Tutorials/Mob farm

Mob farms are structures built to acquire mob drops more easily and in larger numbers. They usually consist of two components: a large, dark room to spawn mobs which are funneled into a central location, and a mob grinder to kill them quickly and efficiently.

Locations
The purpose for the farm is to provide a large area that is a viable spawn position for the intended targets, and to kill the mobs quickly. Due to the rules Minecraft applies to spawning mobs, this makes the choice of a location for the mob farm a difficult problem.

Farms placed on the surface can provide good drop rates during the day, when it is one of the few spots of dark ground, but has a sharp drop in effectiveness during nighttime, when the entire surface is dark enough to support mob spawning.

Farms built underground have a time-independent drop rate, and will remain within your personal spawn range for hostiles when you go mining, but their effectiveness depends on the amount of unlit caverns in your vicinity, which provide alternative places for mobs to spawn.

Farms floating high in the sky can achieve the best spawn rates during the day and night and you are far away from caves, as they represent the only viable spawn ground. However, building one in survival is rather dangerous, and due to their height they will stop working completely when you descend underground to mine resources. To produce loot, you have to stay at the height of the farm.

Farms constructed under an ocean can provide the best during day and night, as the ocean limits the viable spawn locations to open areas underground except for drowned which will spawn within the ocean water. You can also locate your base below it to ensure you will always be close enough to spawn monsters.

Superflat worlds will provide higher spawn rates than other worlds, as the missing air pockets underground reduce the amount of dark places.

The Nether is difficult to farm, as water evaporates and most mobs are immune to fire. This reduces the amount of functioning farm designs considerably. One could try to funnel the mobs through Nether Portals to circumvent the restrictions.

Drops
What a mob farm produces depends on location and the type of grinder used to kill the mobs. Automatic killing prevents certain drops and experience, but is safer as the player is not required to be near the mobs. The following is a table of mobs that can be effectively farmed and their usual and player-caused drops. Player-caused drops and experience can be obtained only when the monster is killed directly by the player.

Note that zombie, skeleton, and creeper heads will only drop if killed by a charged creeper. Wither skeletons, however, have a small chance to drop theirs no matter the cause of death, but still will always drop them when killed via charged creeper.

Designs
When planning a mob farm, one should always consider the necessary size, which depends on where one plans to be in relation to the area. If you plan to be directly beneath the center of the farm, waiting for the items, the radius in which mobs would spawn can be easily used to calculate the size of the farm: floor( sqrt(1024 - (Spawn Floor Height - Collection Floor Height)^2 )) = Spawn Room Radius If you plan to spend your time in a less defined position, it might be easier to stack a simple design several times, ensuring that at least some areas are in the spawn range while limiting complexity.

TIP: If you are making a room that relies on darkness to spawn mobs, cave sounds and bats are good signs that your spawner is dark enough to let hostile mobs spawn.

Sinkhole
The easiest possible design consists of a large, empty area of simple shape, with one or more holes in the ground for the mobs to drop through. The edge of each hole has to be lined with opened trapdoors or gates to trick the Mob AI into believing the hole to be solid ground. Trapdoors can also be controlled with redstone, so one could shut off the farm by closing the holes remotely.

The whole room is closed by a roof to create a minimal light level. A roof height of 3 allows Endermen to spawn, while a roof height of 1 would restrict the farm to spiders.

Sinkhole farms are not very effective, as the chance for a mob to wander into a hole is small, and zero when the player is so far away that the mobs freeze. But they can be built quickly and cheaply and will work in the Nether as well (which other designs requiring water won't, because you can't place water in the Nether).

Canal-style
To improve the chances of a mob falling into the holes, one can add channels filled with flowing water, leading to the central hole. The channels are lined with open trapdoors to trick the mobs into falling in, and the water transports them into the grinders. Such a design requires a bit of planning to ensure that there is no stationary water in which the mobs might get stuck, reducing effectiveness.

Since the system uses water to transport mobs, it will fail to capture Endermen, which teleport away when touching water. Therefore, the roof of the cavern should be 2 blocks above the ground to prevent griefing of your farm by Endermen taking blocks.

Compact canal design
A very easily built design can be made in a 20x20 area, using eight water source blocks to fill the channels, which are exactly eight blocks long so that the water stops exactly at the edges of the central hole. The design can be easily stacked or placed next to each other to increase the effectiveness. This design can be made even smaller as shown in the video (14x14 area). Shrinking the design can be done by using signs to cut off the water flow at the edge of the sinkhole, forcing the mobs to fall into the central hole.

