Bagneux is located some 10 km north by north-west of Soissons and 25 km south-east of Noyon. It can be accessed by road D428 from Épagny in the west running east through the commune to Juvigny. The south and west of the commune are heavily forested slopes with the rest of the commune flat farmland.
Part of the southern border of the commune is formed by Le Plat Ru stream which joins the Ru de Mareuil which is the western border of the commune and continues westwards.
In 2010, the commune had 73 inhabitants. The evolution of the number of inhabitants is known through the population censuses conducted in the commune 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 in 1 January 2009 and was based on the census of 2006.