The Meu forms part of the commune's southern border.
The Rance forms part of the commune's western border, flows east-southeast through the middle of the commune, then forms most of its southeastern border.
Population
Historical population
Year
Pop.
±%
1962
329
—
1968
385
+17.0%
1975
341
−11.4%
1982
337
−1.2%
1990
267
−20.8%
1999
241
−9.7%
2008
233
−3.3%
Inhabitants of Mérillac are called mérillaciens in French.