Catholic population in Great Britain
The independent social research agency, NatCen (www.natcen.ac.uk), conducted a study among adults in Great Britain to establish proportion of population belonging to various Christian churches (inc. Roman Catholic Church). The result, based on 3,311 responses, showed that 9 percent of the surveyed population was Roman Catholic.
Two steps were necessary before this result could be used to approximate number of Catholics at the diocesan level. Firstly, Catholics living in Scotland needed to be excluded, so that the proportion can be used in conjunction with Census 2011 data, which covered England and Wales and secondly, the number of Catholics under 18 in England and Wales needed to be estimated and added to the estimated number of adult Catholics in England and Wales.
Census 2011 (E&W) and Scottish Census 2011 estimated total number of adults at 48.36m. As such, the adult catholic population of Great Britain can be estimated at about 4.35m. Since Scottish Census 2011 collected information about size and age-breakdown of the Catholic population, it is possible to establish total number of adult Catholics in England and Wales at: 3.67m (681.2k in Scotland).
The child to adult ratio in Christian population in both England & Wales and Scotland was similar (approximately 15:85), hence it is reasonable to assume that such ratio in Catholic population will also be similar. Catholic children made up 18 percent of Scottish Catholic population. Applying the same proportion to the E&W population, gives the estimated number of catholic children of 861k, bringing the estimate of the Catholic population in England and Wales to 4.53m, or 8 percent of total population and 13.6 percent of the Christian population of England and Wales.
Catholic population at the diocesan level
The estimates of the catholic population complied by the Pastoral Research Centre Trust (www.prct.org.uk), based on the estimates supplied by parish priests to their Bishops were used to weight the England and Wales estimate for each diocese. The estimated number of Catholics in the Diocese of Salford is 308.5k. The diocese ranked fourth in England and Wales in terms of the size of Catholic population, after Westminster, Southwark and Liverpool. The size of the Christian population in the diocese is estimated at about 1.41m, hence the Catholic population makes up about 21.9 percent of the Christian population (or 12.9 percent of the general population).
There may be considerable regional differences to the proportion of Catholics in Christian and general population below the diocesan-level estimate (i.e. for individual deaneries and parishes), caused mainly by current and historic levels of migration and socio-demographics of the area. Further research may be undertaken to provide proportions at Middle-Layer Super Output Area level (weighing for different parts of the parish).