Start from a dungeon spawner
Natural dungeons with zombie or skeleton spawners are the best first farm: the spawner activates when you are within 16 blocks, spawns in a 9×3×9 area around it, and needs only darkness. Dig around it, expose the spawner room, and plan a collection floor 2 blocks below the spawner.
The water funnel and the drop
Place water in the back corners of the room so currents push mobs toward one point; dig a 1×2 outlet into a falling shaft 20+ blocks deep. The drop leaves mobs at half a heart — you finish them with one punch through a slit, which routes all XP to you. Mobs killed purely by the fall drop loot but no XP.
Light control is the whole build
Torch every dark corner of the room and the shaft ceiling — hostile spawns need light level 0 in modern versions. Stand within 16 blocks of the spawner to keep it active, and AFK-punch from the kill slit for hands-free sessions. Skeletons give bones for bonemeal (crop booster) and arrows; zombies give the iron-farm-adjacent loot and 5 XP each.
Building the dark platform itself is a lighting problem — see the light level guide for why a solid roof at height 3 is what drops sky light to 0.
How a spawner actually works — the four numbers that matter
A monster spawner is not a magic box; it runs on four hard numbers, and a farm that underperforms has almost always broken one of them:
- 16 blocks — the radius a player must stay within for the cage to be active at all. Step further away and it simply stops.
- 9 × 3 × 9 — the volume the cage searches for valid spawn spots, centred on itself: four blocks out on each horizontal side, one block above and one below.
- Block light 0 — the same light rule as natural spawning, so every one of those spots has to be fully dark in current versions.
- A short cycle — after each burst the spawner waits a random interval (roughly ten to forty seconds) before trying again, and keeps producing all day while you stand in range with the floor dark.
Because a spawner does not use up the normal hostile mob cap for the mobs it creates, a single cage can be a steady, low-effort XP source that needs no redstone. The light level and mob spawning guide explains why "fully dark" is easier to reach than it sounds once sky light is blocked.
Digging the chamber: the geometry of a 9 × 9 spawn floor
The cage only ever looks at a 9 × 3 × 9 volume, so the room needs to be exactly that box plus the walls around it. Hollowing out a huge cavern under a spawner does nothing for the farm and only creates more dark floor for mobs to wander across.
The spawnable floor is the 9 × 9 footprint — 81 blocks — minus the cage itself, leaving 80 valid spawn positions. Underneath it you want:
- A one-block-thick floor with a single 1 × 2 outlet at the centre, so mobs have exactly one path down.
- Water channels along the two back edges pushing mobs toward that outlet — one source block at each back corner, running to the middle.
- A two-block-high kill chamber at the bottom of the drop, with a 1 × 1 slit to hit mobs through.
Keep every wall inside the 9 × 9 box: mobs only spawn where the cage can see, so floor outside the box is wasted digging and wasted water.
How far to drop them: the fall-damage formula
A mob that dies from the fall drops loot but gives no XP — XP goes only to a player kill. So the goal is a drop that leaves a mob on a sliver of health so one punch finishes it. Fall damage in Minecraft is:
damage (in health points) = blocks fallen − 3
so a mob with H health points survives a drop of up to H + 2 blocks, and one more than that kills it. That gives a hard ceiling per mob type:
| Mob | Health (HP) | Safe drop | Drop that kills it |
|---|---|---|---|
| Zombie | 20 | 22 blocks | 23 blocks |
| Skeleton | 20 | 22 blocks | 23 blocks |
| Creeper | 20 | 22 blocks | 23 blocks |
| Spider | 16 | 18 blocks | 19 blocks |
A 22-block drop is the classic choice for a zombie or skeleton spawner: 22 − 3 = 19 damage leaves the mob at 1 HP (half a heart), so it always dies to a single hit but never dies to the fall. A 23-block shaft looks identical and quietly halves your XP, because every mob dies on impact.
Three worked examples
Example 1 — a zombie spawner dug out cleanly
Open a 9 × 3 × 9 box around the cage and add a solid roof one block above it, which drops sky light to 0 across the whole footprint. The spawn floor is 81 − 1 = 80 usable blocks, and after roofing every one of them is a candidate. Two corner water sources then sweep the mobs to the centre outlet.
Example 2 — sizing the drop shaft
A zombie has 20 HP, so the shaft is 22 blocks deep: 22 − 3 = 19 damage leaves it at 1 HP. Dig it as a straight 1 × 1 drop with the kill chamber at the bottom. If spiders keep surviving the climb-out, remember their 16 HP means a 19-block fall already kills them — so spider-proofing means a shorter 18-block shaft, not a longer one.
Example 3 — how much XP a short session gives
Each hostile mob killed by a player is worth 5 XP, and reaching level 6 takes 6² + 6 × 6 = 72 XP. That is 72 ÷ 5 = 14.4, so 15 zombies — a couple of minutes at a spawner producing mobs continuously. Had the same 15 zombies been killed by the fall, you would have picked up 15 pieces of loot and 0 XP.
Why a spawner farm stalls — the usual suspects
- You drifted out of range. The cage stops the moment you are more than 16 blocks away, so sit a few blocks inside that radius rather than on its edge.
- Light leaked in. A single unsealed corner or a hole in the roof lifts block light above 0 and freezes the spawner. Check for light shafts first.
- Mobs piled up in the chamber. The spawner will not place a mob where one already stands, so a blocked drop makes the farm choke on its own output — keep the channel and shaft clear.
- The drop is too deep. A 23-block shaft kills every mob on impact and silently converts XP into loot you may not even collect.
- You are standing in the chamber. Your body blocks spawn positions and can shove mobs out of the box entirely.
Where to verify the mechanics
The activation radius, spawn volume and light rule above are the current behaviour in both editions; the authoritative reference is the official Minecraft Help Center and the Minecraft site of record. For the spawn-light rule the dark chamber depends on, see the light level and spawning guide.
Building and running a mob grinder
A grinder is a build first, then a long AFK stretch at the kill slit. These are the items that suit both halves of a spawner farm.
- Gaming mouse — Finishing mobs at the drop is one punch after another; a mouse that takes repeated clicks without complaint is the point here.
- SSD — The dark platform and the shaft mean constant chunk loading while you build; an SSD keeps the world responsive.
- Gaming headset — Spawner farms are played by ear as much as by eye; a headset catches the mobs you would otherwise miss.
- Mechanical keyboard — Building the water funnel and the outlet is hotkey-heavy; responsive keys speed up the repetitive placement.
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Sources and standards
Where these figures come from
Every formula on this page is implemented from the published reference material below. Where a value depends on a regional code, the calculator lets you override it rather than assuming one jurisdiction.
- Minecraft — official site — the game of record for mechanics, versions and release notes.
- Mojang Studios — the developer, and the authority on balance changes between versions.
- Minecraft Help Center — official Mojang/Microsoft support documentation for game mechanics and troubleshooting.
Figures, formulas and reference tables last verified: 25 September 2026.
