Skip to content

The difference between population estimates and population projections

A detailed look at how they are calculated and their uses

The Office for National Statistics (ONS) produces two main measures of population change: estimates and projections. The key difference between these is that estimates consider only past population size and structure whereas projections are concerned with future populations.

What are population estimates?

ONS calculates annual estimates of the usual resident population of England and Wales, and these can be broken down by local authority, sex and age. The most authoritative population estimates come from the Census, which takes place every 10 years in the UK, the most recent being held in March 2011. Population estimates from the Census are updated each year to produce mid-year population estimates for the years in between Censuses.

Who uses population estimates?

Mid-year population estimates currently have a wide variety of uses within central government, as well as being used by local and health authorities, other public bodies, commercial companies, and individuals in the private and academic sector.  For some groups the absolute numbers are of key importance, such as allocating financial resources from central government, planning services and grossing up survey results. Also, the population figures are used as denominators in the calculation of rates, indicators and ratios.

How are population estimates calculated?

Population estimates are made using the cohort component method, which is a standard demographic method and is used by several other national statistics institutions. The starting point for the estimates is the resident population on 30 June in the previous year. This population by single year of age is then aged on by one year (for example all three-year-olds become four-year-olds one year later). Those who have been born during the 12 month period prior to the mid-year point are then added on to the population and all those who have died during the 12 month period are removed according to their age, sex, and their usual place of residence. Numbers of births and deaths come from the Civil Registration System administered by ONS.

Movements of people into and out of the UK (international migration) and movements between different areas in the UK (internal migration) are also accounted for in the population estimates. Internal migration includes both moves between the other countries of the UK and moves between local areas within each part of the UK. Migration is the most difficult part of the process to measure because the UK has no population register so we use the best available proxy data to estimate migration instead. 

What are population projections?

Population projections provide a picture of the population as it may develop in future years, indicating the potential size and age-sex structure. They are not forecasts and do not attempt to predict the impact that future government policies, changing economic circumstances or other factors might have on demographic behaviour. ONS calculates a principal projection and a number of variant projections which are based on alternative, but still realistic, assumptions of future fertility, mortality and migration and are intended to provide users with an indication of the uncertainty surrounding projections.

Who uses population projections?

ONS projections are useful for a number of groups in UK society as they help plan for the future. The national projections are used by government departments to calculate information such as future school place requirements, expected future cost of state pensions and potential demand for health services. They are also used as a base for the sub-national projections, which are used for planning and estimating future provision of education, healthcare and social services.

For example, the Department for Communities and Local Government has in the past used the sub-national projections for England as an input into the funding calculations for local authorities. They are also an input to the official Household Projections, used to help regional house planning and monitoring.

How are population projections calculated?

The starting point for the projections is the latest available mid-year population estimate of the resident population.  Population projections are made using the cohort component method and are made for successive years running from one mid-year to the next. For each age, the starting population, taking into account net migration less the number of deaths, produces the number in the population, one year older, at the end of the year.  The survivors of those projected to be born during the year are then also added.

The numbers of births, deaths and migrants are calculated using assumptions of future levels of fertility, mortality and migration. They are determined by a mixture of trend observation (based on observed levels and change over previous years), and consideration of expert opinion, with actual data included in the calculation for the first year of the projection. The resulting projection is called the principal projection.

Where can I find out more about population statistics?

Population projections are calculated by the Population Projections Unit and population estimates are produced by the Population Estimates Unit. If you would like to find out more about the latest population statistics, you can visit the ONS population theme page or the population estimates methodology and population projections methodology pages. If you have any comments or suggestions, we would like to hear them. Please email us at: or

Categories: Population, Population Change, Population Projections, National Population Projections
Content from the Office for National Statistics.
© Crown Copyright applies unless otherwise stated.