Illinois Route 185 passes through the village, leading northwest 17 miles (27 km) to Vandalia, the county seat, and southeast 6.5 miles (10.5 km) to Farina.

According to the 2021 census gazetteer files, St. Peter has a total area of 0.52 square miles (1.35 km2), all land.

[4] As of the 2020 census[3] there were 322 people, 152 households, and 103 families residing in the village.

There were 144 housing units at an average density of 276.39 per square mile (106.71/km2).

25.66% of all households were made up of individuals, and 17.76% had someone living alone who was 65 years of age or older.