Minimal canal design
If there is not enough space for larger designs, this might be used. Its small size of 10x10 (inner area) makes it able to be incorporated into larger construction projects without trouble. It uses four water source blocks, one in each corner, with the water flowing around 3x3 squares of building material to the central hole. The parts of the water between the wall and the blocks is closed over to provide more spawning area.

Large Chamber Fully Automatic Design
This is a very large farm that requires a lot of resources, it has 6 spawn chambers in each building, and redstone controlling it. It outputs 4 waves of average 10-15 mobs every 16 or so seconds.

Active mob displacement
The Canal-Style System still suffers from the mobs freezing when outside a certain range around your character. Active Systems can correct that problem, using Redstone and Dispensers filled with a Water bucket to flood the spawning grounds repeatedly, flushing all mobs into the channels to be transported to the grinders. Thus, the farm provides area for the mobs to spawn in, but does not rely on mob movement to get them into the grinder. Using this, one can omit the trapdoors needed for the other, passive designs.

Sunlight mob farm
Another very simple design is the sunlight farm. This design takes advantage of the fact that undead mobs will search for protection from the sunlight. This farm is very easy to build and only requires a good amount of opaque blocks and trapdoors. The construction is simple: first, dig a 5x5 pit three blocks deep. On the center block, make a seven block tall tower. From the topmost block, build a 5x5 overhang. This will attract the mobs. Finally, place open trapdoors along the rim of the pit. Once day breaks, any undead mobs will walk toward the farm to escape the sun. Unable to sense the open trapdoors, they will fall into the pit. The player can then easily finish them off. For a fully automatic design, the pit could be filled with magma blocks.

Youtube tutorials
This video explains how the Mob Spawning Algorithm works so that you can design your own farms.

Here is a series of survival-friendly AFK Mob Farm tutorials:

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The older farms in some tutorials still work today, but can be improved using features such as observers.

Using observer blocks
The designs shown in the YouTube videos above are somewhat resource intensive, but this design requires only the following materials:
 * 1 dispenser (per layer)
 * 1 water bucket (per layer)
 * 1 observer (per layer)
 * 112 opaque blocks of choice (per layer)
 * 1 clock (e.g. an Ethonian hopper clock: 2 hoppers, 2 sticky pistons, 2 comparators)
 * A capture layer (just several dozen hoppers leading into a chest)

In addition to being inexpensive, it is also easier to build.

The design uses three different layers that are repeated with a redstone clock added to the top layer. The layers are as follows:
 * 1) The dispenser (with the water bucket in it) facing up, surrounded by opaque blocks (e.g. cobblestone) to hold the water.
 * 2) An observer block, facing up into the dispenser; the rest of this layer is air.
 * 3) This layer is completely air.

For layer one, the blocks must hold all of the water, so go out seven blocks in each direction, then fill in diagonally. Optionally, you can surround these blocks with signs to prevent spiders from climbing up. However, this would be a large amount of work for little benefit.

Now, the trick is that when a dispenser goes off, the observer block emits a signal to the block below it, which will activate the dispenser through the air gap in between (which is then filled with water). This will cascade down through all of the layers (make as many as you like, but anywhere between 3 and 10 should be plenty). To start this cascade, the topmost dispenser needs to be activated. It is recommended to do so using an Ethonian hopper clock with about ten items in it. Place the hopper clock with the redstone block on top of the topmost dispenser (do not place a bucket in the topmost dispenser).

Villager
This method works only on zombies and drowned because they are attracted to villagers. This is tricky, as you need a village to get the villager into the right position. This method works by having one or more villagers trapped in a place, so they can't get out. Any nearby zombies will go to the villagers. You can make a maze that the zombies have to go through so that you need only one trap. However, there needs to be some sort of trap that the zombies will fall in to get to the villagers (e.g. a ton of soul sand in water or in air, fall damage, etc.).

Other designs
Instead of using water to transport mobs, there are various other methods to make them move towards the grinder, all of which may work better with certain mobs, but can have higher resource requirements or lesser gathering rates.

One-way doors
Relying on the random movement of mobs, One-Way Door Designs use doors or pistons to prevent the mobs from wandering backwards. Making several sets of doors can increase the speed with which the mobs move towards the hole.

