There are many different ways to look at how much land is protected in each county. I will start with the technical, but if you read to the end I promise you some pretty impressive data!
The technical: The World Conservation Union (IUCN) defines six management categories of protected areas in two groups.
Group One Lands are totally protected areas that are maintained in a natural state and are closed to extractive uses. They comprise Category I, Strict Nature Reserves/Wilderness Area; Category II, National Park; and Category III, National Monuments.
Group Two Lands are partially protected areas managed for specific uses such as recreation, or to provide optimum conditions for certain species or ecological communities. They comprise Category IV, Habitat/Species Management Areas; Category V, Protected Landscape/Seascapes; and Category VI, Managed resource Protected Areas.
The very easy-to-use "Nation Master" web site ranks countries by the percentage of land in "protected areas" and by the percentage of land that is still "wild" ("wildness").
On this web site "protected areas" seems to combine both Group One and Group Two IUCN land protection definitions -- a pretty good index of aesthetically, culturally or environmental important lands afforded a significant level of government protection.
The "Wildness" index on the Nation Master site, uses the percent of land in a given country with a "very low anthropogenic impact". In other words, this is land with very, very low population densities and not much evidence of human disturbance (often because it is desert or tundra). The "wildness" data is largely from Columbia University's Center for International Earth Science Information Network, with an overlay for human population densities (see footnote on the Nation Master web site).
Now for the really impressive numbers.
The growth in the acreage of formally protected lands in the U.S, Canada, and Mexico is very impressive. Since 1970, North American acreage off limits to development rose from 247 million to 741 million acres — about 15 percent of the continent's land surface. Almost all of this is due to the naming of new wilderness, near-wilderness, and biological reserves.
On the international front, the same tremendous growth in protected lands we see in North America is also occurring overseas.
In 2004, the United Nations reported that there are now over 102,000 environmentally protected areas around the world totaling over 17 million square kilometers of land (another 1.8 million square kilometers is underwater).
To put it another way, about 11.5 per cent of the Earth's land surface - an area the size of South America - is now protected.
For scale, and in comparison, the area of the world's protected areas is now far bigger than the land surface of India and China combined.
It is also larger than all land in the world under permanent, arable, crops.