The northern part of the strait between Streymoy and Eysturoy is called Sundini which transitions into Tangafjørður between Norðuri í Sundum (Kollafjørdur) and Morskranes. The southern end is not precisely defined and merges with Nólsoyarfjørður strait at a line roughly between cape Eystnes (near Æðuvík on Eysturoy) and the islet of Hoyvíksholmur (near Tórshavn, Streymoy). Tangafjørður is split in a distinct northern and a southern half by two side fjords, Skálafjørður and Kaldbaksfjørður.
Tangafjørður's overall length is 12 kilometres (7.5 mi). Its southern section is wider (ca. 4.0 kilometres (2.5 mi)) than the northern section (ca. 3.0 kilometres (1.9 mi)). The deepest points of the northern and southern section are both −85 metres (−279 ft) at locations just offshore of Kolbeinagjógv and Hvítanes respectively. Where Tangafjørður flows into Nólsoyarfjørður the seafloor drops to −120 metres (−390 ft).[3] There are shallow sections (submerged thresholds) in the entrance of the Skála- and Kaldbaksfjørður, but the Tangafjørður itself has no such threshold, governing good nutrient in- and outflow. In the middle of the "intersection" of the Kaldbaks-, Skála- and Tangafjørður are reefs and skerries which break the surface. These Flesjarnar skerries are marked by a lighthouse.
The villages on Streymoy are Kollafjørður, Kaldbak, Hvítanes and Hoyvík, part of Tórshavn. The villages on Eysturoy are Morskranes, Kolbeinagjógv, a modern residential area of Strendur (all part of Sjóvar kommuna), Toftir and Nes. Windmills near Æðuvík can be seen from Tangafjørður. There are fish farms close to Nes, Kolbeinagjógv and south of Kollafjørður.[4]