Input (grade 9): At the height of its power, the Iranian Achaemenid Empire encompassed approximately between 5.5 and 7.5 million square kilometersThe British Museum: Forgotten Empire, The World of Ancient Persia.
Output (grade 9): At the height of its power, the Achaemenid Empire had about 7.5 million square kilometers and was territorially the largest empire of classical antiquity.