Indonesia lies between latitudes 11°S and 6°N, and longitudes 95°E and 141°E. It consists of 17,508 islands, about 6,000 of which are inhabited. These are scattered over both sides of the equator. The largest are Java, Sumatra, Borneo (shared with Brunei and Malaysia), New Guinea (shared with Papua New Guinea), and Sulawesi. Indonesia shares land borders with Malaysia on Borneo, Papua New Guinea on the island of New Guinea, and East Timor on the island of Timor. Indonesia shares maritime borders across narrow straits with Singapore, Malaysia, and the Philippines to the north, and with Australia to the south. The capital, Jakarta, is on Java and is the nation’s largest city, followed by Surabaya, Bandung, Medan, and Semarang.

At 1,919,440 square kilometers (741,050 sq mi), Indonesia is the world’s 16th-largest country in terms of land area. Its average population density is 134 people per square kilometer (347 per sq mi), 79th in the world, although Java, the world’s most populous island, has a population density of 940 people per square kilometer (2,435 per sq mi). At 4,884 meters (16,024 ft), Puncak Jaya in Papua is Indonesia’s highest peak, and Lake Toba in Sumatra its largest lake, with an area of 1,145 square kilometers (442 sq mi). The country’s largest rivers are in Kalimantan, and include the Mahakam and Barito; such rivers are communication and transport links between the island’s river settlements.

Indonesia’s location on the edges of the Pacific, Eurasian, and Australian tectonic plates makes it the site of numerous volcanoes and frequent earthquakes. Indonesia has at least 150 active volcanoes, including Krakatoa and Tambora, both famous for their devastating eruptions in the 19th century. The eruption of the Toba supervolcano, approximately 70,000 years ago, was one of the largest eruptions ever, and a global catastrophe. Recent disasters due to seismic activity include the 2004 tsunami that killed an estimated 167,736 in northern Sumatra, and the Yogyakarta earthquake in 2006. However, volcanic ash is a major contributor to the high agricultural fertility that has historically sustained the high population densities of Java and Bali.

Lying along the equator, Indonesia has a tropical climate, with two distinct monsoonal wet and dry seasons. Average annual rainfall in the lowlands varies from 1,780-3,175 millimeters (70-125 in), and up to 6,100 millimeters (240 in) in mountainous regions. Mountainous areas—particularly in the west coast of Sumatra, West Java, Kalimantan, Sulawesi, and Papua—receive the highest rainfall. Humidity is generally high, averaging about 80%. Temperatures vary little throughout the year; the average daily temperature range of Jakarta is 26–30 °C (79–86 °F).