Thunstetten has an area of 9.66 km² (3.73 sq mi).
 Of this area, 5.2 km² (2.0 sq mi) or 53.9% is used for agricultural purposes, while 2.53 km² (0.98 sq mi) or 26.2% is forested.
  Of the rest of the land, 1.91 km² (0.74 sq mi) or 19.8% is settled (buildings or roads), 0.03 km² (7.4 acres) or 0.3% is either rivers or lakes and 0.01 km² (2.5 acres) or 0.1% is unproductive land.

Of the built up area, industrial buildings made up 2.0% of the total area while housing and buildings made up 9.3% and transportation infrastructure made up 7.4%.
 Out of the forested land, all of the forested land area is covered with heavy forests.
 Of the agricultural land, 38.4% is used for growing crops and  9.9% is pastures, while 5.5% is used for orchards or vine crops.
 All the water in the municipality is in lakes.

The municipality is located in the Oberaargau region.
It consists of the villages of Thunstetten and Bützberg as well as many scattered small settlements.

On 31 December 2009 Amtsbezirk Aarwangen, the municipality's former district, was dissolved.
 On the following day, 1 January 2010, it joined the newly created Verwaltungskreis Oberaargau.
Thunstetten has an area of 9.66 km² (3.73 sq mi).  Of this area, 5.2 km² (2.0 sq mi) or 53.9% is used for agricultural purposes, while 2.53 km² (0.98 sq mi) or 26.2% is forested.   Of the rest of the land, 1.91 km² (0.74 sq mi) or 19.8% is settled (buildings or roads), 0.03 km² (7.4 acres) or 0.3% is either rivers or lakes and 0.01 km² (2.5 acres) or 0.1% is unproductive land.
Of the built up area, industrial buildings made up 2.0% of the total area while housing and buildings made up 9.3% and transportation infrastructure made up 7.4%.  Out of the forested land, all of the forested land area is covered with heavy forests.  Of the agricultural land, 38.4% is used for growing crops and  9.9% is pastures, while 5.5% is used for orchards or vine crops.  All the water in the municipality is in lakes.
The municipality is located in the Oberaargau region. It consists of the villages of Thunstetten and Bützberg as well as many scattered small settlements.
On 31 December 2009 Amtsbezirk Aarwangen, the municipality's former district, was dissolved.  On the following day, 1 January 2010, it joined the newly created Verwaltungskreis Oberaargau.
Thunstetten