This article will discuss how to take some readily available information and use it to estimate the undead population of a town after a zombie apocalypse.  To do this in a way that doesn’t require copious amounts of research, we are going to have to make some broad generalizations.  In addition, because zombie and human interaction trends tend to determine their most likely location, we are going to make and discuss some assumptions about that interaction.

Basic Assumptions

We are going to assume that zombies are of extremely low intelligence.  They do not reason or problem solve.  When they spot an objective (almost assuredly a perceived food source) they will head in an immediate path towards it.  When they do not have an objective they will meander around, somewhat randomly.  In this wandering state, they may or may not be subconsciously drawn towards places and activities they often repeated in life.

Some zombies will inexplicably migrate.  They will start moving in a direction and not stop until they have found some new objective.   These migrating zombies sometimes attract the attention of other zombies, and entire flocks of them occasionally wander off.  The ultimate destination of this migration might just be the other side of town, but they have also been known to head off into rural areas and beyond.  One can assume that a migration that leads out of an urban area, given enough time, could reach a new urban area and join the population there.

Zombies do not heal and do not decay rapidly.   Zombies that take damage remain that way indefinitely.  They do not die from shock, blood loss, or disease, but they do require muscles and a nervous system.  Amputated limbs stay amputated and are incapable of independent motion.  Spinal trauma (or other nervous system damage) can render limbs non-functional.  The state of zombification has also either slowed or completely staved off purification.  Given enough time, a zombie might rot away, but human-kind doesn’t have an easy out by just locking themselves away and waiting for all the dead guys to liquefy in several months.

“There’s a zombie on your lawn.”

However the zombie apocalypse starts, it is assumed that people have time to react to it.  In the first few hours after the major outbreak is identified, the government will be issuing warnings.  Local police and rescue forces will be in emergency mode, attempting to keep the peace and directing civilians to safe locations.  Once society breaks down people will be on their own, but before that it is assumed that most people will either seek safety in their own home or an impromptu disaster shelter.

Change In Population

The first thing we need to consider is how many people have been rendered permanently dead.  This number includes both those living dispatched in a way as to prevent reanimation and the walking dead that have been destroyed.  This number should take into account the fact that after the initial collapse of society there will be a lot of chaos and violence–people are not only going to be victims of zombies, but also horrific accidents and murders (as the result of panic, stupidity, arson, looting, and other savory bits of the human psyche.)  This number can change drastically given different “rules of reanimation” as in some scenarios all dead reanimate and in others only those infected reanimate.  This number can also fluctuate based on how close to the initial outbreak we are making our observations.  At the onset this number will be low, as the zombie population will be low and people will still be relatively calm.  This number will jump significantly in the first days, weeks, and possibly months after the outbreak, and things spin out of control.  At some point, the change in population of zombies and survivors should slow down–there will be less opportunity targets and the remaining people will have learned to survive in the new situation at hand.

For the sake of our exercise, we are going to assume a truly apocalyptic situation in which 50% of the world’s population has been rendered permanently dead.  Let us view the impact on a global, national, and local scale.  With an estimated 6 billion people on the planet, that means that 3 billion are gone.  The United States has a population of about 300 million, meaning 150 million are now dead.  On a local scale, a large city like Chicago has a population around 2 million, 1 million of which are now gone, and a small town like Canton, Ohio has about 39,000 dead of 78,000.

Who Is Left?

Just because 2 billion people are permanently dead doesn’t mean there are 2 billion living people walking around.  A portion of those 2 billion will be zombies.  What percent of that portion lies greatly in the GM’s vision of the apocalypse.  Did people organize, flee the cities, and form structured communities?  Were local law enforcement and military forces efficient in fighting the dead and protecting civilians?  What frequency will your group encounter new NPCs?

For our exercise, we are going to keep things grim and assume that only 1% of the population survived.  This value is not easily applied on a global level as there are plenty of remote communities which might completely escape the brunt of the apocalypse.  On a nation level we can estimate that there are 3 million survivors.  A place like Chicago would have 10,000 survivors and small-town Canton would only have about 390.

Many of the survivors will have fled cities.  Rural residences are an ideal place for survivor communities because of the reduced number of zombie and the availability of cultivatable land and livestock.  We are going to assume that at least half of the survivors flee metropolitan areas, while others stay behind in hiding or under barricades.  Chicago has a living population of about 5,000 people.  A place like Canton isn’t far from rural areas, and might actually retain less survivors as they can easily just pick up and get out.  In small towns we might assume that 75% of the population has fled, leaving Canton with around 98 people.

And What of the Dead?

If you subtract the permanent dead and the survivors, what you have left are the zombies.  This alone is not a solid number.  There are contributing factors to the flux of zombie population in an area.  Firstly, we will assume that 10% of the zombies have been rendered non-ambulatory.  They are injured, disabled, or have some other random factor which results in their inability to move about in any significant way.  Of the remaining ambulatory zombies, a portion have wandered off.  This could either be by migration or just random happenstance that takes the zombie out of the urban area.

Looking at Chicago, of the once 990,000 zombies, 99,000 are non-ambulatory, 89,100 have migrated away, and there are 801,900 bona fied walking dead infesting the streets.  Canton has 3,900 non-ambulatory, 3,510 in migration, and 31,590 dead in the area.

The ambulatory zombies will be distributed unevenly around the area.  As people fled back to homes and shelters, the zombies followed.  As many as 75% of the zombies will be found in residential areas, as this is where the food was.  Residential areas might take up 40% of a city, with things like commercial/industrial, parks and streets taking up the rest.  You can estimate how many zombies will be in a given area based on this skew.

Chicago is 234 square miles.  The average zombie per-capita is about 3,400 per square mile .  If you were in a residential area the per-capita would be about 6,425 per square mile (75% of the zombie population is 601,425 and 40% of the area is 93.6 square miles.)  Elsewhere the per-capita would be about 1,428 per square mile (25% of the zombie population is 200,475 and 60% of the area is 140.4 square miles.)

Canton is 20.6 square miles.  It has an average zombie per-captia of 1,533 per square mile.  Residential areas have a per-captia of 2,875 per square mile, and elsewhere has a per-capita of 639 per square mile.  Not quite the hellish doom of Chicago, but still a very unpleasant place to live.

You can convert the per-capita of an area into smaller units to find out specifics.  For instance, a plot of residential land in Chicago might be 300 square yards.  Take the residential per-captia of 6,425 and multiply that by 0.000096849 (1 square mile to 300 square yards) and you get a 62% chance there is a zombie standing on the lawn.  Expand that to the 8 neighboring properties, and you’ve got 4 zombies in the immediate area, with a 92% chance of one more.  Make a loud noise that travels a quarter of a mile and you have 3,212 zombies that have a chance of hearing you and coming to investigate.

Making It Your Own

Obviously, most GMs will want to tweak with all of these numbers when building their own world.  Maybe a 1% to 99% ratio of survivors to zombies is too much for your tastes?  (Or not enough if your game system allows for massive loads of zombie killing!)  Another important thing to think about is the fact that stories are interesting when they stray from the norm.  An area with more zombies than expected (or maybe an area with no zombies at all) can prove for an interesting evening of gameplay.

Tags: Zombies

This entry was posted on Friday, September 18th, 2009 at 9:54 am and is filed under Fluff/Inspiration, General, Rules Ideas. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.