Throughout most of its history, Istanbul has ranked among the largest cities in the world. By 500 CE, Constantinople had somewhere between 400,000 and 500,000 people, edging out its predecessor, Rome, for world's largest city. Constantinople jostled with other major historical cities, such as Baghdad, Chang'an, Kaifeng and Merv for the position of world's most populous city until the 12th century. It never returned to being the world's largest, but remained Europe's largest city from 1500 to 1750, when it was surpassed by London.
The Turkish Statistical Institute estimates that the population of Istanbul Metropolitan Municipality was 15,701,602 at the end of 2024, hosting 18 percent of the country's population. Then about 97–98% of the inhabitants of the metropolitan municipality were within city limits, up from 89% in 2007 and 61% in 1980. 64.9% of the residents live on the European side and 35.1% on the Asian side. While the city ranks as the world's 5th-largest city proper, it drops to the 24th place as an urban area and to the 18th place as a metro area because the city limits are roughly equivalent to the agglomeration. Today, it forms one of the largest urban agglomerations in Europe, alongside Moscow. The city's annual population growth of 3.45 percent ranks as the highest among the seventy-eight largest metropolises in the Organisation for Economic Co-operation and Development. The high population growth mirrors an urbanization trend across the country, as the second and third fastest-growing OECD metropolises are the Turkish cities of İzmir and Ankara.
