Difficulties in measuring and comparing empires

Empires are all individual in character, having been formed in widely different times under widely different political structures. In fact, the term Empire as stated above does not imply any particular form of government. Whether a nation is or was called an empire is also not relevant to whether it is considered an empire for the purposes of this article.

The calculation of the land area of a particular empire is controversial. In particular, there is the question of whether a particular empire can be considered to have laid claim to an area that is sparsely populated, or not populated at all. In general, this list errs on the side of including any land area that was explored and explicitly claimed, even if the areas were very sparsely populated or unpopulated. For example, a large portion of Northern Siberia is not included in the size of the Mongol Empire. The Mongol Empire's northern border was somewhat ill-defined, but in most places it was simply the natural border between the steppe and the taiga. Occupied areas north of this are included in the area of the empire, but at the time the majority of the taiga and tundra were unexplored and uninhabited. This area was only very sparsely populated by the Russian Empire, but it had been explicitly claimed by the Russian Empire by the 1600s, and its extent had been entirely explored by the late 1800s. Similarly, the northernmost Canadian islands such as Ellesmere Island were explored and claimed by the British Empire by the mid 1800s (virtually the entire mainland was at least sparsely populated well before that). No claims on mainland Antarctica are included in the area of any of the empires.

Due to the historical trend of increasing population and GDP, the list of largest empires in these categories is highly dependent on which relatively recent political entities are defined as empires. The measures of population and GDP as a percentage of the world total take into account this historical growth, although decent GDP data is only available for the last few centuries, accurate only for the last decades.