Redstone Comparator

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A redstone comparator is a block that can produce a redstone signal from its front by reading chests, lecterns, copper bulbs, and similar blocks. It can also repeat a signal without changing its strength.

It can also be set to either stop outputting a signal while its side input is receiving a stronger one (front torch off), or output a weaker signal based on its side input's signal strength (front torch on).

A redstone comparator can be broken instantly with any tool, or by hand, and drops itself as an item.

A redstone comparator is removed and dropped as an item if:

• its attachment block is moved, removed, or destroyed;
• water flows into its space;‌^{[Java Edition only]}
• a piston tries to push it or moves a block into its space.

If lava flows into a redstone comparator's space, the redstone comparator is destroyed without dropping as an item.

2–9 Redstone comparators generate in ancient cities.

A redstone comparator can be placed on the top of any opaque block with a solid full-height top surface (including upside-down slabs and upside-down stairs). In Bedrock Edition, a comparator can also be placed on walls and fences. For more information about placement on transparent blocks, see Opacity/Placement.

The redstone comparator has a front and a back — the arrow on the top of the comparator points to the front. When placed, the comparator faces away from the player. The comparator has two miniature redstone torches at the back and one at the front. The back torches turn on when the comparator's output is greater than zero (the arrow on top also turns red). The front torch has two states that can be toggled by using the comparator:

• Down and unpowered (indicating the comparator is in "comparison mode")
• Up and powered (indicating the comparator is in "subtraction mode")

The redstone comparator can take a signal strength input from its rear as well as from both sides. Side inputs are accepted only from redstone dust, block of redstone‌^{[Java Edition only]}, redstone repeaters, other comparators, and observers in specific scenarios. The redstone comparator's front is its output.

It takes 2 game ticks (0.1 seconds) for signals to move through a redstone comparator, either from the rear or from the sides. This applies to changing signal strengths as well as simply to turning on and off.

Redstone comparators check their power state before their scheduled ticks update. This results in redstone comparators not usually responding to 1-tick fluctuations of power or signal strength — for example, a 1-clock input is treated as always off from the side, and always on from the rear. This happens because the signal changes back to its original state before the redstone comparator checks its input states. However, certain setups such as powering any input with two separate observer pulses at the same time causes a redstone comparator to respond to 2-game-tick pulses.

The redstone comparator has four functions: maintain signal strength, compare signal strength, subtract signal strength, and measure certain block states (primarily the fullness of containers).

A redstone comparator with no powered sides outputs the same signal strength as its rear input, with a 1 redstone tick (= 2 game ticks) delay.

A redstone comparator in comparison mode (front torch down and unpowered) compares its rear input to its two side inputs. If either side input is greater than the rear input, the comparator output turns off. If neither side input is greater than the rear input, the comparator outputs the same signal strength as its rear input.

The formula for calculating the output signal strength is as follows:

output = rear × [left ≤ rear ANDright ≤ rear]

A redstone comparator in subtraction mode (front torch up and powered) subtracts the signal strength of the greater side input from the signal strength of the rear input.

output = max(rear − max(left, right), 0)

For example: if the signal strength is 6 at the left input, 7 at the right input and 4 at the rear, the output signal has a strength of max(4 − max(6, 7), 0) = max(4−7, 0) = max(−3, 0) = 0.

If the signal strength is 9 at the rear, 2 at the right input and 5 at the left input, the output signal has a strength of max(9 − max(2, 5), 0) = max(9−5, 0) = 4.

A redstone comparator treats certain blocks behind it as power sources and outputs a signal strength proportional to the block's state. The comparator may be separated from the measured block by a comparator-conducting block. These blocks include all conductive blocks, but see below.

The comparator disregards the power level of the intervening block in this configuration.

In Java Edition, if the intervening block is powered to signal strength 15, then the comparator outputs 15 no matter the fullness of the container.^[1] This never occurs in Bedrock Edition.

In Bedrock Edition the list of comparator-conducting blocks also includes some non-conductive blocks. This may be useful in contraptions as they do not conduct redstone signals from elsewhere like a regular conductive block would.^[2] These blocks are:

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  Dirt Path and 👁 Image
  Farmland
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  Iron Chain
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  Piston and 👁 Image
  Sticky Piston
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  Copper Grate (all variants)
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  Chorus Plant and 👁 Image
  Chorus Flower
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  Sniffer Egg

A redstone comparator can output a signal indicating how full a container is. (0 for empty, 15 for full, etc.) The table on the right is described more in detail, later in this section.

Note: s means stack.

