The Gulf of Finland has an area of 29,500 km², its length is 428 km and it is up to 120 km wide. The width at the mouth of the gulf is 75 km and the distance from Porkkala to Rohuneeme, outside Tallinn, is 52 km. The gulf narrows in the east, eventually becoming the 10–28 km wide Gulf of Kronstadt. The largest bay on the northern coast is the Gulf of Vyborg, and in the south Narva Bay.
There are several islands in the Gulf of Finland. Gogland, Tyters, Lavansaari and Seiskari are the largest of these, and there are countless islands along the very splintered northern shoreline all the way from the west to the Gulf of Vyborg in the east. The deepest parts of the gulf can be found at the mouth of the gulf, where there is a deep with a depth of 80–100 meters. There are even depths of over 100 meters at the southern coast, while the depth at the northern coast never exceeds 60 meters. Much of the northern shoreline is quite shallow and rocky, making it difficult, even dangerous to navigate coastal waters there without accurate charts. The deepest point, 121 m, is at the Estonian coast, just northeast of Tallinn. About 5% of the water mass in the Baltic Sea is located in the Gulf of Finland.
The ocean currents tend to move clockwise on the northern hemisphere (due to the Coriolis effect), and therefore the currents are moving eastwards in near the Finnish coast, and westwards near the Estonian coast. The water is sweeter further in the gulf, because of the large river Neva, which has its outlet there.