Zombie respawn, population and migration in Project Zomboid

Every part of the map has a target zombie count. Zombies are placed the first time you enter an area you have not visited, and that first spawn has nothing to do with the respawn settings. The target is set by the Population Multiplier, which is the number behind the Zombie Count choice in sandbox settings.

Two more multipliers scale that target over time. The Start Multiplier applies on day 1 and the Peak Multiplier applies from the Peak Day onward, with a ramp between them. On Apocalypse the world goes from 1.0 to 1.5 by day 28, so the map is meant to hold half as many zombies again after four weeks. This growth happens even when respawn is off. Six Months Later runs the other way: it starts at 2.0 and falls to 1.0 by day 5.

The sources used here do not say exactly where the extra zombies from the ramp appear, or whether they can be placed in an area you have already cleared.

Respawn is a separate cycle that puts new zombies into a cell that has lost some. Three settings control it.

• Respawn Hours is the time that must pass before a cell can get zombies back. At 0 the whole cycle is off.
• Respawn Unseen Hours is how long a chunk of that cell must go unseen by any player before zombies can appear in it.
• Respawn Multiplier is the share of the cell's target population that can return in one cycle. 0.1 means 10 percent.

With the game's code defaults of 72, 16 and 0.1, a cell can regain up to 10 percent of its target every 72 in-game hours, and only in chunks nobody has looked at for 16 hours. Zombies do not appear in front of you. A base you live in and look at every day does not respawn zombies inside it, but the far side of the same cell can.

The big Build 42 change is that Apocalypse and Outbreak have respawn switched off. In Build 41 Apocalypse used 72, 16 and 0.1. One hosting guide dates the change to build 42.15 (one source). Rising, Extinction and Six Months Later still respawn. See game modes for the rest of each preset.

Migration moves zombies that already exist. Every Redistribute Hours (12 by default) zombies can shift into empty parts of the same cell. No new zombies are created, but the street you emptied can fill from the next block. Setting it to 0 turns this off.

The guides checked for this page describe redistribution as happening inside a cell. Movement from one cell to another comes from sound: zombies walk toward gunfire, alarms, engines, the helicopter and meta events, up to the Follow Sound Distance (100 tiles by default).

Idle zombies also gather into rally groups. The size setting caps how many join one group, the travel distance is how far a zombie will walk to join, and separation and radius control how far apart groups sit and how tightly they bunch. Six Months Later raises the group size to 200, which is why that preset produces roaming hordes. More on behavior is on the zombies page.

All of these sit under Advanced Zombie Options. The default column is the Build 42 Apocalypse value.

The presets differ mainly in population and respawn.

A custom sandbox or a new server that does not start from a preset uses the code defaults instead: population 1.0, respawn 72 / 16 / 0.1. That means a fresh dedicated server can respawn zombies even though single-player Apocalypse does not.

On Build 42 Apocalypse and Outbreak, a zombie you kill is not replaced by the respawn cycle. A town still does not stay empty by itself, for three reasons: the peak ramp raises the world target until day 28 (day 20 on Outbreak), redistribution spreads the survivors of a cell back over it every 12 hours, and noise pulls in zombies from neighboring cells.

On Rising, Extinction and Six Months Later, clearing is never permanent. Any part of a cell you stop watching for 16 hours can regain zombies every 72 hours.

Opinion For a town you want to keep, such as Muldraugh or Rosewood, clear the whole area and not only your street, so redistribution has nothing left to move. Then keep firearms and car alarms for places you do not live in. Quiet sites are listed under base locations.

New single-player game. Choose Custom Sandbox, open Advanced Zombie Options and set Respawn Hours to 0. Set Redistribute Hours to 0 as well if you want cleared blocks to stay clear. To remove the growth over the first month, make the peak multiplier equal to the start multiplier.

Existing save. The values are fixed when the world is created. Guides suggest a sandbox-editing mod from the Workshop, or backing up the save folder and editing its sandbox file by hand (one source). See mods.

Server. Stop the server, open servername_SandboxVars.lua and edit the ZombieConfig block: RespawnHours, RespawnUnseenHours, RespawnMultiplier and RedistributeHours. Restart, then confirm in the admin panel under Sandbox Options. One hosting guide reports that the older top-level ZombieRespawn dropdown is not read by the 42.20 server, so change the ZombieConfig numbers (one source). Renaming the server creates a new settings file, which looks like a reset. Setup is covered in hosting a server.

Meta events are off-screen noises the game invents, with distant gunshots as the standard example. There is no shooter to find. The sound gives zombies in range a direction to walk, so it works as migration pressure and not as a guaranteed attack. The Meta Event setting has three options: Never, Sometimes and Often. Apocalypse uses Sometimes, and Extinction uses Often (one source). In a server file Sometimes is written as MetaEvent = 2 (one source).

The helicopter is the largest single pull. It makes one pass on Apocalypse and Outbreak, commonly around days 6 to 9 (one source). Rising has it set to Never and Extinction to Sometimes. If it sees you outdoors it follows you, and the engine noise drags zombies along its path. Stay indoors and out of sight, and listen for the air activity warning on the emergency broadcast radio channel.