A previous post by Beau (http://worldbuilding.stackexchange.com/users/21766/beau) mentioned that from a practical perspective the supply of zombies is infinite. While it IS very large in relative terms it is far from infinite. I'm going to make a few assumptions to make the math quick, but if you need to change those the changes to the answer should still be easy to figure.

i'm going to assume that the zombie apocalypse happens rapidly enough that the population of the world (humans + zombies) doesn't increase significantly as zombies don't procreate, they just convert existing humans to the zombie state, and the few remaining humans don't have an appreciable birthrate post-apocalypse since nearly all of them are converted to zombies. If this last part isn't the case, then I'd argue that it wasn't much of an apocalypse, just a heck of a disruption, and things will be getting back on track shortly since the zombie horde probably is dealt with if it doesn't overwhelm humanity right off the bat. I'm also leaving out the possibility of zombie animals; their inclusion would just increase the numbers, so if you like feel free to look up the mass references and redo the arithmetic. I also round a tad for clarity, go nuts recalculating with however many significant figures you find useful.

Therefore, under the assumption that pretty much all the world's population is converted to zombies, we have 632 billion pounds of humanity on the planet according to a recent paper in BMC Public Health, summarized in this article: http://www.livescience.com/36470-human-population-weight.html

convert that to metric and you have 287 billion kg of humanity. since the density of humans (and presumably zombies) is VERY close to that of water, which is 1 kg/liter, that is 287 billion liters of zombie flesh.

The minimum square 50 x 50 meter moat needed to hold this volume is calculated by dividing by 1000 to convert liters to cubic meters, then dividing by the 50x50 cross section of the moat to get the total length, then by 1000 to convert that moat length to kilometers, then by four since there are four sides to this square moat.

287000000000L / 1000L/m^3 / (50m * 50m) / 1000m/km / 4sides =

28.7 kilometers/side or, a little less than 18 miles.

That's still a HUGE moat, but it will be a whale of a lot easier to simply dig a massive great moat like this that it would be to obtain all that mercury! And it's a long way from an infinite problem!

A previous post by Beau (https://worldbuilding.stackexchange.com/users/21766/beau) mentioned that from a practical perspective the supply of zombies is infinite. While it IS very large in relative terms it is far from infinite. I'm going to make a few assumptions to make the math quick, but if you need to change those the changes to the answer should still be easy to figure.

i'm going to assume that the zombie apocalypse happens rapidly enough that the population of the world (humans + zombies) doesn't increase significantly as zombies don't procreate, they just convert existing humans to the zombie state, and the few remaining humans don't have an appreciable birthrate post-apocalypse since nearly all of them are converted to zombies. If this last part isn't the case, then I'd argue that it wasn't much of an apocalypse, just a heck of a disruption, and things will be getting back on track shortly since the zombie horde probably is dealt with if it doesn't overwhelm humanity right off the bat. I'm also leaving out the possibility of zombie animals; their inclusion would just increase the numbers, so if you like feel free to look up the mass references and redo the arithmetic. I also round a tad for clarity, go nuts recalculating with however many significant figures you find useful.

Therefore, under the assumption that pretty much all the world's population is converted to zombies, we have 632 billion pounds of humanity on the planet according to a recent paper in BMC Public Health, summarized in this article: http://www.livescience.com/36470-human-population-weight.html

convert that to metric and you have 287 billion kg of humanity. since the density of humans (and presumably zombies) is VERY close to that of water, which is 1 kg/liter, that is 287 billion liters of zombie flesh.

The minimum square 50 x 50 meter moat needed to hold this volume is calculated by dividing by 1000 to convert liters to cubic meters, then dividing by the 50x50 cross section of the moat to get the total length, then by 1000 to convert that moat length to kilometers, then by four since there are four sides to this square moat.

287000000000L / 1000L/m^3 / (50m * 50m) / 1000m/km / 4sides =

28.7 kilometers/side or, a little less than 18 miles.

That's still a HUGE moat, but it will be a whale of a lot easier to simply dig a massive great moat like this that it would be to obtain all that mercury! And it's a long way from an infinite problem!