After losing one eye, what percentage of the visual field is lost?

This question obviously refers to the combined visual field, since it is self-evident that loss of one eye causes a total loss of that eye's visual field!

Together, the eyes have about a 180 degrees of vision, but there is overlap for most of this. Therefore, the loss of one eye doesn't actually cause the loss of anywhere near half of this field, as you might otherwise expect. Only about 40 degrees aren't covered by both eyes, meaning that the loss of one eye leaves you with a visual field of roughly 140 degrees.

In other words, you lose somewhere between a fifth and a quarter of your visual field if you lose an eye.  Obviously, there are other problems with this - the loss of binocular vision means that your 3D perception isn't as good for close objects, and your overall visual acuity is also decreased.