Reading is 41 miles (66 km) due west of central London, 30 miles (48 km) southeast of Oxford and 40 miles (64 km) east of Swindon. The centre of Reading is on a low ridge between the Rivers Thames and Kennet close to their confluence, reflecting the town's history as a river port. Just before the confluence, the Kennet cuts through a narrow steep-sided gap in the hills forming the southern flank of the Thames flood plain. The absence of a floodplain on the Kennet in this defile enabled the development of wharves.

As Reading has grown, its suburbs have spread in three directions:

to the west between the two rivers into the foothills of the Berkshire Downs,
to the south and south-east on the south side of the Kennet, and
to the north of the Thames into the Chiltern Hills. Reading has its own subregional catchment area, incorporating the towns of Wokingham, Bracknell and Twyford, plus large villages such as Pangbourne, Theale, Winnersh, Burghfield and Shiplake.

Depending on the definition adopted, neither the town nor the urban area are necessarily co-terminous with the borough.

The borough has a population of 144,000 in an area of 40.40 km˛, while the Office for National Statistics' definition of the urban area of Reading is significantly larger at 232,662 people in an area of 55.35 km˛. This latter area – sometimes referred to as Greater Reading – incorporates the town's eastern and western suburbs outside the borough, in the civil parishes of Earley, Woodley, Purley-on-Thames and Tilehurst (see below for further details). This urban area is itself a component of the Reading/Wokingham Urban Area. Reading is the 17th largest settlement in England, based on the population of the urban area. Furthermore, except for London boroughs, it is the most populous settlement that does not have city status.

Historically, the town of Reading was smaller than the current borough, and has had several definitions over the years. Such definitions include the old ecclesiastical parishes of the churches of St Mary, St Laurence and St Giles, or the even smaller pre-19th century borough.

Besides the town centre, Reading comprises a number of suburbs and other districts, both within the borough itself and within the surrounding urban area. The names and location of these suburbs are in general usage but, except where some of the outer suburbs correspond to civil parishes, there are no formally defined boundaries. The borough itself is unparished, and the wards used to elect the borough councillors generally ignore the accepted suburbs and use invented ward names.

Reading Minster, or the Minster Church of St Mary the Virgin as it is more properly known, is Reading's oldest ecclesiastical foundation, known to have been founded by the 9th century and possibly earlier. Although eclipsed in importance by the later Abbey, Reading Minster has regained its importance since the destruction of the Abbey.