comparator read a chest?

Place it with its back against the chest (or a block in front of the chest) and dust on the output. The signal strength scales with fullness: roughly empty = 0, one stack in a chest = 1, completely full = 15.

Why does my item filter overflow or jam?

The filler items in the filter hopper must be ones that stack to 64 and cannot match the sorted item in the wrong slots. If the comparator reads a strength above 3 the filter opens for everything - keep exactly the classic 41-target-item plus 1-1-1-1 filler layout.