This may seem like a ridiculously easy question - the answer is probably staring me in the face - but I wondered if someone could help give me clarity.

I've nearly finished making my first map so am still fairly new to this. I'm trying to work out the scale for a map of an island that I've created. I want the island to be about 600,000 sq km in area.

I found a GIMP script on these forums that measures the perimeter of shapes and it says the perimeter of my island is 21,731 pixels.

What's the best way to work out how many pixels to a kilometer?