Containers that can be measured by a comparator include:

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  Barrel
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  Blast furnace
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  Brewing stand
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  Chest
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  Copper Chest
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  Crafter
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  Decorated pot
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  Dispenser
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  Dropper
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  Furnace
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  Hopper
• 👁 BlockSprite chest-left.png: Sprite image for chest-left in Minecraft
  👁 BlockSprite chest-right.png: Sprite image for chest-right in Minecraft
  Large chest
• 👁 BlockSprite copper-chest-left.png: Sprite image for copper-chest-left in Minecraft
  👁 BlockSprite copper-chest-right.png: Sprite image for copper-chest-right in Minecraft
  Large copper chest
• 👁 BlockSprite chest-left.png: Sprite image for chest-left in Minecraft
  👁 BlockSprite chest-right.png: Sprite image for chest-right in Minecraft
  Large trapped chest
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  Minecart with chest on top of a detector rail
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  Minecart with hopper on top of a detector rail
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  Respawn Anchor
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  Shulker box (any color)
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  Smoker
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  Trapped chest

Generally speaking, the comparator output signal strength represents the average fullness of the slots, based on how many of that item form a full stack (64, 16, or 1 for non-stackable items). A partially filled bundle counts as a full stack.

The Minimum items for container signal strength table (right) shows the minimum full-stack-equivalent (FSE) to produce different signal strengths from common containers. A full-stack-equivalent quantifies how many normal 64-stackable items are needed to output a corresponding signal strength. The 's' is a constant 64, with the additional amount needed following after.

One may also consider the terms: cumulative-weight or weighted-sum instead of full-stack-equivalent.

Items that stack to a max of 16 (snowballs, signs, ender pearls, etc.), contribute +4 to the full-stack-equivalent for each unity (count of 1 item). Similarly, items that stack to 1 (minecart, boat, etc.) contribute +64, and items that stack to 64 contribute +1.

Example 1: 3 ender pearls contribute a 3×4 = 12 full-stack-equivalent.

Example 2: 16 ender pearls and 60 redstone dust contributes a 16×4 + 60×1 = 124 full-stack-equivalent.

Example 3: 1 minecart and 60 redstone dust contributes a 1×64 + 60×1 = 124 full-stack-equivalent.

Example 4: To produce a signal strength of 10 from a hopper, one requires a full-stack-equivalent of at least 3s + 14 = 206 but strictly less than 3s + 37 = 229. This can be done with 3 minecarts, and 14 dirt.

When a comparator measures a large chest or large trapped chest, it measures the entire large chest (54 slots), not just the half directly behind the comparator. in Java Edition, a chest or trapped chest that cannot be opened (either because it has a conductive block or a sitting cat above it) always produces an output of 0 no matter how many items are in the container — shulker boxes can always be measured, even if they cannot open.

Calculating signal strength from items

When a container is empty, the output is off.

When it is not empty, the output signal strength is calculated as follows:

signal strength = floor(1 + ((sum of all slots' fullnesses) ÷ (number of slots in container)) ÷ 14)

fullness of a slot = (number of items in slot) ÷ (max stack size for this type of item)

Example: 300 blocks in a dispenser (which has 9 slots), where each block stacks to a maximum of 64 has a 300 full-stack-equivalent. This produces an output with a signal strength of 8:

1 + ((300 items ÷ 64 items per slot) ÷ 9 slots) × 14 = 8.292, floored is 8

Calculating items from signal strength

It can be useful in redstone circuits to use containers with comparators to create signals of a specific strength. The number of items required in a container to produce a signal of desired strength is calculated as follows:

items required = max(desired signal strength, ceiling(((total slots in container) × 64 ÷ 14 × (desired signal strength − 1)))

Example: To use a furnace (which has 3 slots) to create a strength 9 signal, players need 110 items:

max(9, (3×64÷14) × (9−1)) = 109.714, rounded up is 110

Some non-container blocks can also be measured by a redstone comparator:

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Beehive and 👁 Image
bee nest

A hive or nest outputs a signal strength equal to the amount of honey in the hive/nest up to a maximum of 5.

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Cake

A cake outputs a signal strength relative to the amount of cake remaining. Each slice is worth 2 signal strength, with 7 total slices, for an output of 14 for a full cake.

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Cauldron

A cauldron outputs different signal strengths depending on how much water or powder snow is inside. From completely empty to completely full, the output values are 0, 1, 2, and 3. If lava is inside, the strength is always 3.

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Chiseled bookshelf

A chiseled bookshelf outputs a signal strength between 1 and 6 indicating the last slot interacted with. When no slot has been interacted with yet, it outputs 0.

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Composter

A composter outputs different signal strengths depending on the level inside. From completely empty to completely full, the output values are 0, 1, 2, 3, 4, 5, 6, 7 and 8.

Note that under normal circumstances, a Composter will only temporarily produce power level 7 before automatically updating itself to level 8.

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Copper bulb

A copper bulb at any oxidation stage emits a full signal of 15 when lit and 0 otherwise.

