Aulnay is located some 28 km east by south-east of Arcis-sur-Aube, 16 km north-west of Brienne-le-Château, and 50 km west by south-west of Saint-Dizier. Access to the commune is by the D 35 road from Jasseines in the north which passes through the west of the commune and the village before it continues south to Chalette-sur-Voire. The D 5 road comes from Brillecourt in the west and passes through the village before continuing east to Braux. South of the village is the hamlet of Petit Aulnay. Apart from a belt of trees along the river the commune is entirely farmland.
The Ravet river flows through the commune from east to west forming part of the south-facing border in both the east and west of the commune before continuing west to join the Aube at Brillecourt.
In 2010 the commune had 126 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.