Bailly-le-Franc is located some 20 km south-west of Saint-Dizier and 15 km north-east of Brienne-le-Château. The northern border of the commune is the departmental border between Aube and Marne while the eastern border is the departmental border between Aube and Haute-Marne. Access to the commune is by the D127 road from Margerie-Hancourt in the west which passes through the centre of the commune and continues south-east, changing to the D13 at the commune border, to Droyes. The D655 comes from Outines in the north, changing to the D56 at the communal border and passing through the commune then continuing south-west to Chavanges. The south of the commune is forested with the rest farmland.
The Ruisseau de Chevry flows out of the Étang de Bailly in the west of the commune and passes through the commune towards the east before turning south to form the eastern border of the commune and the department and continuing through the Étang de la Horre to join the Voire. The Ruisseau du Pré Darras forms the northern border of the commune and the department as it flows east to join the Droye.
In 2010 the commune had 28 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 communes 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.