Alaincourt is located some 10 km south-east of Saint-Quentin and some 25 km north-west of Laon. The Motorway from Saint-Quentin – the E17 (A26) – passes directly through the commune heading south-east towards Reims – some 120 km to the south-east. The motorway has no exit in the commune so it must be reached by the road D34 coming north-east from Moy-de-l'Aisne and continuing north-east to Berthenicourt. Apart from small country roads no other roads enter the commune. Alaincourt is the only village in the commune.
Apart from Alaincourt itself, which is quite large, the commune is entirely farmland. East of the village the commune is traversed by the Oise river and the Canal de la Sambre a l'Oise both waterways heading south more-or-less parallel to each other (the canal in a straight line) until the Oise joins the Aisne at Clairoix and continues as the Oise until it joins the Seine at Conflans-Sainte-Honorine in the north-west of Paris. There are a number of ponds and reservoirs near the Oise in the south-east of the commune. Apart from the Oise and a small stream also called the Oise there are no other watercourses in the commune.
In 2010, the commune had 503 inhabitants. The evolution of the number of inhabitants is known through the population censuses conducted in the town 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 on 1 January 2009 and was based on the census of 2006.