How to Make a Mob Farm

Guide Java Bedrock

Quick answer: Build a dark room high in the air (or over the ocean) with spawning platforms and water channels that flush mobs into a central drop chute. Hostile mobs spawn in dark spaces within 128 blocks of you in Java, and a 22-block drop leaves most 20-HP mobs on half a heart, so one hit finishes them for full XP.

What you need

Stacks of building blocks (cobblestone)
Water buckets + signs/trapdoors
Hoppers and chests
Torches/slabs for spawn-proofing

Step by step

Pick the spot. Hostile mobs can only spawn within a 128-block sphere around you in Java (roughly 96 blocks horizontally in Bedrock at simulation distance 6 or higher). Build over deep ocean or high in the sky so you don't have to light up caves.
Build spawning platforms. Make dark platforms with a roof tall enough for the mobs you want: a 3-block roof lets Endermen spawn, a 1-block roof limits the farm to spiders. Line the edges of the central holes with open trapdoors or fence gates, so mobs think the ground is solid and walk in.
Add water flushing. Channels of flowing water on each platform carry mobs to a central hole. Lining the channels with open trapdoors tricks mobs into walking off. Spiders can clog the hole, so keep an eye on it.
Build the kill chute. A drop of 22 blocks deals 19 fall damage (own calculation: distance minus 3), which leaves 20-HP mobs such as zombies, skeletons and creepers on half a heart, so one punch kills them and you get the XP. Put hoppers and a chest at the bottom to collect the drops.
Light up everything else. The farm only works fast if it is the only dark spawnable space in range: light every cave and surface inside your sphere, or stay high above them so they are out of range.

Java vs Bedrock

Java: mobs spawn only inside a 128-block radius sphere around you, and hostile mobs farther than 128 blocks from every player despawn instantly. Bedrock: spawning happens in a shell 24-128 blocks away, limited to roughly 96 blocks horizontally at simulation distance 6 or higher and to 44 blocks at simulation distance 4. Bedrock also has caps on mob density, so farms there give lower rates.

Versions: Dark-room mob farms have worked since Classic. The 1.18 Caves & Cliffs update changed spawning to light level 0 (was 7), which made spawn-proofing far easier and dark-room farms more efficient.

Frequently asked questions

Why is my mob farm so slow?

Usually other dark spaces in range are using up the mob cap, such as unlit caves below you. Light them up or move the farm so they are out of range. Also check that the difficulty is not Peaceful.

How high should the drop be?

22 blocks takes off 19 HP (own calculation), which leaves zombies, skeletons and creepers (20 HP) on half a heart. Spiders (16 HP) die from it and witches (26 HP) survive, so use a longer drop if you want a pure loot farm.

Do mob farms work in Bedrock?

Yes, but Bedrock limits mob density with caps, so rates are lower than in Java. Build over an ocean and mind your simulation distance.