With an area of only 4,186 km², Østfold is Norway’s second smallest county. It is, however, one of the country’s most densely populated, with 265,000 inhabitants, or around 5.6% of the total population.

The number of inhabitants per km² in Østfold is 66, compared to a national average of 12.

Østfold is also one of Norway’s flattest counties (the highest point in Østfold culminates at an unimpressive 336m, or 1,102ft, above sea level), and one of the best regions for agriculture (only 3% of Norway’s surface is arable land, and a great part of this can be found in Østfold). Two thirds of Østfold are covered by forest.

The main towns are: Fredrikstad (72,000 inhabitants), Sarpsborg (51,000 inhabitants), Moss (29,000 inhabitants) and Halden (28,000 inhabitants). Other towns are Mysen and Askim, both in Indre Østfold.