The United Kingdom covers approximately 94,354 square miles (244,376 km²), with a land area of 93,723 square miles (242,741 km²). It occupies the majority of the British Isles and consists of England, Scotland, Wales, and Northern Ireland. The UK also includes numerous smaller islands, with the Isle of Man and the Channel Islands—Jersey and Guernsey—functioning as Crown dependencies. These dependencies are in union with the British monarch but are not officially part of the UK, although the British government oversees their external affairs.

Geographically, the UK lies between the North Atlantic Ocean and the North Sea. Its southeast coast is only 22 miles (35 km) from northern France, separated by the English Channel. The UK is connected to continental Europe via the Channel Tunnel, which at 31 miles (50 km) in length (24 miles or 38 km underwater) is the longest underwater tunnel in the world. Northern Ireland shares a 310-mile (499 km) land border with the Republic of Ireland and has a coastline of 404 miles (650 km). Great Britain and its surrounding islands have a combined coastline of approximately 19,491 miles (31,368 km), though measurements vary due to the coastline paradox.

The Royal Greenwich Observatory in London defines the Prime Meridian, established at the International Meridian Conference in 1884. The UK spans latitudes 49° to 61° N and longitudes 9° W to 2° E, encompassing diverse terrains from lowlands and rolling hills to rugged highlands and coastal cliffs. Its natural environment includes four terrestrial ecoregions: Celtic broadleaf forests, English Lowlands beech forests, North Atlantic moist mixed forests, and Caledonian conifer forests. Woodland areas cover an estimated 3.25 million hectares, accounting for around 13% of the nation’s land area, supporting rich biodiversity across the islands.