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Copper Golem Statue

A copper golem statue emits a signal strength from 1 to 4 based on its pose.

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Command block

A command block stores the "success count" of the last command executed, which represents the number of times the most recently used command of this command block succeeded. A "success" is defined by the command's success conditions: if a red error message is returned in the chat, the command was not successful.

Most commands can succeed once per execution, but certain commands (such as those that accept players as arguments) can succeed multiple times, and the comparator outputs the number of times it succeeded (maximum 15 when sent to redstone dust, but in the code it is able to go up to the 32-bit integer limit, and can be used in contraptions with no redstone dust with those values).

A command block continues to store the success count of the last command executed until it executes its command again, thus the comparator continues to output the same signal strength even after the command block is no longer being activated (it doesn't turn off when the signal to the command block turns off).

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Crafter

A crafter outputs a signal strength equal to the number of crafting slots that are either disabled or occupied by an item. An empty crafter with no disabled slots does not output any signal through a comparator. A crafter with at least one slot disabled or filled outputs a signal through a comparator, with strength corresponding to the number of occupied slots.

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Creaking heart

An active creaking heart with a creaking linked to it outputs a signal strength dependent on the Euclidean distance between the heart and its creaking. The strength is calculated as follows:

output = 15 − floor(distance ÷ 32 × 15)

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End portal frame

An end portal frame outputs a full signal of 15 if it contains an eye of ender and 0 otherwise.

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Item frame and 👁 Image
glow item frame

A comparator can measure the state of an item frame's contents. An item frame comparator outputs 0 if the item frame is empty, or 1 to 8 for any item depending on its rotation: 1 at initial placement, plus 1 for each 45° of rotation for a maximum of 8.

A map placed inside of an item frame rotates 90° per use instead of 45°, but a comparator still outputs power levels 1 through 8. It takes two full rotations to cycle through all comparator outputs, and each orientation of the map corresponds to two output levels that differ by 4.

The item frame must be attached to a full block, and the comparator must be placed behind this block, facing away from the item frame.

If the item frame occupies the same space as another block that the comparator can measure,‌^{[JE only]} such as a chest, lectern, brewing stand, detector rail, decorative pot, copper golem statue, or cake, the comparator behind the item frame outputs whichever signal is stronger.

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Jukebox

A jukebox outputs a signal strength indicating which music disc is currently playing. See the Minimum Items for Container Signal Strength table above.

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Lectern

A lectern outputs a signal strength that depends on which page of the lectern's book is opened. The calculation used is:

signal strength = floor(1 + ((current page − 1) ÷ (number of pages in book − 1)) × 14)

This results in page 1 having a signal strength of 1, and the last page having a signal strength of 15. The exception is a single page book, which outputs a signal strength of 15.

For example, a book with 15 pages outputs a signal equal to the current page number. A book with 5 pages outputs signal strengths of 1, 4, 8, 11 and 15 for the different pages. A book with 100 pages has the signal strength increase to the next level on pages 1, 9, 16, 23, 30, 37, 44, 51, 58, 65, 72, 79, 86, 93 and 100.

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Respawn anchor

A respawn anchor outputs a signal strength of 0, 3, 7, 11, or 15, depending on the "charged" value.

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Sculk sensor

A sculk sensor outputs a signal strength depending on the type of vibration that is detected.

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Shelf

A shelf outputs a signal strength of 1, 2, or 4 if an item is in the left, middle, or right slot respectively. Shelves with items in multiple slots sum the signal strengths, up to a total of 7.

Java Edition:

Bedrock Edition:

Java Edition:

Bedrock Edition:

Java Edition:

Bedrock Edition:

Java Edition:

Bedrock Edition:

A redstone comparator has a block entity associated with it that holds additional data about the block.

Java Edition:

Bedrock Edition:

See Bedrock Edition level format/Block entity format.

Issues relating to "Comparator*" are maintained on the bug tracker. Issues should be reported and viewed there.

• Comparators do not emit redstone particles when powered, unlike redstone torches and repeaters.^[5]

• 👁 Dinnerbone showing how comparators work.
  
  Dinnerbone showing how comparators work.
• 👁 Dinnerbone showing how comparators work.
  
  Dinnerbone showing how comparators work.
• 👁 Dinnerbone showing how comparators work.
  
  Dinnerbone showing how comparators work.
• 👁 A contraption incorporating comparators.
  
  A contraption incorporating comparators.
• 👁 Comparators in action.
  
  Comparators in action.
• 👁 Output specific signals.
  
  Output specific signals.
• 👁 Another comparator in use.
  
  Another comparator in use.
• 👁 Rotating the torch in the item frame adjusts the comparator's output.
  
  Rotating the torch in the item frame adjusts the comparator's output.