It is the fourth largest lake located entirely within Vermont, and has a 1,375 acres (5.56 km2) surface area, an average depth of 20 ft (6.1 m) and a maximum depth of 33 ft (10 m).

[1] Lake Carmi supports many species of fish, including northern pike and walleye.

The lake drains northwards into the Pike River in Quebec.

In the thousands of years since the last ice age, the southern end of the original lake has silted in, creating wetland forests and the third largest peat bog in Vermont.

The name Lake Carmi was officially adopted in 1910 in honor of Carmi L. Marsh, a resident of Franklin who enlisted in the Union Army for the Civil War and was commissioned as a second lieutenant in the 13th Vermont Infantry.