Andelain is located some 25 km south-east of Saint Quentin and 5 km east of Tergnier. It can be accessed by the D13 road from Charmes in the north-east passing through the village and continuing to Deuillet in the south-west. The D1032 road also runs through the north of the commune from west to east and the D553 road cuts through the south-western corner of the commune. Apart from the village the commune consists entirely of farmland with no other villages or hamlets.
The Oise river forms a small part of the north-western border of the commune and a stream flows through the north-western part of the commune into the Oise river.
In 2010, the commune had 186 inhabitants. The evolution of the number of inhabitants is known through the population censuses conducted in the town since 1793. From the 21st century, a census of municipalities with fewer than 10,000 inhabitants is held every five years, unlike larger towns that have a sample survey every year.[Note 1]
^At the beginning of the 21st century, the methods of identification have been modified by law No. 2002-276 of 27 February 2002 , the so-called "law of local democracy" and in particular Title V "census operations" which allow, after a transitional period running from 2004 to 2008, the annual publication of the legal population of the different French administrative districts. For municipalities with a population greater than 10,000 inhabitants, a sample survey is conducted annually, the entire territory of these municipalities is taken into account at the end of the period of five years. The first "legal population" after 1999 under this new law came into force on 1 January 2009 and was based on the census of 2006.