The easiest configuration consists of a wall of doors (Iron on Hard difficulty to prevent Zombies from smashing them, or fencegates, or maybe trapdoors somehow. Use wooden doors on other difficulties) with a row of pressure plates in front of them. When the mobs walk onto the pressure plates, the door opens, letting them through, but once on the other side, they can't open the door since there is no switch.

With Pistons, the arrangement is reversed, with the pressure plates on the desired side, and the pistons pointing upwards, so that they block the path of the mobs when extended. This design works only with a 2-block high roof to prevent the mobs from jumping, and even then might fail on spiders. However, it requires considerably more common ingredients than the iron door variant.

Such systems can also be used to "store" mobs after gathering them from the farm, so that they later can be killed for the rare drops and experience.

Slime
This farm design uses large slimes to push mobs off ledges. The simplest way to get these slimes are through a slime farm. As for the rates, it gets over 20,000 items per hour, as said in the video title.

Transporting mobs
After collecting the mobs from the farm, it might be beneficial to transport them to a different location before grinding them, so that you can access the items more easily while remaining at a close-to optimal position for the spawning of your farms.

Horizontal transportation
Horizontal Transportation can be done easily using flowing water, with a drop of 1 block vertically for every 8 block traveled horizontally. Build a tunnel with a height of 3 blocks and a width equal to the size of your gathering holes (usually 2). Mine forward to a length of 8 Blocks, so that water placed at the start ends exactly at the drop. Repeat, but one block lower, so that the mobs fall from one funnel part to the next. Mine out the roof at the higher level for 2 or 3 more blocks to prevent spider clinging to the walls from clogging up the funnel.

Alternatively, one can use a cactus to grind the spiders immediately. To do this, make the first funnel part only 7 block long and place only one water source block instead of two. Place a sand block and a cactus on top on the same side as the source block. To place the cactus, you have to mine out the block next to it. To ensure the water flow on the next part, mine out the wall block next to the sand and add the water sources there and next to the sand, where there would usually be one.

Downward transportation
Transporting mobs and items downward is trivial, simply let them drop down a chute with a water brake at the bottom. Or omit the water brake when you want the mobs to die from fall damage.

Upward transportation
Upward transportation uses the fact that nearly all mobs will attempt to swim in water, moving upwards. Therefore, to get the mobs moving upwards, one has to provide only a column of water with enough air holes to prevent them from drowning. This can be done by arranging signs or ladders and water source blocks in the following vertical configuration:

This can be repeated indefinitely in any direction for a mob elevator. When arriving at the right height, flowing water on top of the topmost ladder is enough to dislodge them.

Slimes will not swim in versions prior to Java Edition 1.8, which can be used to separate them from other mobs.

As of Java Edition 1.13 and Bedrock Edition 1.5.0, a block of soul sand can be placed at the bottom of a column of water source blocks to force entities quickly upwards, while also supplying air for long distances. If soul sand is not used, undead mobs can be separated from the others because they will not swim upwards.

Grinding
The last part of a mob farm is to kill the gathered mobs and collect their items. This can be done in different ways, using the variety of damage available in Minecraft, like falling, suffocating, drowning, burning, sunlight (for undead), touching cacti, simply player-applied damage from weapons or lava.

Passive Systems do not change to kill the mobs, they rely on constantly available damage sources. The most common mob grinders, the "Lava Blade" and "Drowning Trap" fall in this category.

Active Systems have to change configuration to actually kill mobs, usually in the form of redstone devices. The most known might be the "Piston Grinder", which applies suffocation damage by pushing an opaque block into the head of the mob. While possibly faster than passive systems in killing, active systems usually have a limited capacity, and higher amounts of mobs might clog or even jam the system. The "Minecart Grinder" uses Minecarts to carry the mobs into one block high space, suffocating them. This method can jam up too, but is fairly amusing.

EXP farms are systems that capture and soften up the mobs, but rely on the player to deliver the killing blow, so that the rare drops and experience can be gathered. An example would be a nearly-lethal falling height which would reduce the majority of mobs to half a heart, easily killed even without a weapon. However, such systems are only as good as the player using them, and in constant danger of being destroyed by creepers if not built out of obsidian, or if the player is out of sight of the creeper, in which case it will not explode.

With the Hopper, it becomes easy to collect the dropped items, even without player interference. This makes fully automatic mob farms possible where the loot will be deposited in a chest for easy accessibility.

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