St Isaac's Cathedral is the world's fourth largest domed building in Europe, inferior in its dimensions only to St Peter's in Rime, St Paul's in London and Santa Maria del Fiore in Florence. It is 101.5 m high, 111.2 m long and 97.6 m wide; the height of the monolithic granite columns of its porticoes is 17 m. The cathedral's central dome is one of the largest in the world; its external diameter is 25.8 m and the internal one is 22.15 m.