Figuring out land area in GIMP?

[acrosome]

I'm trying to figure out how much of my world is land and how much is ocean.

I made a black & white equirectangular sea mask of my world, loaded it into GProjector and exported it as Mollweide (which is, obviously, an equal-area projection). I loaded this into GIMP, added an alpha channel, and selected and deleted everything that was border. So now theoretically all I should have to do is look at the color histogram to figure out how much is black and how much is white, right? Only they don't add up. I get a total of something like 96% between them.

What am I doing wrong?

EDIT-- Granted, when I compressed the sea mask to make it small enough to load into GProjector I necessarily created some gray pixels. Would that be enough to mess this up? If so, how can I do it better?

EDIT EDIT-- Dammit, it was those gray pixels. I figured it out. I have to raise my frustration threshold or something.

[johnvanvliet]

if it is a black ( 0 ) and white ( 255) only image
open /color / info / histogram

the mean average is related to that
128 is 50% ,50% black and white
the higher the average the more white it is

then it is a simple ratio