Arignac is located 2 km north of Tarascon-sur-Ariège and some 11 km south of Foix. Access to the commune is by road D88 from Tarascon-sur-Ariège in the south passing through the village and continuing north-east to join National Highway N20 at Exit 14. The N20 passes through the south-east of the commune but the nearest exit is Exit 14 just east of the commune. The town covers about 10% of the commune with the rest mostly forest and hill country with a few farms in the south.
The Ariège river forms the south-eastern border of the commune and the Saurat flows south through the commune and the town to join the Ariège.
In 2009 the commune had 706 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.