Arnicourt is located some 6 km north by north-west of Rethel and 8 km south-west of Novion-Porcien. Access to the commune is by the D10 road from Sorbon in the south passing through the commune and the village continuing to Sery in the north. The commune is entirely farmland with a small patch of forest in the north.
The Plumion river flows through the north of the commune from north to south-west joining the Rayee on the south-western border. The Rayee continues south-west forming a small part of the south-western border.
In 2009 the commune had 164 inhabitants. The evolution of the number of inhabitants is known from the population censuses conducted in the commune since 1793. From the 21st century, a census of communes 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" allows, after a transitional period running from 2004 to 2008, the annual publication of the legal population of the different French administrative districts. For communes with a population greater than 10,000 inhabitants, a sample survey is conducted annually, the entire territory of these communes 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.