Isan covers 160,000 km² (62,000 sq mi) making it about half the size of Germany and roughly the size of England and Wales.
It is roughly coterminous with the Khorat Plateau, which tilts gently from its northwestern corner where it is about 213 m (700 feet) above sea level to the southeast where the elevation is only about 62 metres (200 feet).
Except for a few hills in the northeastern corner, the region is primarily gently undulating land, most of it varying in elevation from 90–180 m (300–600 feet), tilting from the Phetchabun Mountains in the west down toward the Mekong River.
The plateau consists of two plains: the southern Khorat plain is drained by the Mun and Chi rivers, while the northern Sakon Nakhon plain is drained by the Loei and Songkhram rivers.
The two plains are separated by the Phu Phan Mountains.
The soil is mostly sandy, with substantial salt deposits.

The Mekong forms most of the border between Thailand and Laos to the north and east of Isan, while the south of the region borders on Cambodia.
The Mekong's main Thai tributary is the Mun River, which rises in the Khao Yai National Park near Nakhon Ratchasima Province and runs east, joining the Mekong in Ubon Ratchathani Province.
The other main river in Isan is the Chi River, which flows through central Isan before turning south to meet the Mun in Sisaket Province.
The smaller Loei and Songkhram rivers are also tributaries of the Mekong, the former flowing north through Loei Province and the latter flowing east through Udon Thani, Sakon Nakhon, Nakhon Phanom, and Nong Khai Provinces.

The average temperature range is from 30.2 °C (86.4 °F) to 19.6 °C (67.3 °F).
The highest temperature recorded was 43.9 °C (111.0 °F) in Udon Thani, the lowest −1.4 °C (29.5 °F) at Sakhon Nakhon Agro Station.

Rainfall is unpredictable, but is concentrated in the rainy season from May to October.
Average annual precipitation varies from 2,000 mm (79 in) in some areas to 1,270 mm (50 in) in the southwestern provinces of Nakhon Ratchasima, Buriram, Maha Sarakham, Khon Kaen, and Chaiyaphum.
The rainy season begins with occasional short but heavy showers, eventually raining heavily for longer periods almost every day, usually in the late afternoon or at night, until it ends abruptly at the onset of the cool season.

The cool season runs from October to February and the hot season from February to May with the peak of high temperatures in April.
Isan covers 160,000 km² (62,000 sq mi) making it about half the size of Germany and roughly the size of England and Wales. It is roughly coterminous with the Khorat Plateau, which tilts gently from its northwestern corner where it is about 213 m (700 feet) above sea level to the southeast where the elevation is only about 62 metres (200 feet). Except for a few hills in the northeastern corner, the region is primarily gently undulating land, most of it varying in elevation from 90–180 m (300–600 feet), tilting from the Phetchabun Mountains in the west down toward the Mekong River. The plateau consists of two plains: the southern Khorat plain is drained by the Mun and Chi rivers, while the northern Sakon Nakhon plain is drained by the Loei and Songkhram rivers. The two plains are separated by the Phu Phan Mountains. The soil is mostly sandy, with substantial salt deposits.
The Mekong forms most of the border between Thailand and Laos to the north and east of Isan, while the south of the region borders on Cambodia. The Mekong's main Thai tributary is the Mun River, which rises in the Khao Yai National Park near Nakhon Ratchasima Province and runs east, joining the Mekong in Ubon Ratchathani Province. The other main river in Isan is the Chi River, which flows through central Isan before turning south to meet the Mun in Sisaket Province. The smaller Loei and Songkhram rivers are also tributaries of the Mekong, the former flowing north through Loei Province and the latter flowing east through Udon Thani, Sakon Nakhon, Nakhon Phanom, and Nong Khai Provinces.
The average temperature range is from 30.2 °C (86.4 °F) to 19.6 °C (67.3 °F). The highest temperature recorded was 43.9 °C (111.0 °F) in Udon Thani, the lowest −1.4 °C (29.5 °F) at Sakhon Nakhon Agro Station.
Rainfall is unpredictable, but is concentrated in the rainy season from May to October. Average annual precipitation varies from 2,000 mm (79 in) in some areas to 1,270 mm (50 in) in the southwestern provinces of Nakhon Ratchasima, Buriram, Maha Sarakham, Khon Kaen, and Chaiyaphum. The rainy season begins with occasional short but heavy showers, eventually raining heavily for longer periods almost every day, usually in the late afternoon or at night, until it ends abruptly at the onset of the cool season.
The cool season runs from October to February and the hot season from February to May with the peak of high temperatures in April.
Isan