In the north it is bounded by the Phi Pan Nam Range, one of the hilly systems of northern Thailand.

The four-region system includes provinces only as far north as Chai Nat, Sing Buri and Lopburi and extends west and east to the borders of Myanmar and Cambodia.

In the geographic six-region system, the central region includes the following 22 provinces, divided into three groups: The four-region system includes 26 provinces in its definition of Central Thailand.

Especially for statistical purposes, they are divided into four groups:[3] The eastern region is sometimes listed as a separate region distinct from Central Thailand: sometimes only the four coastal provincesand sometimes the above list excluding Nakhon Nayok.

For economic statistics of Central Thailand by National Statistical Office (NSO) the following six provinces are listed: 1.Ang Thong 2.Ayutthaya 3.Chai Nat 4.Lopburi 5.Saraburi 6.Sing Buri However Nakhon Nayok province is listed by eastern Thailand.