Zombie hordes feature prominently in Surviving The Evacuation. I never intended this to happen, but the more I wrote, the more logical their inclusion was. The undead are attracted to noise. In a dead world, with no machines, few animals, and fewer people, the loudest sound would be that of the living dead. They would gravitate towards one another. One or two, or one or two hundred, might become trapped behind some drystone wall out in the wilds of Lancashire. Their numbers would grow until even that rigid barrier broke. Thousands would drift across the countryside. As they did, shoes and feet would kick at stone, chipping away at sole and skin as much as they wore down masonry, but eventually the wall would be ground to dust. All it would take is time. And over time, that thousand zombies would meet another thousand, and another, and another, and more still until, with their trampling feet louder than a storm, they slouched through the countryside, turning all to mud and ruin.
But how big would a horde get? Well, it may help if you imagine yourself as a zombie. Wherever you are right now, at home, at work, out in the street (especially if you are out in the street), pretend to be one of the living dead. Stagger a bit, paw at the air, snap your teeth. And as you are doing that, notice how your arms are waving around. Notice how far you are moving. (It might help if you affix some kind of powder to the soles of your shoes. If you are at work, why not try toner from a copier.) Now keep going, staggering back and forth. Go on. I’ll tell you when to stop.
No, not yet.
Keep going. Keep those arms waving and pawing, those feet stumbling and tripping, almost but not quite, about to fall. A little longer - well, actually a lot longer. Keep it going for another hour or so (thinking about it, this could be the next get-fit craze. Zombi-cise. I must remember to trademark that). Okay, so stop pretending to be a zombie (and apologise to any passers-by or co-workers you have hopefully terrified) and let’s imagine there are two of zombie-you. If your left arm collides with the other creature, and remembering that you are devoid of all thought and reason, you’ll knock this other creature to the left or yourself to the right, in line with the Newton’s Third Law of Motion. And thus is the direction the horde travels decided.
Most of the horde can only see the decaying back of the creatures in front. They can only hear the deafeningly loud, irregular stamp of necrotic feet, but they think that there is prey ahead. They are a great undulating mass of death, with about... check the area on the ground now covered in toner. It’s an irregular circle with a diameter of about 12 feet, right? (This is the average I got in my many, many experiments.)
When there is no prey, the undead squat or stand motionless. But remember that the horde isn’t stationary, they act in pursuit of this unseen prey. The laws of physics tell us* that they will form a great undulating, pulsating mass of death, pushing and shoving at one another as they heedlessly slouch across the countryside.
An irregular circle of 12 feet in diameter is approximately 452 square feet. Of course, as we’ve stated, this is an irregular circle for one of the creatures. The area that they occupy will overlap with their neighbours, so this is only giving us a rough estimate. Caveat over. How big is the horde?
There are 27,878,400 square feet in a square mile. Dividing that by 452 gives us 61,677. 87. Now, technically speaking you can have a fraction of a zombie, depending on how much of its body has been crushed, battered, or broken. For the purposes of this exercise, let’s round it up to an even number. 61,678 zombies per square mile. That’s the best-case scenario, and assumes the undead are all being respectful of one another’s personal space. The sad reality is that it is likely to be closer to twice that number. And that’s just in one square mile.
* For the record, the above is why I don’t teach physics or maths.